Bump github/codeql-action/analyze from 4.36.2 to 4.37.9

Bumps [github/codeql-action/analyze](https://github.com/github/codeql-action) from 4.36.2 to 4.37.9. - [Release notes](https://github.com/github/codeql-action/releases) - [Changelog](https://github.com/github/codeql-action/blob/main/CHANGELOG.md) - [Commits](https://github.com/github/codeql-action/compare/8aad20d150bbac5944a9f9d289da16a4b0d87c1e...cdf488f595d80d6e07e03d4674febd5ab45fa938) --- updated-dependencies: - dependency-name: github/codeql-action/analyze dependency-version: 4.37.9 dependency-type: direct:production update-type: version-update:semver-minor ... Signed-off-by: dependabot[bot] <support@github.com>
Fix float round/2, floor/2, ceil/2 raises wrong exception for invalid precision (#15872 )
2026-09-08 21:54:46 +00:00 · 2026-09-08 18:06:30 +02:00 · 2026-09-08 14:54:59 +02:00 · 2026-09-08 13:11:39 +02:00 · 2026-09-08 08:53:16 +09:00 · 2026-09-07 16:24:15 +02:00
389 changed files with 21493 additions and 10239 deletions
@@ -3,8 +3,7 @@

 ---
 name: Report an issue
-description:
-  Tell us about something that is not working the way we (probably) intend
+description: Tell us about something that is not working the way we (probably) intend
 body:
  - type: markdown
    attributes:
@@ -13,11 +12,17 @@ body:


        Please, do not use this form for guidance, questions or support.
-        Try instead in [Elixir Forum](https://elixirforum.com),
-        the [IRC Chat](https://web.libera.chat/#elixir),
-        [Stack Overflow](https://stackoverflow.com/questions/tagged/elixir),
-        [Slack](https://elixir-slackin.herokuapp.com),
-        [Discord](https://discord.gg/elixir) or in other online communities.
+        Try instead in [Elixir Forum](https://elixirforum.com) or any of
+        our online communities (Slack, Discord, etc).
+
+  - type: checkboxes
+    id: existing-issue
+    attributes:
+      label: Existing issue
+      description: Please search [existing issues](https://github.com/elixir-lang/elixir/issues) before continuing.
+      options:
+        - label: I have searched existing issues and could not find a duplicate.
+          required: true

  - type: textarea
    id: elixir-and-otp-version
@@ -0,0 +1,4 @@
+* Describe here the reasons behind the pull request.
+* Make sure you have read the CONTRIBUTING.md file.
+* Make sure any relevant documentation and tests have been added/updated.
+* Do not submit Draft pull requests unless previously asked/agreed.
@@ -26,33 +26,27 @@ jobs:
      fail-fast: false
      matrix:
        include:
-          - otp_version: "28.1"
+          - otp_version: "29.0"
            deterministic: true
-          - otp_version: "28.1"
-            erlc_opts: "warnings_as_errors"
+          - otp_version: "28.4"
            docs: true
            coverage: true
+          - otp_version: "28.1"
          - otp_version: "27.3"
-            erlc_opts: "warnings_as_errors"
          - otp_version: "27.0"
-            erlc_opts: "warnings_as_errors"
-          - otp_version: "26.0"
          - otp_version: master
            development: true
          - otp_version: maint
            development: true

-    # Earlier Erlang/OTP versions ignored compiler directives
-    # when using warnings_as_errors. So we only set ERLC_OPTS
-    # from Erlang/OTP 27+.
    env:
-      ERLC_OPTS: ${{ matrix.erlc_opts || '' }}
+      ERLC_OPTS: "warnings_as_errors"
    steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false

-      - uses: erlef/setup-beam@e6d7c94229049569db56a7ad5a540c051a010af9 # v1.20.4
+      - uses: erlef/setup-beam@54075bcc5e249e4758d363f27d099f55d843f124 # v1.24.1
        with:
          otp-version: ${{ matrix.otp_version }}

@@ -98,7 +92,7 @@ jobs:

      - name: "Upload Coverage Artifact"
        if: ${{ matrix.coverage }}
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
        with:
          name: TestCoverage
          path: cover/*
@@ -121,19 +115,19 @@ jobs:
    strategy:
      matrix:
        otp_version:
+          - "29.0"
          - "28.1"
          - "27.3"
-          - "26.2"

    steps:
      - name: Configure Git
        run: git config --global core.autocrlf input

-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false

-      - uses: erlef/setup-beam@e6d7c94229049569db56a7ad5a540c051a010af9 # v1.20.4
+      - uses: erlef/setup-beam@54075bcc5e249e4758d363f27d099f55d843f124 # v1.24.1
        with:
          otp-version: ${{ matrix.otp_version }}

@@ -28,16 +28,16 @@ jobs:
            build-mode: none
    steps:
      - name: Checkout repository
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false
      - name: Initialize CodeQL
-        uses: github/codeql-action/init@89a39a4e59826350b863aa6b6252a07ad50cf83e # v4.32.4
+        uses: github/codeql-action/init@8aad20d150bbac5944a9f9d289da16a4b0d87c1e # v4.36.2
        with:
          languages: ${{ matrix.language }}
          build-mode: ${{ matrix.build-mode }}
      - name: Perform CodeQL Analysis
-        uses: github/codeql-action/analyze@89a39a4e59826350b863aa6b6252a07ad50cf83e # v4.32.4
+        uses: github/codeql-action/analyze@cdf488f595d80d6e07e03d4674febd5ab45fa938 # v4.37.9
        with:
          category: "/language:${{matrix.language}}"

@@ -48,8 +48,8 @@ jobs:
      security-events: write
    steps:
      - name: Checkout repository
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false
      - name: Run zizmor
-        uses: zizmorcore/zizmor-action@0dce2577a4760a2749d8cfb7a84b7d5585ebcb7d # v0.5.0
+        uses: zizmorcore/zizmor-action@3dc1ecc9bcb9e94e9b2c709687979e1298497054 # v0.6.2
@@ -27,7 +27,7 @@ jobs:

      - name: Checkout project
        id: checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false

@@ -33,9 +33,9 @@ jobs:
      fail-fast: false

    steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false

      - name: Run markdownlint-cli2
-        uses: DavidAnson/markdownlint-cli2-action@07035fd053f7be764496c0f8d8f9f41f98305101 # v22.0.0
+        uses: DavidAnson/markdownlint-cli2-action@21c1be1b93ad9ed58fa840aacc3f279cde2a72ff # v24.2.0
@@ -74,6 +74,6 @@ unless System.get_env("DRYRUN") do
    "api-username" => "Elixir"
  }

-  resp = Req.post!("https://elixirforum.com/posts.json", {:json, post}, headers: headers)
+  resp = Req.post!("https://forum.elixirforum.com/posts.json", {:json, post}, headers: headers)
  IO.puts("#{resp.status} Elixir Forum\n#{inspect(resp.body)}")
 end
@@ -89,9 +89,9 @@ runs:

    - name: Run OSS Review Toolkit
      id: ort
-      uses: oss-review-toolkit/ort-ci-github-action@1805edcf1f4f55f35ae6e4d2d9795ccfb29b6021 # v1.1.0
+      uses: oss-review-toolkit/ort-ci-github-action@086d928d24ef1653dc0777296b312fda5faaaf52 # v1.2.0
      with:
-        image: ghcr.io/oss-review-toolkit/ort-minimal:65.0.0
+        image: ghcr.io/oss-review-toolkit/ort:92.2.0
        run: >-
          labels,
          cache-dependencies,
@@ -107,4 +107,6 @@ runs:
        ort-cli-report-args: >-
          -O CycloneDX=output.file.formats=json,xml
          -O SpdxDocument=outputFileFormats=JSON,YAML
+        ort-cli-scan-args: >-
+          --scanners Provenant
        sw-version: "${{ inputs.version }}"
@@ -32,7 +32,7 @@ jobs:
      fail-fast: false

    steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false

@@ -43,7 +43,7 @@ jobs:
            --draft \
            "$GITHUB_REF_NAME"

-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        # zizmor: ignore[artipacked]
        if: github.ref_type == 'branch'

@@ -71,9 +71,6 @@ jobs:
      fail-fast: true
      matrix:
        include:
-          - otp: 26
-            otp_version: "26.0"
-
          - otp: 27
            otp_version: "27.0"

@@ -81,8 +78,11 @@ jobs:
            otp_version: "28.0"
            build_docs: build_docs

+          - otp: 29
+            otp_version: "29.0"
+
    steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false

@@ -100,19 +100,19 @@ jobs:
          shasum -a 256 Docs.zip > Docs.zip.sha256sum

      - name: "Upload Linux release artifacts"
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
        with:
          name: build-linux-elixir-otp-${{ matrix.otp }}
          path: elixir-otp-${{ matrix.otp }}.zip

      - name: "Upload Windows release artifacts"
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
        with:
          name: build-windows-elixir-otp-${{ matrix.otp }}
          path: elixir-otp-${{ matrix.otp }}.exe

      - name: "Upload doc artifacts"
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
        if: matrix.build_docs
        with:
          name: Docs
@@ -125,7 +125,7 @@ jobs:
    strategy:
      fail-fast: true
      matrix:
-        otp: [26, 27, 28]
+        otp: [27, 28, 29]
        flavor: [windows, linux]

    env:
@@ -139,20 +139,20 @@ jobs:

    steps:
      - name: "Download build"
-        uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
        with:
          name: build-${{ matrix.flavor }}-elixir-otp-${{ matrix.otp }}

      - name: Log in to Azure
        if: ${{ matrix.flavor == 'windows' && vars.AZURE_TRUSTED_SIGNING_ACCOUNT_NAME }}
-        uses: azure/login@a457da9ea143d694b1b9c7c869ebb04ebe844ef5 # v2.3.0
+        uses: azure/login@f5d393ae46f8fde4be8b75f32e3fc50e654ad0ca # v3.0.1
        with:
          client-id: ${{ secrets.AZURE_CLIENT_ID }}
          tenant-id: ${{ secrets.AZURE_TENANT_ID }}
          subscription-id: ${{ secrets.AZURE_SUBSCRIPTION_ID }}

      - name: "Sign files with Trusted Signing"
-        uses: azure/trusted-signing-action@87c2e83e6868da99d3380aa309851b32ed9a8346 # v1.1.0
+        uses: azure/trusted-signing-action@c7ab2a863ab5f9a846ddb8265964877ef296ee82 # v2.0.0
        if: ${{ matrix.flavor == 'windows' && vars.AZURE_TRUSTED_SIGNING_ACCOUNT_NAME }}
        with:
          endpoint: https://eus.codesigning.azure.net/
@@ -182,7 +182,7 @@ jobs:
          shasum -a 256 "$RELEASE_FILE" > "${RELEASE_FILE}.sha256sum"

      - name: "Upload Linux release artifacts"
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
        with:
          name: sign-${{ matrix.flavor }}-elixir-otp-${{ matrix.otp }}
          path: ${{ env.RELEASE_FILE }}*
@@ -205,13 +205,13 @@ jobs:

      - name: Checkout project
        id: checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false

      - name: "Download Build Artifacts"
        id: download-build-artifacts
-        uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
        with:
          pattern: "{sign-*-elixir-otp-*,Docs}"
          merge-multiple: true
@@ -226,7 +226,7 @@ jobs:

      - name: Attest Distribution Assets with SBoM
        id: attest-sbom
-        uses: actions/attest-sbom@4651f806c01d8637787e274ac3bdf724ef169f34 # v3.0.0
+        uses: actions/attest-sbom@c604332985a26aa8cf1bdc465b92731239ec6b9e # v4.1.0
        with:
          subject-path: |
            /tmp/build-artifacts/{elixir-otp-*.*,Docs.zip}
@@ -258,7 +258,7 @@ jobs:
          SBOM_SPDX_JSON: "${{ steps.ort.outputs.results-sbom-spdx-json-path }}"

      - name: "Assemble Release SBoM Artifacts"
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
        with:
          name: "SBoM"
          path: |
@@ -268,7 +268,7 @@ jobs:
            ${{ steps.ort.outputs.results-sbom-spdx-json-path }}

      - name: "Assemble Distribution Attestations"
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
        with:
          name: "Attestations"
          path: "attestations/*.sigstore"
@@ -282,7 +282,7 @@ jobs:
      contents: write

    steps:
-      - uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0
+      - uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
        with:
          pattern: "{sign-*-elixir-otp-*,Docs,SBoM,Attestations}"
          merge-multiple: true
@@ -333,7 +333,7 @@ jobs:
          echo "Required variables for uploading to hex.pm are not set up, skipping..."
          exit 1

-      - uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0
+      - uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
        with:
          pattern: "{sign-*-elixir-otp-*,Docs}"
          merge-multiple: true
@@ -17,11 +17,11 @@ jobs:
    name: Notify

    steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          persist-credentials: false

-      - uses: erlef/setup-beam@e6d7c94229049569db56a7ad5a540c051a010af9 # v1.20.4
+      - uses: erlef/setup-beam@54075bcc5e249e4758d363f27d099f55d843f124 # v1.24.1
        with:
          otp-version: "27.3"
          elixir-version: "1.18.3"
@@ -5,7 +5,8 @@
    "**/*.md"
  ],
  "ignores": [
-    ".git/**"
+    ".git/**",
+    ".github/**"
  ],
  "gitignore": true,
  "config": {
@@ -37,6 +37,11 @@ curations:
      comment: "Apply Trademark Policy to VERSION file"
      detected_license: "NONE"
      concluded_license: "Apache-2.0"
+    - path: ".github/pull_request_template.md"
+      reason: "NOT_DETECTED"
+      comment: "Apply default license to GitHub pull request template"
+      detected_license: "NONE"
+      concluded_license: "Apache-2.0"

    # Wrongly Identified
    - path: ".gitignore"
@@ -0,0 +1 @@
+CONTRIBUTING.md
@@ -4,290 +4,108 @@
  SPDX-FileCopyrightText: 2012 Plataformatec
 -->

-# Changelog for Elixir v1.20
+# Changelog for Elixir v1.21

-## Type system improvements
-
-This release includes type inference of all constructs.
-
-### Type inference of function definitions
-
-Elixir now performs inference of whole functions. The best way to show the new capabilities are with examples. Take the following code:
-
-```elixir
-def add_foo_and_bar(data) do
-  data.foo + data.bar
-end
-```
-
-Elixir now infers that the function expects a `map` as first argument, and the map must have the keys `.foo` and `.bar` whose values are either `integer()` or `float()`. The return type will be either `integer()` or `float()`.
-
-Here is another example:
-
-```elixir
-def sum_to_string(a, b) do
-  Integer.to_string(a + b)
-end
-```
-
-Even though the `+` operator works with both integers and floats, Elixir infers that `a` and `b` must be both integers, as the result of `+` is given to a function that expects an integer. The inferred type information is then used during type checking to find possible typing errors.
-
-### Type inference of guards
-
-This release also performs inference of guards! Let's see some examples:
-
-```elixir
-def example(x, y) when is_list(x) and is_integer(y)
-```
-
-The code above correctly infers `x` is a list and `y` is an integer.
-
-```elixir
-def example({:ok, x} = y) when is_binary(x) or is_integer(x)
-```
-
-The one above infers x is a binary or an integer, and `y` is a two element tuple with `:ok` as first element and a binary or integer as second.
-
-```elixir
-def example(x) when is_map_key(x, :foo)
-```
-
-The code above infers `x` is a map which has the `:foo` key, represented as `%{..., foo: dynamic()}`. Remember the leading `...` indicates the map may have other keys.
-
-```elixir
-def example(x) when not is_map_key(x, :foo)
-```
-
-And the code above infers `x` does not have the `:foo` key (hence `x.foo` will raise a typing violation), which has the type: `%{..., foo: not_set()}`.
-
-You can also have expressions that assert on the size of data structures:
-
-```elixir
-def example(x) when tuple_size(x) < 3
-```
-
-Elixir will correctly track the tuple has at most two elements, and therefore accessing `elem(x, 3)` will emit a typing violation. In other words, Elixir can look at complex guards, infer types, and use this information to find bugs in our code, without a need to introduce type signatures (yet).
-
-### Typing across clauses
-
-Elixir now infers the type of a given clause based on previous clauses. Let's see an example:
-
-```elixir
-case System.get_env("SOME_VAR") do
-  nil -> :not_found
-  value -> {:ok, String.upcase(value)}
-end
-```
-
-`System.get_env("SOME_VAR")` returns either `nil` or a `binary()`. Because the first clause matches on `nil`, the type system now knows `value` can no longer be `nil`, and therefore it must only be a `binary()`, which allows the second clause to also type check without violations.
-
-This type inference across clauses also helps the type system find redundant clauses and dead code in existing codebases.
-
-### Complete typing of maps keys
-
-Maps were one of the first data-structures we implemented within the Elixir type system however, up to this point, they only supported atom keys. If they had additional keys, those keys were simply marked as `dynamic()`.
-
-As of Elixir v1.20, we can track all possible domains as map keys. For example, the map:
-
-```elixir
-%{123 => "hello", 456.0 => :ok}
-```
-
-will have the type:
-
-```elixir
-%{integer() => binary(), float() => :ok}
-```
-
-It is also possible to mix domain keys, as above, with atom keys, yielding the following:
-
-```elixir
-%{integer() => integer(), root: integer()}
-```
-
-This system is an implementation of [Typing Records, Maps, and Structs, by Giuseppe Castagna (2023)](https://www.irif.fr/~gc/papers/icfp23.pdf).
-
-### Typing of map operations
-
-We have typed the majority of the functions in the `Map` module, allowing the type system to track how keys are added, updated, and removed across all possible key types.
-
-For example, imagine we are calling the following `Map` functions with a variable `map`, which we don't know the exact shape of, and an atom key:
-
-```elixir
-Map.put(map, :key, 123)
-#=> returns type %{..., key: integer()}
-
-Map.delete(map, :key)
-#=> returns type %{..., key: not_set()}
-```
-
-As you can see, we track when keys are set and also when they are removed.
-
-Some operations, like `Map.replace/3`, only replace the key if it exists, and that is also propagated by the type system:
-
-```elixir
-Map.replace(map, :key, 123)
-#=> returns type %{..., key: if_set(integer())}
-```
-
-In other words, if the key exists, it would have been replaced by an integer value. Furthermore, whenever calling a function in the `Map` module and the given key is statically proven to never exist in the map, an error is emitted.
-
-By combining full type inference with bang operations like `Map.fetch!/2`, `Map.pop!/2`, `Map.replace!/3`, and `Map.update!/3`, Elixir is able to propagate information about the desired keys. Take this module:
-
-```elixir
-defmodule User do
-  def name(map), do: Map.fetch!(map, :name)
-end
-
-defmodule CallsUser do
-  def calls_name do
-    User.name(%{})
-  end
-end
-```
-
-The code above has a type violation, which is now caught by the type system:
-
-```text
-    warning: incompatible types given to User.name/1:
-
-        User.name(%{})
-
-    given types:
-
-        %{name: not_set()}
-
-    but expected one of:
-
-        dynamic(%{..., name: term()})
-
-    type warning found at:
-    │
- 16 │     User.name(%{})
-    │         ~
-    │
-    └─ lib/calls_user.ex:7:5: CallsUser.calls_name/0
-```
-
-### Acknowledgements
-
-The type system was made possible thanks to a partnership between [CNRS](https://www.cnrs.fr/) and [Remote](https://remote.com/). The development work is currently sponsored by [Fresha](https://www.fresha.com/) and [Tidewave](https://tidewave.ai/).
-
-## v1.20.0-rc.3 (2026-03-09)
+## v1.21.0-dev

 ### 1. Enhancements

-#### IEx
+#### EEx

-  * [IEx] Optimize autocompleting modules
+  * [EEx] Support splitting middle expressions across EEx clauses
+
+#### Elixir
+
+  * [Access] Add support for keyword lists in `Access.key/2` and `Access.key!/1`
+  * [Code] Add support for the `:erlc_options` compiler option
+  * [Code.Formatter] Add a `:migrate_atom_interpolations` option
+  * [Kernel] Improve performance of type constructors and complex intersections
+  * [Kernel] Warn on binary patterns with segments that are not byte-aligned
+  * [Kernel.ParallelCompiler] Add a hint when spawned processes cannot load modules defined during compilation
+  * [Keyword] Optimize `Keyword.pop/3`, `Keyword.pop!/2`, and `Keyword.pop_lazy/3`
+  * [List] Add `List.to_existing_atom/2` and `List.to_unsafe_atom/1`
+  * [MapSet] Optimize `MapSet.symmetric_difference/2` when set sizes differ
+  * [Path] Add `Path.safe_join/2`
+  * [Registry] Optimize exact key matching in lookups
+  * [String] Optimize `String.bag_distance/2`
+  * [String] Add `String.to_existing_atom/2` and `String.to_unsafe_atom/1`
+  * [URI] Optimize percent-decoding and `URI.to_string/1`
+
+#### ExUnit
+
+  * [ExUnit.Assertions] Add `trace/3` helper

 ### 2. Bug fixes

 #### Elixir

-  * [Enum] Fix `Enum.slice/2` for ranges with step > 1 sliced by step > 1
-  * [File] Preserve directory permissions in `File.cp_r/3`
-  * [File] Fix `File.cp_r/3` infinite loop with symlink cycles
-  * [File] Fix `File.cp_r/3` infinite loop when copying into subdirectory of source
-  * [File] Warn when defining `@type record()`, fixes CI on Erlang/OTP 29
-  * [File] Fix `File.Stream` `Enumerable.count` for files without trailing newline
-  * [Float] Fix `Float.parse/1` inconsistent error handling for non-scientific notation overflow
-  * [Kernel] Process fields even when structs are unknown (regression)
-  * [Kernel] Improve performance on several corner cases in the type system (regression)
-  * [Kernel] Fix regression when using `Kernel.in/2` in defguard (regression)
-
-## v1.20.0-rc.2 (2026-03-04)
-
-### 1. Enhancements
-
-#### Elixir
-
-  * [Code] Add `module_definition: :interpreted` option to `Code` which allows module definitions to be evaluated instead of compiled. In some applications/architectures, this can lead to drastic improvements to compilation times. Note this does not affect the generated `.beam` file, which will have the same performance/behaviour as before
-  * [Code] Make module purging opt-in and move temporary module deletion to the background to speed up compilation times
-  * [Integer] Add `Integer.popcount/1`
-  * [Kernel] Move struct validation in patterns and updates to type checker, this means adding and remove struct fields will cause fewer files to be recompiled
-  * [Kernel] Add type inference across clauses. For example, if one clause says `x when is_integer(x)`, then the next clause may no longer be an integer
-  * [Kernel] Detect and warn on redundant clauses
-  * [List] Add `List.first!/1` and `List.last!/1`
-  * Add Software Bill of Materials guide to the Documentation
-
-#### Mix
-
-  * [mix compile] Add `module_definition: :interpreted` option to `Code` which allows module definitions to be evaluated instead of compiled. In some applications/architectures, this can lead to drastic improvements to compilation times. Note this does not affect the generated `.beam` file, which will have the same performance/behaviour as before
-  * [mix deps] Parallelize dep lock status checks during `deps.loadpaths`, improving boot times in projects with many git dependencies
-
-### 2. Potential breaking changes
-
-#### Elixir
-
-  * `map.foo()` (accessing a map field with parens) and `mod.foo` (invoking a function without parens) will now raise instead of emitting runtime warnings, aligning themselves with the type system behaviour
-
-### 3. Bug fixes
-
-#### IEx
-
-  * [IEx] Ensure warnings emitted during IEx parsing are properly displayed/printed
-  * [IEx] Ensure pry works across remote nodes
-
-#### Mix
-
-  * [mix compile.erlang] Topsort Erlang modules before compilation for proper dependency resolution
-
-## v1.20.0-rc.1 (2026-01-13)
-
-### 1. Bug fixes
-
-#### Elixir
-
-  * [Kernel] Do not crash on map types with struct keys when performing type operations (regression)
-  * [Kernel] Mark the outcome of bitstring types as dynamic (regression)
-  * [Kernel] `<<expr::bitstring>>` will have type `binary` instead of `bitstring` if `expr` is a binary (regression)
-  * [Kernel] Do not crash on conditional variables when calling a function on a module which is represented by a variable (regression)
-
-## v1.20.0-rc.0 (2026-01-09)
-
-### 1. Enhancements
-
-#### Elixir
-
-  * [Calendar] Optimize `date_from_iso_days` by using the Neri-Schneider algorithm
-  * [Enum] Add `Enum.min_max` sorter
-  * [Integer] Add `Integer.ceil_div/2`
-  * [IO] Add `IO.iodata_empty?/1`
-  * [File] Skip device, named pipes, etc in `File.cp_r/3` instead of erroring with reason `:eio`
-  * [Kernel] Print intermediate results of `dbg` for pipes
-  * [Kernel] Warn on unused requires
-  * [Regex] Add `Regex.import/1` to import regexes defined with `/E`
+  * [Calendar] Fix `Calendar.strftime/3` formatting of negative years with `%y`
+  * [Calendar] Fix rounding for `:day`, `:hour`, and `:minute` units in `DateTime.diff/3`, `NaiveDateTime.diff/3`, and `Time.diff/3`
+  * [Calendar.ISO] Fix `Calendar.ISO.valid_time?/4` to reject non-integer microsecond precision
+  * [Calendar.ISO] Reject negative zero UTC offsets in basic formats
+  * [Code.Formatter] Fix rendering calls where `do` is followed by non-block keyword arguments
+  * [Code.Fragment] Fix cursor completion when operator keywords such as `in`, `when`, `and`, `or`, and `not` follow another operator
+  * [Date] Preserve the `:format` option in `Date.to_iso8601/2` with custom calendars
+  * [Date.Range] Fix slicing date ranges with stepped ranges
+  * [Duration] Reject duplicate seconds in `Duration.from_iso8601/1`
+  * [Enum] Fix `Enum.min/2,3` and `Enum.max/2,3` with custom sorters on ranges
+  * [IO.ANSI.Docs] Recognize additional punctuation delimiters when rendering Markdown
+  * [Kernel] Fix expansion of rebound variables in bitstring size expressions
+  * [Kernel] Expand `defguard` macros separately in guard and body contexts, preserving `and`/`or` error semantics outside guards
+  * [Kernel] Fix inferred stacktrace types to allow arbitrary keyword metadata
+  * [Kernel] Fix inferred types for functions with non-returning clauses
+  * [Kernel] Fix map field type inference in the presence of empty map types
+  * [Kernel] Fix tuple fetch and deletion type operations across equivalent tuple types
+  * [Kernel] Fix variables defined in one default argument leaking into subsequent default arguments
+  * [Kernel] Improve the error message for non-atom struct keys
+  * [Kernel] Raise when `|` is used in guards
+  * [Kernel.Typespec] Preserve metadata when proxying to Elixir typespecs
+  * [Keyword] Delete duplicate keys when `Keyword.get_and_update/3` and `Keyword.get_and_update!/3` return `:pop`
+  * [Macro] Properly escape C1 control characters and Unicode noncharacters
+  * [NaiveDateTime] Fix `NaiveDateTime.diff/3` over-counting incomplete units
+  * [Range] Fix `Range.disjoint?/2` for ranges beyond floating-point precision
+  * [Range] Fix `Range.disjoint?/2` for single-element ranges with a negative step
+  * [String] Fix `String.reverse/1` grapheme ordering around invalid UTF-8 bytes
+  * [String] Return `1.0` from `String.bag_distance/2` for two empty strings
+  * [Time] Validate microseconds in `Time.from_seconds_after_midnight/3`

 #### ExUnit

-  * [ExUnit.CaptureLog] Add `:formatter` option for custom log formatting
+  * [ExUnit.Assertions] Fix `refute_in_delta/4` at the delta boundary and with negative deltas
+  * [ExUnit.CaptureIO] Stop `StringIO` processes when capturing a named device fails
+
+#### IEx
+
+  * [IEx.Autocomplete] Fix completion crashes on maps with non-atom keys
+  * [IEx.Evaluator] Recognize `**` and `not in` as continuation operators
+  * [IEx.Helpers] Fix `r/1` when multiple modules are defined in the same file
+  * [IEx.Helpers] Fix heap and stack memory calculations in `process_info/1`

 #### Mix

-  * [mix deps] Support filtering `mix deps` output
-  * [mix compile] Enforce `:elixirc_paths` to be a list of strings to avoid paths from being discarded (the only documented type was lists of strings)
-  * [mix test] Add `mix test --dry-run`
-
-### 2. Potential breaking changes
-
-#### Elixir
-
-  * `require SomeModule` no longer expands to the given module at compile-time, but it still returns the module at runtime. Note that while Elixir does not guarantee macros will expand to certain constructs, but since this can break code relying on the previous behaviour, such as `require(SomeMod).some_macro()`, we are adding this note to the CHANGELOG
+  * [Mix] Prevent synchronization lock files from being overwritten with empty contents
+  * [Mix.Release] Accept chardata paths in `Mix.Release.make_boot_script/4`
+  * [Mix.SCM.Git] Raise if Git refspecs start with `-`
+  * [mix deps] Recompile path and fetchable dependencies when one of the dependencies they were compiled with is removed
+  * [mix deps] Mark fetchable dependencies for compilation when their build exists but their SCM manifest is missing
+  * [mix deps.compile] Preserve code paths and compiler options across OS partitions
+  * [mix format] Pass `:sigils` to plugins invoked for sigils, allowing nested sigils to be formatted
+  * [mix new] Avoid trailing whitespace in generated files

 ### 3. Hard deprecations

 #### Elixir

-  * [File] `File.stream!(path, modes, lines_or_bytes)` is deprecated in favor of `File.stream!(path, lines_or_bytes, modes)`
-  * [Kernel] Matching on the size inside a bit pattern now requires the pin operator for consistency, such as `<<x::size(^existing_var)>>`
-  * [Kernel.ParallelCompiler] `Kernel.ParallelCompiler.async/1` is deprecated in favor of `Kernel.ParallelCompiler.pmap/2`, which is more performant and addresses known limitations
+  * [Macro.Env] `Macro.Env.fetch_alias/2` and `Macro.Env.fetch_macro_alias/2` are deprecated, use `Macro.Env.expand_alias/4` instead

-#### Logger
+### 4. Soft deprecations

-  * [Logger] `Logger.*_backend` functions are deprecated in favor of handlers. If you really want to keep on using backends, see the `:logger_backends` package
-  * [Logger] `Logger.enable/1` and `Logger.disable/1` have been deprecated in favor of `Logger.put_process_level/2` and `Logger.delete_process_level/1`
+#### Elixir

-## v1.19
+  * [Kernel] Atom interpolation (`:"foo_#{bar}"`) is deprecated in favor of explicit `String.to_unsafe_atom/1`
+  * [List] `List.to_atom/1` is deprecated in favor of `List.to_unsafe_atom/1`
+  * [String] `String.to_atom/1` is deprecated in favor of `String.to_unsafe_atom/1`

-The CHANGELOG for v1.19 releases can be found [in the v1.19 branch](https://github.com/elixir-lang/elixir/blob/v1.19/CHANGELOG.md).
+## v1.20
+
+The CHANGELOG for v1.20 releases can be found [in the v1.20 branch](https://github.com/elixir-lang/elixir/blob/v1.20/CHANGELOG.md).
@@ -55,7 +55,7 @@ To recompile all (including Erlang modules):
 make compile
 ```

-After your changes are done, please remember to run `make format` to guarantee
+After your changes are done, run `make format` to guarantee
 all files are properly formatted, then run the full suite with
 `make test`.

@@ -71,53 +71,67 @@ updating an existing checkout, run `make clean compile`. You can check
 [the official build status](https://github.com/elixir-lang/elixir/actions/workflows/ci.yml).
 More tasks can be found by reading the [Makefile](Makefile).

-We encourage contributors to write tests that capture both existing and newly
-introduced behavior, especially for bug fixes and major changes:
+## Sending a pull request

-  * **Bug Fixes:** If you are fixing a bug, please try to include a test that
-    *fails* before your change and *passes* afterward. This makes it easier to
-    confirm that the fix addresses the underlying issue and helps prevent
-    regressions in the future.
+Contributions are done [via pull request](https://help.github.com/articles/using-pull-requests/)
+and must include tests and other relevant proof of work:
+
+  * **Bug Fixes:** If you are fixing a bug, include a test that *fails* before
+    your change and *passes* afterward. This makes it easier to confirm that the
+    fix addresses the underlying issue and helps prevent regressions in the future.

  * **New Features or Major Changes:** If you are adding a new feature or making
-    major changes to existing functionality, please add tests that cover the
-    major parts of that functionality. Aim to have the best code coverage possible.
+    major changes to existing functionality, please add assocaited tests. Aim to
+    have the best code coverage possible.

-With tests running and passing, you are ready to contribute to Elixir and
-[send a pull request](https://help.github.com/articles/using-pull-requests/).
-We have saved some excellent pull requests we have received in the past in
-case you are looking for some examples:
+  * **Performance improvements:** For performance improvements, please include the
+    benchmark script, with inputs and results, in the pull request description.
+    We recommend using [benchee](https://github.com/bencheeorg/benchee).

-  * [Implement Enum.member? - Pull request](https://github.com/elixir-lang/elixir/pull/992)
+Here are some pull requests we have received in the past you can use as reference:

-  * [Add String.valid? - Pull request](https://github.com/elixir-lang/elixir/pull/1058)
+  * [Implement Enum.member?](https://github.com/elixir-lang/elixir/pull/992)

-  * [Implement capture_io for ExUnit - Pull request](https://github.com/elixir-lang/elixir/pull/1059)
+  * [Add String.valid?](https://github.com/elixir-lang/elixir/pull/1058)
+
+  * [Implement capture_io for ExUnit](https://github.com/elixir-lang/elixir/pull/1059)

 ## Reviewing changes

 Once a pull request is sent, the Elixir team will review your changes.
-We outline our process below to clarify the roles of everyone involved.
-
-All pull requests must be approved by two committers before being merged into
-the repository. If changes are necessary, the team will leave appropriate
-comments requesting changes to the code. Unfortunately, we cannot guarantee a
-pull request will be merged, even when modifications are requested, as the Elixir
-team will re-evaluate the contribution as it changes.
-
-Committers may also push style changes directly to your branch. If you would
-rather manage all changes yourself, you can disable the "Allow edits from maintainers"
-feature when submitting your pull request.
-
-The Elixir team may optionally assign someone to review a pull request.
-If someone is assigned, they must explicitly approve the code before
-another team member can merge it.
+If changes are necessary, the team will leave appropriate comments requesting
+changes to the code. Unfortunately, we cannot guarantee a pull request will
+be merged, even when modifications are requested, as the Elixir team will
+re-evaluate the contribution as it changes.

 When the review finishes, your pull request will be squashed and merged
 into the repository. If you have carefully organized your commits and
 believe they should be merged without squashing, please mention it in
 a comment.

+## Building documentation
+
+Building the documentation requires that [ExDoc](https://github.com/elixir-lang/ex_doc)
+is cloned and compiled alongside Elixir.
+
+After cloning and compiling Elixir, run:
+
+```sh
+elixir_dir=$(pwd)
+cd .. && git clone https://github.com/elixir-lang/ex_doc.git
+cd ex_doc && "${elixir_dir}/bin/elixir" "${elixir_dir}/bin/mix" do deps.get + compile
+
+# Now we will go back to Elixir's root directory,
+cd "${elixir_dir}"
+
+# and generate HTML and EPUB documents:
+make docs
+```
+
+This will produce documentation sets for `elixir`, `eex`, `ex_unit`, `iex`, `logger`,
+and `mix` under the `doc` directory. If you are planning to contribute documentation,
+[please check our best practices for writing documentation](https://elixir.hexdocs.pm/writing-documentation.html).
+
 ## Licensing and Compliance Requirements

 Please review our [Open Source Policy](OPEN_SOURCE_POLICY.md) for complete
@@ -181,25 +195,50 @@ affecting **all external contributors**:
    See <https://developercertificate.org/> for a copy of the Developer Certificate
    of Origin license.

-## Building documentation
+## Using AI and coding agents

-Building the documentation requires that [ExDoc](https://github.com/elixir-lang/ex_doc)
-is installed and built alongside Elixir.
+While we allow the use of AI on contributions and discussions, please be mindful
+when doing so. Generally speaking, Elixir maintainers already have access to AI
+(like many other developers). Therefore, if we need the feedback or help of a
+coding agent, we can request so ourselves. For this reason, we often find
+the point of view of the human behind the agent more valuable.

-After cloning and compiling Elixir, run:
+That said, here are examples of how one might (or might not) use AI and coding
+agents in Elixir spaces:

-```sh
-elixir_dir=$(pwd)
-cd .. && git clone https://github.com/elixir-lang/ex_doc.git
-cd ex_doc && "${elixir_dir}/bin/elixir" "${elixir_dir}/bin/mix" do deps.get + compile
+  * When it comes to discussions, using AI to help express yourself is welcome,
+    but avoid directly copy and pasting AI generated content. If there is a language
+    barrier, use AI to translate, review, and improve your text, but do not use AI
+    to respond on your behalf.

-# Now we will go back to Elixir's root directory,
-cd "${elixir_dir}"
+  * Do not use coding agents to tackle existing issues unless they have the
+    "Contributions Welcome" label.

-# and generate HTML and EPUB documents:
-make docs
-```
+  * If you request a feature on the mailing list and it is accepted, you may
+    use coding agents to implement it, as long as it follows the AI Contributions
+    guidelines below.

-This will produce documentation sets for `elixir`, `eex`, `ex_unit`, `iex`, `logger`,
-and `mix` under the `doc` directory. If you are planning to contribute documentation,
-[please check our best practices for writing documentation](https://hexdocs.pm/elixir/writing-documentation.html).
+  * When automating AI usage on the Elixir codebase for performance improvements,
+    security fixes, or correctness changes to the compiler or type system, pair it
+    with a separate set of agents whose job is to argue against and try to invalidate
+    any proposed change. And treat their approval as advisory: a human must still
+    validate it before opening issues or pull requests.
+
+If any code is written by AI, then you must follow the guidelines below.
+
+### AI contributions
+
+AI agents MUST NOT add Signed-off-by tags. Only humans can legally certify the Developer
+Certificate of Origin (DCO). The human submitter is responsible for:
+
+  * Reviewing all AI-generated code
+  * Ensuring compliance with licensing requirements
+  * Adding their own Signed-off-by tag to certify the DCO
+  * Taking full responsibility for the contribution
+  * Disclosing use of AI for comments and code contributions
+
+When AI tools contribute to Elixir, proper attribution helps track the evolving role of
+AI in the development process. Contributions should include an Assisted-by tag in the
+following format:
+
+    Assisted-by: AGENT_NAME:MODEL_VERSION
@@ -32,9 +32,9 @@ SOURCE_DATE_EPOCH_FILE = $(SOURCE_DATE_EPOCH_PATH)/SOURCE_DATE_EPOCH
 #==> Functions

 define CHECK_ERLANG_RELEASE
-	erl -noshell -eval '{V,_} = string:to_integer(erlang:system_info(otp_release)), io:fwrite("~s", [is_integer(V) and (V >= 26)])' -s erlang halt | grep -q '^true'; \
+	erl -noshell -eval '{V,_} = string:to_integer(erlang:system_info(otp_release)), io:fwrite("~s", [is_integer(V) and (V >= 27)])' -s erlang halt | grep -q '^true'; \
 		if [ $$? != 0 ]; then \
-		  echo "At least Erlang/OTP 26.0 is required to build Elixir"; \
+		  echo "At least Erlang/OTP 27.0 is required to build Elixir"; \
 		  exit 1; \
 		fi
 endef
@@ -189,7 +189,7 @@ clean_elixir:
 #==> Documentation tasks

 SOURCE_REF = $(shell tag="$(call GIT_TAG)" revision="$(call GIT_REVISION)"; echo "$${tag:-$$revision}")
-DOCS_COMPILE = CANONICAL=$(CANONICAL) bin/elixir ../ex_doc/bin/ex_doc "$(1)" "$(VERSION)" "lib/$(2)/ebin" --main "$(3)" --source-url "https://github.com/elixir-lang/elixir" --source-ref "$(call SOURCE_REF)" --logo lib/elixir/pages/images/logo.png --output doc/$(2) --canonical "https://hexdocs.pm/$(2)/$(CANONICAL)" --homepage-url "https://elixir-lang.org/docs.html" $(DOCS_OPTIONS) $(4)
+DOCS_COMPILE = CANONICAL=$(CANONICAL) bin/elixir ../ex_doc/bin/ex_doc "$(1)" "$(VERSION)" "lib/$(2)/ebin" --main "$(3)" --source-url "https://github.com/elixir-lang/elixir" --source-ref "$(call SOURCE_REF)" --logo lib/elixir/pages/images/logo.png --output doc/$(2) --canonical "https://$(2).hexdocs.pm/$(CANONICAL)" --homepage-url "https://elixir-lang.org/docs.html" $(DOCS_OPTIONS) $(4)
 DOCS_CONFIG = bin/elixir lib/elixir/scripts/docs_config.exs "$(1)"

 docs: compile ../ex_doc/bin/ex_doc docs_elixir docs_eex docs_mix docs_iex docs_ex_unit docs_logger
@@ -7,14 +7,14 @@
 <h1>
 <picture>
  <source media="(prefers-color-scheme: dark)" srcset="https://github.com/elixir-lang/elixir-lang.github.com/raw/main/images/logo/logo-dark.png">
-  <img alt="Elixir logo" src="https://github.com/elixir-lang/elixir-lang.github.com/raw/main/images/logo/logo.png" width="200">
+  <img alt="Elixir logo" src="https://github.com/elixir-lang/elixir-lang.github.com/raw/main/public/images/logo/logo.png" width="200">
 </picture>
 </h1>

 [![CI](https://github.com/elixir-lang/elixir/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/elixir-lang/elixir/actions/workflows/ci.yml?query=branch%3Amain)
 [![OpenSSF Best Practices](https://www.bestpractices.dev/projects/10187/badge)](https://www.bestpractices.dev/projects/10187)

-Elixir is a dynamic, functional language designed for building scalable
+Elixir is a programming language designed for building scalable
 and maintainable applications.

 For more about Elixir, installation and documentation,
@@ -122,9 +122,10 @@ variable `ERL_COMPILER_OPTIONS=deterministic`.
 Contributions to Elixir are always welcome! Before you get started, please check
 out our [CONTRIBUTING.md](CONTRIBUTING.md) file. There you will find detailed
 guidelines on how to set up your environment, run the test suite, format your
-code, and submit pull requests. We also include information on our review
-process, licensing requirements, and helpful tips to ensure a smooth
-contribution experience.
+code, and submit pull requests.
+
+Note you must disclose the use of coding agents and AI written code in your
+contributions. See the "Using AI and coding agents" in [CONTRIBUTING.md](CONTRIBUTING.md).

 ## Development links

@@ -12,12 +12,12 @@ Elixir applies bug fixes only to the latest minor branch. Security patches are a

 Elixir version | Support
 :------------- | :-----------------------------
-1.20           | Development
-1.19           | Bug fixes and security patches
+1.21           | Development
+1.20           | Bug fixes and security patches
+1.19           | Security patches only
 1.18           | Security patches only
 1.17           | Security patches only
 1.16           | Security patches only
-1.15           | Security patches only

 ## Announcements

@@ -1 +1 @@
-1.20.0-rc.3
+1.21.0-dev
@@ -6,7 +6,7 @@

 set -e

-ELIXIR_VERSION=1.20.0-rc.3
+ELIXIR_VERSION=1.21.0-dev

 if [ $# -eq 0 ] || { [ $# -eq 1 ] && { [ "$1" = "--help" ] || [ "$1" = "-h" ]; }; }; then
  cat <<USAGE >&2
@@ -4,7 +4,7 @@
 :: SPDX-FileCopyrightText: 2021 The Elixir Team
 :: SPDX-FileCopyrightText: 2012 Plataformatec

-set ELIXIR_VERSION=1.20.0-rc.3
+set ELIXIR_VERSION=1.21.0-dev

 if    ""%1""==""""                if ""%2""=="""" goto documentation
 if /I ""%1""==""--help""          if ""%2""=="""" goto documentation
@@ -293,7 +293,7 @@ defmodule EEx do
      "foo baz"

  """
-  @spec eval_string(String.t(), keyword, [compile_opt]) :: term()
+  @spec eval_string(String.t(), Code.binding(), [compile_opt]) :: term()
  def eval_string(source, bindings \\ [], options \\ [])
      when is_binary(source) and is_list(bindings) and is_list(options) do
    compiled = compile_string(source, options)
@@ -315,7 +315,7 @@ defmodule EEx do
      #=> "foo baz"

  """
-  @spec eval_file(Path.t(), keyword, [compile_opt]) :: String.t()
+  @spec eval_file(Path.t(), Code.binding(), [compile_opt]) :: String.t()
  def eval_file(filename, bindings \\ [], options \\ [])
      when is_list(bindings) and is_list(options) do
    filename = IO.chardata_to_string(filename)
@@ -344,6 +344,7 @@ defmodule EEx do

  It returns `{:ok, [token]}` where a token is one of:

+    * `{:comment, content, %{column: column, line: line}}`
    * `{:text, content, %{column: column, line: line}}`
    * `{:expr, marker, content, %{column: column, line: line}}`
    * `{:start_expr, marker, content, %{column: column, line: line}}`
@@ -79,7 +79,7 @@ defmodule EEx.Compiler do
        {:error, message <> code_snippet(state.source, state.indentation, meta), meta}

      {:ok, expr, new_line, new_column, rest} ->
-        {key, expr} =
+        {key, expr, extra_meta} =
          case :elixir_tokenizer.tokenize(expr, 1, file: "eex", check_terminators: false) do
            {:ok, _line, _column, _warnings, rev_tokens, []} ->
              # We ignore warnings because the code will be tokenized
@@ -87,7 +87,7 @@ defmodule EEx.Compiler do
              token_key(rev_tokens, expr)

            {:error, _, _, _, _} ->
-              {:expr, expr}
+              {:expr, expr, %{}}
          end

        marker =
@@ -102,8 +102,8 @@ defmodule EEx.Compiler do
            marker
          end

-        token = {key, marker, expr, %{line: line, column: column}}
-        trim_and_tokenize(rest, new_line, new_column, state, buffer, acc, &[token | &1])
+        token = {key, marker, expr, Map.merge(%{line: line, column: column}, extra_meta)}
+        trim_and_tokenize(rest, new_line, new_column, state, buffer, acc, &merge_token(token, &1))
    end
  end

@@ -127,6 +127,27 @@ defmodule EEx.Compiler do
    tokenize(rest, line, column, state, [{line, column}], fun.(acc))
  end

+  # Merge middle expressions separated only by whitespace so the whitespace is
+  # part of the Elixir expression, not a separate EEx body.
+  defp merge_token(
+         {:middle_expr, ~c"", chars, meta},
+         [{:text, text, text_meta}, {:middle_expr, ~c"", prev_chars, prev_meta} | acc]
+       ) do
+    if only_spaces?(text) and clause_block_identifier?(prev_meta) do
+      [{:middle_expr, ~c"", prev_chars ++ text ++ chars, prev_meta} | acc]
+    else
+      [
+        {:middle_expr, ~c"", chars, meta},
+        {:text, text, text_meta},
+        {:middle_expr, ~c"", prev_chars, prev_meta} | acc
+      ]
+    end
+  end
+
+  defp merge_token(token, acc) do
+    [token | acc]
+  end
+
  # Retrieve marker for <%

  defp retrieve_marker([marker | t]) when marker in [?=, ?/, ?|] do
@@ -177,35 +198,37 @@ defmodule EEx.Compiler do
  defp token_key(rev_tokens, expr) do
    case {Enum.reverse(rev_tokens), drop_eol(rev_tokens)} do
      {[{:end, _} | _], [{:do, _} | _]} ->
-        {:middle_expr, expr}
+        {:middle_expr, expr, %{}}

      {_, [{:do, _} | _]} ->
-        {:start_expr, maybe_append_space(expr)}
+        {:start_expr, maybe_append_space(expr), %{}}

-      {_, [{:block_identifier, _, _} | _]} ->
-        {:middle_expr, maybe_append_space(expr)}
+      {_, [{:block_identifier, _, identifier} | _]} ->
+        {:middle_expr, maybe_append_space(expr), %{block_identifier: identifier}}

      {[{:end, _} | _], [{:stab_op, _, _} | _]} ->
-        {:middle_expr, expr}
+        {:middle_expr, expr, %{}}

-      {_, [{:stab_op, _, _} | reverse_tokens]} ->
-        fn_index = Enum.find_index(reverse_tokens, &match?({:fn, _}, &1)) || :infinity
-        end_index = Enum.find_index(reverse_tokens, &match?({:end, _}, &1)) || :infinity
-
-        if end_index > fn_index do
-          {:start_expr, expr}
+      {_, [{:stab_op, _, _} | rev_tokens]} ->
+        if fn_before_end?(rev_tokens) do
+          {:start_expr, expr, %{}}
        else
-          {:middle_expr, expr}
+          {:middle_expr, expr, %{}}
        end

      {tokens, _} ->
        case Enum.drop_while(tokens, &closing_bracket?/1) do
-          [{:end, _} | _] -> {:end_expr, expr}
-          _ -> {:expr, expr}
+          [{:end, _} | _] -> {:end_expr, expr, %{}}
+          _ -> {:expr, expr, %{}}
        end
    end
  end

+  defp fn_before_end?([{:fn, _} | _]), do: true
+  defp fn_before_end?([{:end, _} | _]), do: false
+  defp fn_before_end?([_ | rev_tokens]), do: fn_before_end?(rev_tokens)
+  defp fn_before_end?([]), do: false
+
  defp drop_eol([{:eol, _} | rest]), do: drop_eol(rest)
  defp drop_eol(rest), do: rest

@@ -303,7 +326,7 @@ defmodule EEx.Compiler do
      file: file,
      source: source,
      line: line,
-      quoted: [],
+      quoted: %{},
      parser_options: [indentation: indentation] ++ parser_options,
      indentation: indentation
    }
@@ -347,7 +370,7 @@ defmodule EEx.Compiler do
        state.parser_options

    expr = Code.string_to_quoted!(chars, options)
-    buffer = state.engine.handle_expr(buffer, IO.chardata_to_string(mark), expr)
+    buffer = handle_expr(buffer, mark, expr, meta, state)
    generate_buffer(rest, buffer, scope, state)
  end

@@ -366,7 +389,7 @@ defmodule EEx.Compiler do
        rest,
        state.engine.handle_begin(buffer),
        [{contents, start_line, start_column} | scope],
-        %{state | quoted: [], line: line}
+        %{state | quoted: %{}, line: line}
      )

    if mark == ~c"" and not match?({:=, _, [_, _]}, contents) do
@@ -376,7 +399,7 @@ defmodule EEx.Compiler do
      IO.warn(message, file: state.file, line: meta.line, column: meta.column)
    end

-    buffer = state.engine.handle_expr(buffer, IO.chardata_to_string(mark), contents)
+    buffer = handle_expr(buffer, mark, contents, meta, state)
    generate_buffer(rest, buffer, scope, state)
  end

@@ -410,7 +433,7 @@ defmodule EEx.Compiler do
       ) do
    {wrapped, state} = wrap_expr(current, meta.line, buffer, chars, state)
    options = [file: state.file, line: line, column: column] ++ state.parser_options
-    tuples = Code.string_to_quoted!(wrapped, options)
+    tuples = Code.string_to_quoted!(:lists.flatten(wrapped), options)
    buffer = insert_quoted(tuples, state.quoted)
    {buffer, rest}
  end
@@ -426,7 +449,7 @@ defmodule EEx.Compiler do

  defp generate_buffer([{:eof, _meta}], _buffer, [{content, line, column} | _scope], state) do
    message = "expected a closing '<% end %>' for block expression in EEx"
-    expr_meta = non_whitespace_meta(content, line, column, state)
+    expr_meta = non_whitespace_meta(:lists.flatten(content), line, column, state)
    syntax_error!(message, expr_meta, state)
  end

@@ -443,10 +466,10 @@ defmodule EEx.Compiler do

  defp wrap_expr(current, line, buffer, chars, state) do
    new_lines = List.duplicate(?\n, line - state.line)
-    key = length(state.quoted)
-    placeholder = ~c"__EEX__(" ++ Integer.to_charlist(key) ++ ~c");"
-    count = current ++ placeholder ++ new_lines ++ chars
-    new_state = %{state | quoted: [{key, state.engine.handle_end(buffer)} | state.quoted]}
+    key = map_size(state.quoted)
+    placeholder = [~c"__EEX__(", Integer.to_charlist(key), ~c");"]
+    count = [current, placeholder, new_lines, chars]
+    new_state = %{state | quoted: Map.put(state.quoted, key, state.engine.handle_end(buffer))}

    {count, new_state}
  end
@@ -476,11 +499,16 @@ defmodule EEx.Compiler do
    Enum.all?(chars, &(&1 in @all_spaces))
  end

+  defp clause_block_identifier?(%{block_identifier: identifier}) do
+    identifier in [:else, :rescue, :catch]
+  end
+
+  defp clause_block_identifier?(_meta), do: false
+
  # Changes placeholder to real expression

  defp insert_quoted({:__EEX__, _, [key]}, quoted) do
-    {^key, value} = List.keyfind(quoted, key, 0)
-    value
+    Map.fetch!(quoted, key)
  end

  defp insert_quoted({left, line, right}, quoted) do
@@ -513,6 +541,20 @@ defmodule EEx.Compiler do
      column: meta.column
  end

+  defp handle_expr(buffer, mark, expr, meta, state) do
+    state.engine.handle_expr(buffer, IO.chardata_to_string(mark), expr)
+  rescue
+    e in EEx.SyntaxError ->
+      reraise %{
+                e
+                | file: e.file || state.file,
+                  line: e.line || meta.line,
+                  column: e.column || meta.column,
+                  snippet: e.snippet || code_snippet(state.source, state.indentation, meta)
+              },
+              __STACKTRACE__
+  end
+
  defp code_snippet(source, indentation, meta) do
    line_start = max(meta.line - 3, 1)
    line_end = meta.line
@@ -195,7 +195,7 @@ defmodule EEx.Engine do
  def handle_expr(state, "=", ast) do
    check_state!(state)
    %{binary: binary, dynamic: dynamic, vars_count: vars_count} = state
-    var = Macro.var(:"arg#{vars_count}", __MODULE__)
+    var = Macro.var(String.to_unsafe_atom("arg#{vars_count}"), __MODULE__)

    ast =
      quote do
@@ -270,7 +270,7 @@ defmodule EEx.TokenizerTest do
      {:text, ~c"foo ", %{column: 1, line: 1}},
      {:start_expr, ~c"", ~c" if true do ", %{column: 5, line: 1}},
      {:text, ~c"bar", %{column: 21, line: 1}},
-      {:middle_expr, ~c"", ~c" else ", %{column: 24, line: 1}},
+      {:middle_expr, ~c"", ~c" else ", %{block_identifier: :else, column: 24, line: 1}},
      {:text, ~c"baz", %{column: 34, line: 1}},
      {:end_expr, ~c"", ~c" end ", %{column: 37, line: 1}},
      {:eof, %{column: 46, line: 1}}
@@ -286,7 +286,7 @@ defmodule EEx.TokenizerTest do
    exprs = [
      {:start_expr, ~c"=", ~c" if true do ", %{column: 2, line: 1}},
      {:text, ~c"\n TRUE \n", %{column: 20, line: 1}},
-      {:middle_expr, ~c"", ~c" else ", %{column: 3, line: 3}},
+      {:middle_expr, ~c"", ~c" else ", %{block_identifier: :else, column: 3, line: 3}},
      {:text, ~c"\n FALSE \n", %{column: 13, line: 3}},
      {:end_expr, ~c"", ~c" end ", %{column: 3, line: 5}},
      {:eof, %{column: 3, line: 7}}
@@ -262,6 +262,37 @@ defmodule EExTest do
      assert_eval("foo 1,2,3", "foo <% require Enum, as: E %><%= E.join [1, 2, 3], \",\" %>")
    end

+    test "with expression with else clause split across tags" do
+      template = """
+      <%= with {:ok, x} <- @res do %>
+        <p><%= x %></p>
+      <% else %>
+        <% _ -> %>
+          <p>bad</p>
+      <% end %>
+      """
+
+      assert_eval("\n  <p>ok</p>\n\n", template, [assigns: [res: {:ok, "ok"}]],
+        engine: EEx.SmartEngine
+      )
+
+      assert_eval("\n    <p>bad</p>\n\n", template, [assigns: [res: :error]],
+        engine: EEx.SmartEngine
+      )
+    end
+
+    test "empty clauses separated by whitespace" do
+      template = """
+      <%= case x do %>
+        <% :foo -> %>
+        <% :bar -> %>
+      <% end %>
+      """
+
+      assert_eval("\n  \n", template, x: :foo)
+      assert_eval("\n\n", template, x: :bar)
+    end
+
    test "with end of token" do
      assert_eval("foo bar %>", "foo bar %>")
    end
@@ -522,7 +553,7 @@ defmodule EExTest do

      assert message |> Exception.message() |> strip_ansi() =~ """
                  │
-              514 │                   true && @some[\s
+              #{line + 2} │                   true && @some[\s
                  │                                │ └ missing closing delimiter (expected "]")
                  │                                └ unclosed delimiter
             """
@@ -575,6 +606,18 @@ defmodule EExTest do
        EEx.compile_string("foo <%= bar", file: "my_file.eex")
      end
    end
+
+    test "unsupported marker error carries template location metadata" do
+      error =
+        assert_raise EEx.SyntaxError, fn ->
+          EEx.compile_string("<%/ true %>", file: "sample.eex", line: 7)
+        end
+
+      assert error.file == "sample.eex"
+      assert error.line == 7
+      assert error.column == 1
+      assert Exception.message(error) =~ "sample.eex:7:1:"
+    end
  end

  describe "warnings" do
@@ -10,8 +10,8 @@ defmodule Access do
  keys of any type in a data structure via the `data[key]` syntax.

  `Access` supports keyword lists (`Keyword`) and maps (`Map`) out
-  of the box. Keywords supports only atoms keys, keys for maps can
-  be of any type. Both return `nil` if the key does not exist:
+  of the box. Keyword lists support only atom keys, while keys for maps
+  can be of any type. Both return `nil` if the key does not exist:

      iex> keywords = [a: 1, b: 2]
      iex> keywords[:a]
@@ -33,7 +33,7 @@ defmodule Access do
      iex> keywords[:c][:unknown]
      nil

-  This works because accessing anything on a `nil` value, returns
+  This works because accessing anything on a `nil` value returns
  `nil` itself:

      iex> nil[:a]
@@ -226,6 +226,8 @@ defmodule Access do
    end
  end

+  defguardp is_probably_keyword(list) when list == [] or is_atom(elem(hd(list), 0))
+
  @doc """
  Fetches the value for the given key in a container (a map, keyword
  list, or struct that implements the `Access` behaviour).
@@ -485,7 +487,7 @@ defmodule Access do
  ## Accessors

  @doc """
-  Returns a function that accesses the given key in a map/struct.
+  Returns a function that accesses the given key in a map/struct/keyword list.

  The returned function is typically passed as an accessor to `Kernel.get_in/2`,
  `Kernel.get_and_update_in/3`, and friends.
@@ -514,30 +516,56 @@ defmodule Access do
      iex> pop_in(map, [Access.key(:user), Access.key(:name)])
      {"john", %{user: %{}}}

-  An error is raised if the accessed structure is not a map or a struct:
+      iex> keyword = [user: [name: "john"]]
+      iex> get_in(keyword, [Access.key(:unknown, []), Access.key(:name, "john")])
+      "john"
+      iex> get_and_update_in(keyword, [Access.key(:user), Access.key(:name)], fn prev ->
+      ...>   {prev, String.upcase(prev)}
+      ...> end)
+      {"john", [user: [name: "JOHN"]]}
+      iex> pop_in(keyword, [Access.key(:user), Access.key(:name)])
+      {"john", [user: []]}

-      iex> get_in([], [Access.key(:foo)])
-      ** (BadMapError) expected a map, got:
-      ...
+  An error is raised if the accessed structure is not a map, struct, or keyword list:
+
+      iex> get_in(123, [Access.key(:foo)])
+      ** (RuntimeError) Access.key/2 expected a map/struct/keyword list, got: ...
+
+      iex> put_in([1, 2, 3], [Access.key(:foo)], :bar)
+      ** (RuntimeError) Access.key/2 expected a map/struct/keyword list, got: ...
  """
-  @spec key(key, term) :: access_fun(data :: struct | map, current_value :: term)
+  @spec key(key, term) :: access_fun(data :: struct | map | keyword, current_value :: term)
  def key(key, default \\ nil) do
    fn
-      :get, data, next ->
+      :get, %{} = data, next ->
        next.(Map.get(data, key, default))

-      :get_and_update, data, next ->
+      :get_and_update, %{} = data, next ->
        value = Map.get(data, key, default)

        case next.(value) do
          {get, update} -> {get, Map.put(data, key, update)}
          :pop -> {value, Map.delete(data, key)}
        end
+
+      :get, data, next when is_probably_keyword(data) ->
+        next.(Keyword.get(data, key, default))
+
+      :get_and_update, data, next when is_probably_keyword(data) ->
+        value = Keyword.get(data, key, default)
+
+        case next.(value) do
+          {get, update} -> {get, Keyword.put(data, key, update)}
+          :pop -> {value, Keyword.delete(data, key)}
+        end
+
+      _op, data, _next ->
+        raise "Access.key/2 expected a map/struct/keyword list, got: #{inspect(data)}"
    end
  end

  @doc """
-  Returns a function that accesses the given key in a map/struct.
+  Returns a function that accesses the given key in a map/struct/keyword list.

  The returned function is typically passed as an accessor to `Kernel.get_in/2`,
  `Kernel.get_and_update_in/3`, and friends.
@@ -546,6 +574,19 @@ defmodule Access do

  ## Examples

+      iex> keyword = [user: [name: "john"]]
+      iex> get_in(keyword, [Access.key!(:user), Access.key!(:name)])
+      "john"
+      iex> get_and_update_in(keyword, [Access.key!(:user), Access.key!(:name)], fn prev ->
+      ...>   {prev, String.upcase(prev)}
+      ...> end)
+      {"john", [user: [name: "JOHN"]]}
+      iex> pop_in(keyword, [Access.key!(:user), Access.key!(:name)])
+      {"john", [user: []]}
+      iex> get_in(keyword, [Access.key!(:user), Access.key!(:unknown)])
+      ** (KeyError) key :unknown not found in:
+      ...
+
      iex> map = %{user: %{name: "john"}}
      iex> get_in(map, [Access.key!(:user), Access.key!(:name)])
      "john"
@@ -574,13 +615,15 @@ defmodule Access do
  `Access.key!/1` is useful when the key is not known in advance
  and must be accessed dynamically.

-  An error is raised if the accessed structure is not a map/struct:
+  An error is raised if the accessed structure is not a map/struct/keyword list:

-      iex> get_in([], [Access.key!(:foo)])
-      ** (RuntimeError) Access.key!/1 expected a map/struct, got: []
+      iex> get_in(123, [Access.key!(:foo)])
+      ** (RuntimeError) Access.key!/1 expected a map/struct/keyword list, got: 123

+      iex> put_in([1, 2, 3], [Access.key!(:foo)], :bar)
+      ** (RuntimeError) Access.key!/1 expected a map/struct/keyword list, got: ...
  """
-  @spec key!(key) :: access_fun(data :: struct | map, current_value :: term)
+  @spec key!(key) :: access_fun(data :: struct | map | keyword, current_value :: term)
  def key!(key) do
    fn
      :get, %{} = data, next ->
@@ -594,8 +637,19 @@ defmodule Access do
          :pop -> {value, Map.delete(data, key)}
        end

+      :get, data, next when is_probably_keyword(data) ->
+        next.(Keyword.fetch!(data, key))
+
+      :get_and_update, data, next when is_probably_keyword(data) ->
+        value = Keyword.fetch!(data, key)
+
+        case next.(value) do
+          {get, update} -> {get, Keyword.put(data, key, update)}
+          :pop -> {value, Keyword.delete(data, key)}
+        end
+
      _op, data, _next ->
-        raise "Access.key!/1 expected a map/struct, got: #{inspect(data)}"
+        raise "Access.key!/1 expected a map/struct/keyword list, got: #{inspect(data)}"
    end
  end

@@ -863,7 +917,7 @@ defmodule Access do
      iex> pop_in(list, [Access.filter(&(&1.salary >= 20)), :name])
      {["francine"], [%{name: "john", salary: 10}, %{salary: 30}]}

-  When no match is found, an empty list is returned and the update function is never called
+  When no match is found, an empty list is returned and the update function is never called:

      iex> list = [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]
      iex> get_in(list, [Access.filter(&(&1.salary >= 50)), :name])
@@ -880,7 +934,7 @@ defmodule Access do

  """
  @doc since: "1.6.0"
-  @spec filter((term -> boolean)) :: access_fun(data :: list, current_value :: list)
+  @spec filter((term -> as_boolean(term))) :: access_fun(data :: list, current_value :: list)
  def filter(func) when is_function(func) do
    fn op, data, next -> filter(op, data, func, next) end
  end
@@ -981,12 +1035,12 @@ defmodule Access do
  end

  defp slice(:get_and_update, data, range, next) when is_list(data) do
-    range = normalize_range(range, data)
+    %Range{first: first, last: last, step: step} = normalize_range(range, data)

-    if range.first > range.last do
+    if first > last do
      {[], data}
    else
-      get_and_update_slice(data, range, next, [], [], 0)
+      get_and_update_slice(data, first, last, step, next, [], [], 0)
    end
  end

@@ -1091,16 +1145,23 @@ defmodule Access do

  defp normalize_range(range, _list), do: range

-  defp get_and_update_slice([head | rest], range, next, updates, gets, index) do
-    if index in range do
+  defp get_and_update_slice(rest, _first, last, _step, _next, updates, gets, index)
+       when index > last do
+    {:lists.reverse(gets), :lists.reverse(updates, rest)}
+  end
+
+  defp get_and_update_slice([head | rest], first, last, step, next, updates, gets, index) do
+    if index >= first and rem(index - first, step) == 0 do
      case next.(head) do
        :pop ->
-          get_and_update_slice(rest, range, next, updates, [head | gets], index + 1)
+          get_and_update_slice(rest, first, last, step, next, updates, [head | gets], index + 1)

        {get, update} ->
          get_and_update_slice(
            rest,
-            range,
+            first,
+            last,
+            step,
            next,
            [update | updates],
            [get | gets],
@@ -1108,11 +1169,11 @@ defmodule Access do
          )
      end
    else
-      get_and_update_slice(rest, range, next, [head | updates], gets, index + 1)
+      get_and_update_slice(rest, first, last, step, next, [head | updates], gets, index + 1)
    end
  end

-  defp get_and_update_slice([], _range, _next, updates, gets, _index) do
+  defp get_and_update_slice([], _first, _last, _step, _next, updates, gets, _index) do
    {:lists.reverse(gets), :lists.reverse(updates)}
  end

@@ -1139,7 +1200,7 @@ defmodule Access do
      iex> pop_in(list, [Access.find(&(&1.salary <= 40))])
      {%{name: "john", salary: 10}, [%{name: "francine", salary: 30}]}

-  When no match is found, nil is returned and the update function is never called
+  When no match is found, nil is returned and the update function is never called:

      iex> list = [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]
      iex> get_in(list, [Access.find(&(&1.salary >= 50)), :name])
@@ -327,7 +327,7 @@ defmodule Agent do
  passing the agent state. The result of the function invocation is
  returned from this function.

-  `timeout` is an integer greater than zero which specifies how many
+  `timeout` is a non-negative integer which specifies how many
  milliseconds are allowed before the agent executes the function and returns
  the result value, or the atom `:infinity` to wait indefinitely. If no result
  is received within the specified time, the function call fails and the caller
@@ -366,7 +366,7 @@ defmodule Agent do
  elements, the first being the value to return (that is, the "get" value)
  and the second one being the new state of the agent.

-  `timeout` is an integer greater than zero which specifies how many
+  `timeout` is a non-negative integer which specifies how many
  milliseconds are allowed before the agent executes the function and returns
  the result value, or the atom `:infinity` to wait indefinitely. If no result
  is received within the specified time, the function call fails and the caller
@@ -407,7 +407,7 @@ defmodule Agent do

  This function always returns `:ok`.

-  `timeout` is an integer greater than zero which specifies how many
+  `timeout` is a non-negative integer which specifies how many
  milliseconds are allowed before the agent executes the function and returns
  the result value, or the atom `:infinity` to wait indefinitely. If no result
  is received within the specified time, the function call fails and the caller
@@ -996,7 +996,7 @@ defmodule Application do
  end

  @doc """
-  Gets the directory for app.
+  Gets the directory for `app`.

  This information is returned based on the code path. Here is an
  example:
@@ -1077,7 +1077,7 @@ defmodule Application do
  @doc """
  Formats the error reason returned by `start/2`,
  `ensure_started/2`, `stop/1`, `load/1` and `unload/1`,
-  returns a string.
+  and returns a string.
  """
  @spec format_error(any) :: String.t()
  def format_error(reason) do
@@ -155,6 +155,255 @@ defmodule Base do
    for <<char::8 <- string>>, char not in ~c"\s\t\r\n", into: <<>>, do: <<char::8>>
  end

+  # SWAR (SIMD Within A Register) fast paths for valid16?/2 and valid32?/2
+  # (non-hex). Each chunk of 8 bytes is validated in one guard: 7 bytes via
+  # bitwise arithmetic on a single 56-bit integer, plus a per-byte range
+  # check for the 8th byte. 56 bits is the largest width that fits in a BEAM
+  # small int on 64-bit (fixnum range is 59-bit signed); at 64 bits every
+  # `w + 0x80..` would allocate a bignum on the heap and the optimisation
+  # would collapse. See https://github.com/erlang/otp/pull/10938 for the
+  # corresponding pattern in OTP.
+  @swar_mask80 0x80808080808080
+
+  # Per-range SWAR constants, broadcast across 7 lanes. Naming convention:
+  #   @swar_ge_X = 0x80 - X  → high bit of `(w + @swar_ge_X)` lane is set
+  #                            iff that byte is ≥ X
+  #   @swar_gt_X = 0x7F - X  → high bit of `(w + @swar_gt_X)` lane is set
+  #                            iff that byte is > X
+  # A byte is in range [lo, hi] iff
+  #   `bxor(w + @swar_ge_lo, w + @swar_gt_hi)` has its high bit set.
+  @swar_ge_0 0x50505050505050
+  @swar_gt_9 0x46464646464646
+  @swar_ge_2 0x4E4E4E4E4E4E4E
+  @swar_gt_7 0x48484848484848
+  @swar_ge_A 0x3F3F3F3F3F3F3F
+  @swar_gt_F 0x39393939393939
+  @swar_gt_V 0x29292929292929
+  @swar_gt_Z 0x25252525252525
+  @swar_ge_a 0x1F1F1F1F1F1F1F
+  @swar_gt_f 0x19191919191919
+  @swar_gt_v 0x09090909090909
+  @swar_gt_z 0x05050505050505
+
+  # For base64 standard, '/' (0x2F) sits exactly one below '0' (0x30), so we
+  # extend the digit range to [0x2F, 0x39], which absorbs '/' into one range
+  # check — saves one singleton term. Trick lifted from
+  # https://lemire.me/blog/2025/04/13/detect-control-characters-quotes-and-backslashes-efficiently-using-swar/
+  @swar_ge_slash 0x51515151515151
+
+  # base64's '+', '-' and '_' border no other range, so each is checked as the
+  # degenerate range [c, c]. Mycroft zero-byte detection is one operation
+  # cheaper per singleton but not exact per lane: subtracting 0x01..01 makes a
+  # zero lane borrow from the lane above it, and a lane holding 1 turns that
+  # borrow into 0xFF, so bytes equal to `c bxor 0x01` read as matches too.
+  @swar_ge_plus 0x55555555555555
+  @swar_gt_plus 0x54545454545454
+  @swar_ge_dash 0x53535353535353
+  @swar_gt_dash 0x52525252525252
+  @swar_ge_underscore 0x21212121212121
+  @swar_gt_underscore 0x20202020202020
+
+  # Per-byte validity checks (used in both the SWAR clauses for the 8th byte
+  # of each stride and in the body of the sub-8-byte tail clauses).
+  @compile {:inline,
+            valid_char16upper?: 1,
+            valid_char16lower?: 1,
+            valid_char16mixed?: 1,
+            valid_char32upper?: 1,
+            valid_char32lower?: 1,
+            valid_char32mixed?: 1,
+            valid_char32hexupper?: 1,
+            valid_char32hexlower?: 1,
+            valid_char32hexmixed?: 1,
+            valid_char64base?: 1,
+            valid_char64url?: 1,
+            valid_word16upper?: 1,
+            valid_word16lower?: 1,
+            valid_word16mixed?: 1,
+            valid_word32upper?: 1,
+            valid_word32lower?: 1,
+            valid_word32mixed?: 1,
+            valid_word32hexupper?: 1,
+            valid_word32hexlower?: 1,
+            valid_word32hexmixed?: 1,
+            valid_word64base?: 1,
+            valid_word64url?: 1}
+
+  defp valid_char16upper?(c), do: c in ?0..?9 or c in ?A..?F
+  defp valid_char16lower?(c), do: c in ?0..?9 or c in ?a..?f
+  defp valid_char16mixed?(c), do: c in ?0..?9 or c in ?A..?F or c in ?a..?f
+
+  defp valid_char32upper?(c), do: c in ?A..?Z or c in ?2..?7
+  defp valid_char32lower?(c), do: c in ?a..?z or c in ?2..?7
+  defp valid_char32mixed?(c), do: c in ?A..?Z or c in ?a..?z or c in ?2..?7
+
+  # Most common range first — letters dominate (22/32) over digits (10/32)
+  # in hex base32, so letters go first in the OR short-circuit.
+  defp valid_char32hexupper?(c), do: c in ?A..?V or c in ?0..?9
+  defp valid_char32hexlower?(c), do: c in ?a..?v or c in ?0..?9
+  defp valid_char32hexmixed?(c), do: c in ?A..?V or c in ?a..?v or c in ?0..?9
+
+  defp valid_char64base?(c),
+    do: c in ?A..?Z or c in ?a..?z or c in ?0..?9 or c == ?+ or c == ?/
+
+  defp valid_char64url?(c),
+    do: c in ?A..?Z or c in ?a..?z or c in ?0..?9 or c == ?- or c == ?_
+
+  # SWAR 7-byte word validity. Structure for each function:
+  #   1. ASCII gate `band(w, MASK80) == 0` — every byte < 0x80 so the
+  #      additions below cannot carry across lanes.
+  #   2. "Each byte is in range A OR range B (OR range C)" gate — OR per-
+  #      range XOR masks (high bit set in lane iff byte in that range), AND
+  #      with MASK80, demand all 7 high bits set.
+  defp valid_word16upper?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bxor(w + @swar_ge_0, w + @swar_gt_9),
+          bxor(w + @swar_ge_A, w + @swar_gt_F)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  defp valid_word16lower?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bxor(w + @swar_ge_0, w + @swar_gt_9),
+          bxor(w + @swar_ge_a, w + @swar_gt_f)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  defp valid_word16mixed?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bor(
+            bxor(w + @swar_ge_0, w + @swar_gt_9),
+            bxor(w + @swar_ge_A, w + @swar_gt_F)
+          ),
+          bxor(w + @swar_ge_a, w + @swar_gt_f)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  defp valid_word32upper?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bxor(w + @swar_ge_A, w + @swar_gt_Z),
+          bxor(w + @swar_ge_2, w + @swar_gt_7)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  defp valid_word32lower?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bxor(w + @swar_ge_a, w + @swar_gt_z),
+          bxor(w + @swar_ge_2, w + @swar_gt_7)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  defp valid_word32mixed?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bor(
+            bxor(w + @swar_ge_A, w + @swar_gt_Z),
+            bxor(w + @swar_ge_a, w + @swar_gt_z)
+          ),
+          bxor(w + @swar_ge_2, w + @swar_gt_7)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  defp valid_word32hexupper?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bxor(w + @swar_ge_0, w + @swar_gt_9),
+          bxor(w + @swar_ge_A, w + @swar_gt_V)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  defp valid_word32hexlower?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bxor(w + @swar_ge_0, w + @swar_gt_9),
+          bxor(w + @swar_ge_a, w + @swar_gt_v)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  defp valid_word32hexmixed?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bor(
+            bxor(w + @swar_ge_0, w + @swar_gt_9),
+            bxor(w + @swar_ge_A, w + @swar_gt_V)
+          ),
+          bxor(w + @swar_ge_a, w + @swar_gt_v)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  # base64 SWAR word validity: 3 ranges (A-Z, a-z, 0-9) OR'd with singletons.
+  # For base, the digit range is extended to [0x2F, 0x39] to absorb '/' as
+  # part of one range (Lemire merge), leaving only '+' as a singleton. For url,
+  # '-' and '_' are two more singletons.
+  defp valid_word64base?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bor(
+            bor(
+              bxor(w + @swar_ge_A, w + @swar_gt_Z),
+              bxor(w + @swar_ge_a, w + @swar_gt_z)
+            ),
+            bxor(w + @swar_ge_slash, w + @swar_gt_9)
+          ),
+          bxor(w + @swar_ge_plus, w + @swar_gt_plus)
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
+  defp valid_word64url?(w) do
+    band(w, @swar_mask80) == 0 and
+      band(
+        bor(
+          bor(
+            bor(
+              bxor(w + @swar_ge_A, w + @swar_gt_Z),
+              bxor(w + @swar_ge_a, w + @swar_gt_z)
+            ),
+            bxor(w + @swar_ge_0, w + @swar_gt_9)
+          ),
+          bor(
+            bxor(w + @swar_ge_dash, w + @swar_gt_dash),
+            bxor(w + @swar_ge_underscore, w + @swar_gt_underscore)
+          )
+        ),
+        @swar_mask80
+      ) == @swar_mask80
+  end
+
  @doc """
  Encodes a binary string into a base 16 encoded string.

@@ -187,7 +436,7 @@ defmodule Base do
  end

  for {base, alphabet} <- [upper: b16_alphabet, lower: to_lower_enc.(b16_alphabet)] do
-    name = :"encode16#{base}"
+    name = String.to_unsafe_atom("encode16#{base}")
    encoded = to_encode_list.(alphabet)

    @compile {:inline, [{name, 1}]}
@@ -287,7 +536,7 @@ defmodule Base do
    * `:lower` - only allows lower case characters
    * `:mixed` - allows mixed case characters

-  An `ArgumentError` exception is raised if the padding is incorrect or
+  An `ArgumentError` exception is raised if the string has an odd length or
  a non-alphabet character is present in the string.

  ## Examples
@@ -368,28 +617,24 @@ defmodule Base do
  upper = Enum.with_index(b16_alphabet)

  for {base, alphabet} <- [upper: upper, lower: to_lower_dec.(upper), mixed: to_mixed_dec.(upper)] do
-    decode_name = :"decode16#{base}!"
-    validate_name = :"validate16#{base}?"
-    valid_char_name = :"valid_char16#{base}?"
+    decode_name = String.to_unsafe_atom("decode16#{base}!")
+    validate_name = String.to_unsafe_atom("validate16#{base}?")
+    valid_char_name = String.to_unsafe_atom("valid_char16#{base}?")
+    valid_word_name = String.to_unsafe_atom("valid_word16#{base}?")

    {min, decoded} = to_decode_list.(alphabet)

+    # SWAR fast path: 7 bytes per stride, validated entirely via
+    # `valid_word16<base>?` in the body. The `and` short-circuits when SWAR
+    # fails on any byte. Tail bytes (1-6 leftover) recurse through the
+    # single-byte clause below.
+    defp unquote(validate_name)(<<w::56, rest::binary>>),
+      do: unquote(valid_word_name)(w) and unquote(validate_name)(rest)
+
    defp unquote(validate_name)(<<>>), do: true

-    defp unquote(validate_name)(<<c1, c2, rest::binary>>) do
-      unquote(valid_char_name)(c1) and
-        unquote(valid_char_name)(c2) and
-        unquote(validate_name)(rest)
-    end
-
-    defp unquote(validate_name)(<<_char, _rest::binary>>), do: false
-
-    @compile {:inline, [{valid_char_name, 1}]}
-    defp unquote(valid_char_name)(char)
-         when elem({unquote_splicing(decoded)}, char - unquote(min)) != nil,
-         do: true
-
-    defp unquote(valid_char_name)(_char), do: false
+    defp unquote(validate_name)(<<char, rest::binary>>),
+      do: unquote(valid_char_name)(char) and unquote(validate_name)(rest)

    defp unquote(decode_name)(char) do
      index = char - unquote(min)
@@ -490,7 +735,7 @@ defmodule Base do
  end

  for {base, alphabet} <- [base: b64_alphabet, url: b64url_alphabet] do
-    name = :"encode64#{base}"
+    name = String.to_unsafe_atom("encode64#{base}")
    encoded = to_encode_list.(alphabet)

    @compile {:inline, [{name, 1}]}
@@ -756,28 +1001,24 @@ defmodule Base do
  end

  for {base, alphabet} <- [base: b64_alphabet, url: b64url_alphabet] do
-    decode_name = :"decode64#{base}!"
+    decode_name = String.to_unsafe_atom("decode64#{base}!")

-    validate_name = :"validate64#{base}?"
-    validate_main_name = :"validate_main64#{validate_name}?"
-    valid_char_name = :"valid_char64#{base}?"
+    validate_name = String.to_unsafe_atom("validate64#{base}?")
+    validate_main_name = String.to_unsafe_atom("validate_main64#{validate_name}?")
+    valid_char_name = String.to_unsafe_atom("valid_char64#{base}?")
+    valid_word_name = String.to_unsafe_atom("valid_word64#{base}?")
    {min, decoded} = alphabet |> Enum.with_index() |> to_decode_list.()

+    # SWAR fast path: 7 bytes per stride, validated via `valid_word64<base>?`
+    # in the body. Tail leftover (1-6 bytes after a 7-byte stride hits an
+    # 8-byte-multiple `main`) recurses through the single-byte clause.
+    defp unquote(validate_main_name)(<<w::56, rest::binary>>),
+      do: unquote(valid_word_name)(w) and unquote(validate_main_name)(rest)
+
    defp unquote(validate_main_name)(<<>>), do: true

-    defp unquote(validate_main_name)(
-           <<c1::8, c2::8, c3::8, c4::8, c5::8, c6::8, c7::8, c8::8, rest::binary>>
-         ) do
-      unquote(valid_char_name)(c1) and
-        unquote(valid_char_name)(c2) and
-        unquote(valid_char_name)(c3) and
-        unquote(valid_char_name)(c4) and
-        unquote(valid_char_name)(c5) and
-        unquote(valid_char_name)(c6) and
-        unquote(valid_char_name)(c7) and
-        unquote(valid_char_name)(c8) and
-        unquote(validate_main_name)(rest)
-    end
+    defp unquote(validate_main_name)(<<char, rest::binary>>),
+      do: unquote(valid_char_name)(char) and unquote(validate_main_name)(rest)

    defp unquote(validate_name)(<<>>, _pad?), do: true

@@ -863,13 +1104,6 @@ defmodule Base do
      end
    end

-    @compile {:inline, [{valid_char_name, 1}]}
-    defp unquote(valid_char_name)(char)
-         when elem({unquote_splicing(decoded)}, char - unquote(min)) != nil,
-         do: true
-
-    defp unquote(valid_char_name)(_char), do: false
-
    defp unquote(decode_name)(char) do
      index = char - unquote(min)

@@ -1080,7 +1314,7 @@ defmodule Base do
        hexupper: b32hex_alphabet,
        hexlower: to_lower_enc.(b32hex_alphabet)
      ] do
-    name = :"encode32#{base}"
+    name = String.to_unsafe_atom("encode32#{base}")
    encoded = to_encode_list.(alphabet)

    @compile {:inline, [{name, 1}]}
@@ -1154,7 +1388,7 @@ defmodule Base do

  The values for `:case` can be:

-    * `:upper` - only allows  upper case characters (default)
+    * `:upper` - only allows upper case characters (default)
    * `:lower` - only allows lower case characters
    * `:mixed` - allows mixed case characters

@@ -1419,27 +1653,24 @@ defmodule Base do
        hexlower: to_lower_dec.(hexupper),
        hexmixed: to_mixed_dec.(hexupper)
      ] do
-    decode_name = :"decode32#{base}!"
-    validate_name = :"validate32#{base}?"
-    validate_main_name = :"validate_main32#{validate_name}?"
-    valid_char_name = :"valid_char32#{base}?"
+    decode_name = String.to_unsafe_atom("decode32#{base}!")
+    validate_name = String.to_unsafe_atom("validate32#{base}?")
+    validate_main_name = String.to_unsafe_atom("validate_main32#{validate_name}?")
+    valid_char_name = String.to_unsafe_atom("valid_char32#{base}?")
    {min, decoded} = to_decode_list.(alphabet)

+    # SWAR fast path: 7 bytes per stride, validated via `valid_word32<base>?`
+    # in the body. Tail leftover (1-6 bytes after a 7-byte stride hits an
+    # 8-byte-multiple `main`) recurses through the single-byte clause.
+    valid_word_name = String.to_unsafe_atom("valid_word32#{base}?")
+
+    defp unquote(validate_main_name)(<<w::56, rest::binary>>),
+      do: unquote(valid_word_name)(w) and unquote(validate_main_name)(rest)
+
    defp unquote(validate_main_name)(<<>>), do: true

-    defp unquote(validate_main_name)(
-           <<c1::8, c2::8, c3::8, c4::8, c5::8, c6::8, c7::8, c8::8, rest::binary>>
-         ) do
-      unquote(valid_char_name)(c1) and
-        unquote(valid_char_name)(c2) and
-        unquote(valid_char_name)(c3) and
-        unquote(valid_char_name)(c4) and
-        unquote(valid_char_name)(c5) and
-        unquote(valid_char_name)(c6) and
-        unquote(valid_char_name)(c7) and
-        unquote(valid_char_name)(c8) and
-        unquote(validate_main_name)(rest)
-    end
+    defp unquote(validate_main_name)(<<char, rest::binary>>),
+      do: unquote(valid_char_name)(char) and unquote(validate_main_name)(rest)

    defp unquote(validate_name)(<<>>, _pad?), do: true

@@ -1519,13 +1750,6 @@ defmodule Base do
      end
    end

-    @compile {:inline, [{valid_char_name, 1}]}
-    defp unquote(valid_char_name)(char)
-         when elem({unquote_splicing(decoded)}, char - unquote(min)) != nil,
-         do: true
-
-    defp unquote(valid_char_name)(_char), do: false
-
    defp unquote(decode_name)(char) do
      index = char - unquote(min)

@@ -3,6 +3,8 @@
 # SPDX-FileCopyrightText: 2012 Plataformatec

 defmodule Calendar do
+  @strftime_max_width 1024
+
  @moduledoc """
  This module defines the responsibilities for working with
  calendars, dates, times and datetimes in Elixir.
@@ -104,6 +106,7 @@ defmodule Calendar do
  @typedoc "Any map or struct that contains the time fields."
  @type time :: %{
          optional(any) => any,
+          calendar: calendar,
          hour: hour,
          minute: minute,
          second: second,
@@ -293,7 +296,7 @@ defmodule Calendar do
  @callback time_from_day_fraction(day_fraction) :: {hour, minute, second, microsecond}

  @doc """
-  Define the rollover moment for the calendar.
+  Defines the rollover moment for the calendar.

  This is the moment, in your calendar, when the current day ends
  and the next day starts.
@@ -379,13 +382,13 @@ defmodule Calendar do
  @callback iso_days_to_end_of_day(iso_days) :: iso_days

  @doc """
-  Shifts date by given duration according to its calendar.
+  Shifts date by the given duration according to its calendar.
  """
  @doc since: "1.17.0"
  @callback shift_date(year, month, day, Duration.t()) :: {year, month, day}

  @doc """
-  Shifts naive datetime by given duration according to its calendar.
+  Shifts naive datetime by the given duration according to its calendar.
  """
  @doc since: "1.17.0"
  @callback shift_naive_datetime(
@@ -400,7 +403,7 @@ defmodule Calendar do
            ) :: {year, month, day, hour, minute, second, microsecond}

  @doc """
-  Shifts time by given duration according to its calendar.
+  Shifts time by the given duration according to its calendar.
  """
  @doc since: "1.17.0"
  @callback shift_time(hour, minute, second, microsecond, Duration.t()) ::
@@ -529,6 +532,7 @@ defmodule Calendar do
    * `%`: indicates the start of a formatted section
    * `<padding>`: set the padding (see below)
    * `<width>`: a number indicating the minimum size of the formatted section
+      (maximum #{@strftime_max_width})
    * `<format>`: the format itself (see below)

  ### Accepted padding options
@@ -559,11 +563,11 @@ defmodule Calendar do
  P      | "am" or "pm" (noon is "pm", midnight as "am")                           | am, pm
  q      | Quarter                                                                 | 1, 2, 3, 4
  s      | Number of seconds since the Epoch, 1970-01-01 00:00:00+0000 (UTC)       | 1565888877
-  S      | Second                                                                  | 00, 59, 60
+  S      | Second                                                                  | 00, 59
  u      | Day of the week                                                         | 1 (Monday), 7 (Sunday)
  x      | Preferred date (without time) representation                            | 2018-10-17
  X      | Preferred time (without date) representation                            | 12:34:56
-  y      | Year as 2-digits                                                        | 01, 01, 86, 18
+  y      | Year as 2-digits                                                        | -01, 01, 86, 18
  Y      | Year                                                                    | -0001, 0001, 1986
  z      | +hhmm/-hhmm time zone offset from UTC (empty string if naive)           | +0300, -0530
  Z      | Time zone abbreviation (empty string if naive)                          | CET, BRST
@@ -620,7 +624,7 @@ defmodule Calendar do
      ...>)
      "серпень"

-   Microsecond formatting:
+    Microsecond formatting:

      iex> Calendar.strftime(~U[2019-08-26 13:52:06Z], "%y-%m-%d %H:%M:%S.%f")
      "19-08-26 13:52:06.0"
@@ -667,9 +671,13 @@ defmodule Calendar do
  end

  defp parse_modifiers(<<digit, rest::binary>>, width, pad, parser_data) when digit in ?0..?9 do
-    new_width = (width || 0) * 10 + (digit - ?0)
+    width = (width || 0) * 10 + (digit - ?0)

-    parse_modifiers(rest, new_width, pad, parser_data)
+    if width > @strftime_max_width do
+      raise ArgumentError, "invalid strftime format: width must be at most #{@strftime_max_width}"
+    end
+
+    parse_modifiers(rest, width, pad, parser_data)
  end

  # set default padding if none was specified
@@ -704,7 +712,7 @@ defmodule Calendar do

  # Literally just %
  defp format_modifiers("%" <> rest, width, pad, datetime, format_options, acc) do
-    parse(rest, datetime, format_options, [pad_leading("%", width, pad) | acc])
+    parse(rest, datetime, format_options, [pad_leading_ascii("%", width, pad) | acc])
  end

  # Abbreviated name of day
@@ -773,7 +781,7 @@ defmodule Calendar do

  # Day of the month
  defp format_modifiers("d" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime.day |> Integer.to_string() |> pad_leading(width, pad)
+    result = datetime.day |> Integer.to_string() |> pad_leading_ascii(width, pad)
    parse(rest, datetime, format_options, [result | acc])
  end

@@ -792,31 +800,35 @@ defmodule Calendar do

  # Hour using a 24-hour clock
  defp format_modifiers("H" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime.hour |> Integer.to_string() |> pad_leading(width, pad)
+    result = datetime.hour |> Integer.to_string() |> pad_leading_ascii(width, pad)
    parse(rest, datetime, format_options, [result | acc])
  end

  # Hour using a 12-hour clock
  defp format_modifiers("I" <> rest, width, pad, datetime, format_options, acc) do
-    result = (rem(datetime.hour + 23, 12) + 1) |> Integer.to_string() |> pad_leading(width, pad)
+    result =
+      (rem(datetime.hour + 23, 12) + 1) |> Integer.to_string() |> pad_leading_ascii(width, pad)
+
    parse(rest, datetime, format_options, [result | acc])
  end

  # Day of the year
  defp format_modifiers("j" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime |> Date.day_of_year() |> Integer.to_string() |> pad_leading(width, pad)
+    result =
+      datetime |> Date.day_of_year() |> Integer.to_string() |> pad_leading_ascii(width, pad)
+
    parse(rest, datetime, format_options, [result | acc])
  end

  # Month
  defp format_modifiers("m" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime.month |> Integer.to_string() |> pad_leading(width, pad)
+    result = datetime.month |> Integer.to_string() |> pad_leading_ascii(width, pad)
    parse(rest, datetime, format_options, [result | acc])
  end

  # Minute
  defp format_modifiers("M" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime.minute |> Integer.to_string() |> pad_leading(width, pad)
+    result = datetime.minute |> Integer.to_string() |> pad_leading_ascii(width, pad)
    parse(rest, datetime, format_options, [result | acc])
  end

@@ -844,19 +856,23 @@ defmodule Calendar do

  # Quarter
  defp format_modifiers("q" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime |> Date.quarter_of_year() |> Integer.to_string() |> pad_leading(width, pad)
+    result =
+      datetime |> Date.quarter_of_year() |> Integer.to_string() |> pad_leading_ascii(width, pad)
+
    parse(rest, datetime, format_options, [result | acc])
  end

  # Second
  defp format_modifiers("S" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime.second |> Integer.to_string() |> pad_leading(width, pad)
+    result = datetime.second |> Integer.to_string() |> pad_leading_ascii(width, pad)
    parse(rest, datetime, format_options, [result | acc])
  end

  # Day of the week
  defp format_modifiers("u" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime |> Date.day_of_week() |> Integer.to_string() |> pad_leading(width, pad)
+    result =
+      datetime |> Date.day_of_week() |> Integer.to_string() |> pad_leading_ascii(width, pad)
+
    parse(rest, datetime, format_options, [result | acc])
  end

@@ -906,20 +922,25 @@ defmodule Calendar do

  # Year as 2-digits
  defp format_modifiers("y" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime.year |> rem(100) |> Integer.to_string() |> pad_leading(width, pad)
+    result =
+      if datetime.year < 0 do
+        [?- | -datetime.year |> rem(100) |> Integer.to_string() |> pad_leading_ascii(width, pad)]
+      else
+        datetime.year |> rem(100) |> Integer.to_string() |> pad_leading_ascii(width, pad)
+      end
+
    parse(rest, datetime, format_options, [result | acc])
  end

  # Year
  defp format_modifiers("Y" <> rest, width, pad, datetime, format_options, acc) do
-    {sign, year} =
+    result =
      if datetime.year < 0 do
-        {?-, -datetime.year}
+        [?- | -datetime.year |> Integer.to_string() |> pad_leading_ascii(width, pad)]
      else
-        {[], datetime.year}
+        datetime.year |> Integer.to_string() |> pad_leading_ascii(width, pad)
      end

-    result = [sign | year |> Integer.to_string() |> pad_leading(width, pad)]
    parse(rest, datetime, format_options, [result | acc])
  end

@@ -966,7 +987,7 @@ defmodule Calendar do
      Integer.to_string(div(absolute_offset, 3600) * 100 + rem(div(absolute_offset, 60), 60))

    sign = if utc_offset + std_offset >= 0, do: "+", else: "-"
-    result = "#{sign}#{pad_leading(offset_number, width, pad)}"
+    result = "#{sign}#{pad_leading_ascii(offset_number, width, pad)}"
    parse(rest, datetime, format_options, [result | acc])
  end

@@ -985,13 +1006,19 @@ defmodule Calendar do
    raise ArgumentError, "invalid strftime format: %#{next}"
  end

-  defp pad_preferred(result, width, pad) when length(result) < width do
-    pad_preferred([pad | result], width, pad)
+  defp pad_preferred(result, width, pad) do
+    result
+    |> IO.iodata_to_binary()
+    |> pad_leading(width, pad)
  end

-  defp pad_preferred(result, _width, _pad), do: result
-
  defp pad_leading(string, count, padding) do
+    to_pad = count - String.length(string)
+    if to_pad > 0, do: do_pad_leading(to_pad, padding, string), else: string
+  end
+
+  # Similar to `pad_leading/3`, but only for strings that always ASCII-only
+  defp pad_leading_ascii(string, count, padding) do
    to_pad = count - byte_size(string)
    if to_pad > 0, do: do_pad_leading(to_pad, padding, string), else: string
  end
@@ -81,7 +81,7 @@ defmodule Date do

  Ranges of dates can be increasing (`first <= last`) and are
  always inclusive. For a decreasing range, use `range/3` with
-  a step of -1 as first argument.
+  a step of -1 as third argument.

  ## Examples

@@ -160,7 +160,7 @@ defmodule Date do
      ) do
    raise ArgumentError,
          "both dates must have matching calendar and the step must be a " <>
-            "non-zero integer, got: #{inspect(first)}, #{inspect(last)}, #{step}"
+            "non-zero integer, got: #{inspect(first)}, #{inspect(last)}, #{inspect(step)}"
  end

  defp range(first, first_days, last, last_days, calendar, step) do
@@ -422,7 +422,7 @@ defmodule Date do
  def to_iso8601(%{calendar: _} = date, format) when format in [:basic, :extended] do
    date
    |> convert!(Calendar.ISO)
-    |> to_iso8601()
+    |> to_iso8601(format)
  end

  @doc """
@@ -556,14 +556,18 @@ defmodule Date do
  """
  @doc since: "1.4.0"
  @spec compare(Calendar.date(), Calendar.date()) :: :lt | :eq | :gt
-  def compare(%{calendar: calendar} = date1, %{calendar: calendar} = date2) do
-    %{year: year1, month: month1, day: day1} = date1
-    %{year: year2, month: month2, day: day2} = date2
-
-    case {{year1, month1, day1}, {year2, month2, day2}} do
-      {first, second} when first > second -> :gt
-      {first, second} when first < second -> :lt
-      _ -> :eq
+  def compare(
+        %{year: year1, month: month1, day: day1, calendar: calendar},
+        %{year: year2, month: month2, day: day2, calendar: calendar}
+      ) do
+    cond do
+      year1 > year2 -> :gt
+      year1 < year2 -> :lt
+      month1 > month2 -> :gt
+      month1 < month2 -> :lt
+      day1 > day2 -> :gt
+      day1 < day2 -> :lt
+      true -> :eq
    end
  end

@@ -1047,7 +1051,7 @@ defmodule Date do
  @doc """
  Calculates the quarter of the year of a given `date`.

-  Returns the day of the year as an integer. For the ISO 8601
+  Returns the quarter of the year as an integer. For the ISO 8601
  calendar (the default), it is an integer from 1 to 4.

  ## Examples
@@ -37,21 +37,19 @@ defmodule Date.Range do
            first_in_iso_days: first_days,
            last_in_iso_days: last_days,
            step: step
-          } = range,
+          },
          %Date{calendar: calendar} = date
        ) do
      {days, _} = Date.to_iso_days(date)

-      cond do
-        empty?(range) ->
-          {:ok, false}
+      in_range? =
+        if step > 0 do
+          first_days <= days and days <= last_days and rem(days - first_days, step) == 0
+        else
+          last_days <= days and days <= first_days and rem(days - first_days, step) == 0
+        end

-        first_days <= last_days ->
-          {:ok, first_days <= days and days <= last_days and rem(days - first_days, step) == 0}
-
-        true ->
-          {:ok, last_days <= days and days <= first_days and rem(days - first_days, step) == 0}
-      end
+      {:ok, in_range?}
    end

    def member?(%Date.Range{step: _}, _) do
@@ -88,7 +86,7 @@ defmodule Date.Range do
            step: step
          } = range
        ) do
-      {:ok, size(range), &slice(first + &1 * step, step + &3 - 1, &2, calendar)}
+      {:ok, size(range), &slice(first + &1 * step, step * &3, &2, calendar)}
    end

    # TODO: Remove me on v2.0
@@ -185,7 +183,7 @@ defmodule Date.Range do
           last_in_iso_days: last_days,
           step: step
         }),
-         do: abs(div(last_days - first_days, step)) + 1
+         do: div(last_days - first_days, step) + 1

    # TODO: Remove me on v2.0
    defp size(
@@ -195,33 +193,6 @@ defmodule Date.Range do
      step = if first_days <= last_days, do: 1, else: -1
      size(Map.put(date_range, :step, step))
    end
-
-    defp empty?(%Date.Range{
-           first_in_iso_days: first_days,
-           last_in_iso_days: last_days,
-           step: step
-         })
-         when step > 0 and first_days > last_days,
-         do: true
-
-    defp empty?(%Date.Range{
-           first_in_iso_days: first_days,
-           last_in_iso_days: last_days,
-           step: step
-         })
-         when step < 0 and first_days < last_days,
-         do: true
-
-    defp empty?(%Date.Range{step: _}), do: false
-
-    # TODO: Remove me on v2.0
-    defp empty?(
-           %{__struct__: Date.Range, first_in_iso_days: first_days, last_in_iso_days: last_days} =
-             date_range
-         ) do
-      step = if first_days <= last_days, do: 1, else: -1
-      empty?(Map.put(date_range, :step, step))
-    end
  end

  defimpl Inspect do
@@ -180,7 +180,7 @@ defmodule DateTime do
  since v1.15.0.

  The default unit if none gets passed is `:native`,
-  which results on a default resolution of microseconds.
+  which results in a default resolution of microseconds.

  ## Examples

@@ -720,6 +720,9 @@ defmodule DateTime do
  Other time zone databases can be passed as argument or set globally.
  See the "Time zone database" section in the module docs.

+  Shifting to the `"Etc/UTC"` time zone always succeeds without
+  consulting the `time_zone_database`.
+
  ## Examples

      iex> {:ok, pacific_datetime} = DateTime.shift_zone(~U[2018-07-16 10:00:00Z], "America/Los_Angeles", FakeTimeZoneDatabase)
@@ -753,6 +756,28 @@ defmodule DateTime do
    |> shift_zone_for_iso_days_utc(calendar, precision, time_zone, time_zone_database)
  end

+  defp shift_zone_for_iso_days_utc(iso_days_utc, calendar, precision, "Etc/UTC", _time_zone_db) do
+    {year, month, day, hour, minute, second, {microsecond, _}} =
+      calendar.naive_datetime_from_iso_days(iso_days_utc)
+
+    datetime = %DateTime{
+      calendar: calendar,
+      year: year,
+      month: month,
+      day: day,
+      hour: hour,
+      minute: minute,
+      second: second,
+      microsecond: {microsecond, precision},
+      std_offset: 0,
+      utc_offset: 0,
+      zone_abbr: "UTC",
+      time_zone: "Etc/UTC"
+    }
+
+    {:ok, datetime}
+  end
+
  defp shift_zone_for_iso_days_utc(iso_days_utc, calendar, precision, time_zone, time_zone_db) do
    case time_zone_db.time_zone_period_from_utc_iso_days(iso_days_utc, time_zone) do
      {:ok, %{std_offset: std_offset, utc_offset: utc_offset, zone_abbr: zone_abbr}} ->
@@ -1177,7 +1202,7 @@ defmodule DateTime do
    datetime
    |> to_iso_days()
    # Subtract total original offset in order to get UTC and add the new offset
-    |> Calendar.ISO.add_day_fraction_to_iso_days(offset - total_offset, 86400)
+    |> Calendar.ISO.add_time_unit_to_iso_days(offset - total_offset, :second)
    |> calendar.naive_datetime_from_iso_days()
  end

@@ -1296,9 +1321,9 @@ defmodule DateTime do
  end

  @doc """
-  Converts a number of gregorian seconds to a `DateTime` struct.
+  Converts a number of Gregorian seconds to a `DateTime` struct.

-  The returned `DateTime` will have `UTC` timezone, if you want other timezone, please use
+  The returned `DateTime` will have `UTC` timezone, if you want another timezone, please use
  `DateTime.shift_zone/3`.

  ## Examples
@@ -1341,7 +1366,7 @@ defmodule DateTime do
  end

  @doc """
-  Converts a `DateTime` struct to a number of gregorian seconds and microseconds.
+  Converts a `DateTime` struct to a number of Gregorian seconds and microseconds.

  ## Examples

@@ -1576,15 +1601,15 @@ defmodule DateTime do
  def diff(datetime1, datetime2, unit \\ :second)

  def diff(datetime1, datetime2, :day) do
-    diff(datetime1, datetime2, :second) |> div(86400)
+    diff(datetime1, datetime2, :microsecond) |> div(86_400_000_000)
  end

  def diff(datetime1, datetime2, :hour) do
-    diff(datetime1, datetime2, :second) |> div(3600)
+    diff(datetime1, datetime2, :microsecond) |> div(3_600_000_000)
  end

  def diff(datetime1, datetime2, :minute) do
-    diff(datetime1, datetime2, :second) |> div(60)
+    diff(datetime1, datetime2, :microsecond) |> div(60_000_000)
  end

  def diff(
@@ -1626,6 +1651,9 @@ defmodule DateTime do
  all converted to microseconds. Negative values will move backwards
  in time and the default precision is `:second`.

+  If the datetime is in the `"Etc/UTC"` time zone, this function
+  always succeeds without consulting the `time_zone_database`.
+
  This function relies on a contiguous representation of time,
  ignoring timezone changes. For example, if you add one day when there
  are summer time/daylight saving time changes, it will also change the
@@ -1744,6 +1772,9 @@ defmodule DateTime do

  Allowed units are: `:year`, `:month`, `:week`, `:day`, `:hour`, `:minute`, `:second`, `:microsecond`.

+  If the datetime is in the `"Etc/UTC"` time zone, this function
+  always succeeds without consulting the `time_zone_database`.
+
  This operation is equivalent to shifting the datetime wall clock
  (in other words, the value as someone in that timezone would see
  on their watch), then applying the time zone offset to convert it
@@ -1812,44 +1843,6 @@ defmodule DateTime do
  @spec shift(Calendar.datetime(), Duration.duration(), Calendar.time_zone_database()) :: t
  def shift(datetime, duration, time_zone_database \\ Calendar.get_time_zone_database())

-  def shift(%{calendar: calendar, time_zone: "Etc/UTC"} = datetime, duration, _time_zone_database) do
-    %{
-      year: year,
-      month: month,
-      day: day,
-      hour: hour,
-      minute: minute,
-      second: second,
-      microsecond: microsecond
-    } = datetime
-
-    {year, month, day, hour, minute, second, microsecond} =
-      calendar.shift_naive_datetime(
-        year,
-        month,
-        day,
-        hour,
-        minute,
-        second,
-        microsecond,
-        __duration__!(duration)
-      )
-
-    %DateTime{
-      year: year,
-      month: month,
-      day: day,
-      hour: hour,
-      minute: minute,
-      second: second,
-      microsecond: microsecond,
-      time_zone: "Etc/UTC",
-      zone_abbr: "UTC",
-      std_offset: 0,
-      utc_offset: 0
-    }
-  end
-
  def shift(%{calendar: calendar} = datetime, duration, time_zone_database) do
    %{
      year: year,
@@ -2060,11 +2053,12 @@ defmodule DateTime do
  end

  defp apply_tz_offset(iso_days, offset) do
-    Calendar.ISO.add_day_fraction_to_iso_days(iso_days, -offset, 86400)
+    Calendar.ISO.add_time_unit_to_iso_days(iso_days, -offset, :second)
  end

  defp from_map(%{} = datetime_map) do
    %DateTime{
+      calendar: datetime_map.calendar,
      year: datetime_map.year,
      month: datetime_map.month,
      day: datetime_map.day,
@@ -72,7 +72,7 @@ defmodule Duration do

  However, once again, it is important to remember that shifting a duration is not
  arithmetic, so you may want to use the functions in this module depending on what
-  you to achieve. Compare the results of both examples below:
+  you want to achieve. Compare the results of both examples below:

      # Adding one month after the other
      iex> date = ~D[2016-01-31]
@@ -129,6 +129,16 @@ defmodule Duration do
            second: 0,
            microsecond: {0, 0}

+  @typedoc """
+  The microsecond component of a duration.
+
+  Unlike `t:Calendar.microsecond/0`, the value may be negative, as
+  durations may represent negative amounts of time. The precision is
+  an integer from 0 to 6 holding the number of significant digits,
+  as in the calendar types.
+  """
+  @type microsecond :: {value :: integer, precision :: 0..6}
+
  @typedoc """
  The duration struct type.
  """
@@ -140,7 +150,7 @@ defmodule Duration do
          hour: integer,
          minute: integer,
          second: integer,
-          microsecond: Calendar.microsecond()
+          microsecond: microsecond()
        }

  @typedoc """
@@ -154,7 +164,7 @@ defmodule Duration do
          | {:hour, integer}
          | {:minute, integer}
          | {:second, integer}
-          | {:microsecond, Calendar.microsecond()}
+          | {:microsecond, microsecond()}

  @typedoc """
  The duration type specifies a `%Duration{}` struct or a keyword list of valid duration unit pairs.
@@ -243,19 +253,19 @@ defmodule Duration do

  """
  @spec add(t, t) :: t
-  def add(%Duration{} = d1, %Duration{} = d2) do
-    {m1, p1} = d1.microsecond
-    {m2, p2} = d2.microsecond
+  def add(%Duration{microsecond: {ms1, p1}} = d1, %Duration{microsecond: {ms2, p2}} = d2) do
+    %{year: y1, month: mo1, week: w1, day: day1, hour: h1, minute: mi1, second: s1} = d1
+    %{year: y2, month: mo2, week: w2, day: day2, hour: h2, minute: mi2, second: s2} = d2

    %Duration{
-      year: d1.year + d2.year,
-      month: d1.month + d2.month,
-      week: d1.week + d2.week,
-      day: d1.day + d2.day,
-      hour: d1.hour + d2.hour,
-      minute: d1.minute + d2.minute,
-      second: d1.second + d2.second,
-      microsecond: {m1 + m2, max(p1, p2)}
+      year: y1 + y2,
+      month: mo1 + mo2,
+      week: w1 + w2,
+      day: day1 + day2,
+      hour: h1 + h2,
+      minute: mi1 + mi2,
+      second: s1 + s2,
+      microsecond: {ms1 + ms2, max(p1, p2)}
    }
  end

@@ -273,19 +283,19 @@ defmodule Duration do

  """
  @spec subtract(t, t) :: t
-  def subtract(%Duration{} = d1, %Duration{} = d2) do
-    {m1, p1} = d1.microsecond
-    {m2, p2} = d2.microsecond
+  def subtract(%Duration{microsecond: {ms1, p1}} = d1, %Duration{microsecond: {ms2, p2}} = d2) do
+    %{year: y1, month: mo1, week: w1, day: day1, hour: h1, minute: mi1, second: s1} = d1
+    %{year: y2, month: mo2, week: w2, day: day2, hour: h2, minute: mi2, second: s2} = d2

    %Duration{
-      year: d1.year - d2.year,
-      month: d1.month - d2.month,
-      week: d1.week - d2.week,
-      day: d1.day - d2.day,
-      hour: d1.hour - d2.hour,
-      minute: d1.minute - d2.minute,
-      second: d1.second - d2.second,
-      microsecond: {m1 - m2, max(p1, p2)}
+      year: y1 - y2,
+      month: mo1 - mo2,
+      week: w1 - w2,
+      day: day1 - day2,
+      hour: h1 - h2,
+      minute: mi1 - mi2,
+      second: s1 - s2,
+      microsecond: {ms1 - ms2, max(p1, p2)}
    }
  end

@@ -302,14 +312,16 @@ defmodule Duration do
  """
  @spec multiply(t, integer) :: t
  def multiply(%Duration{microsecond: {ms, p}} = duration, integer) when is_integer(integer) do
+    %{year: y, month: mo, week: w, day: d, hour: h, minute: mi, second: s} = duration
+
    %Duration{
-      year: duration.year * integer,
-      month: duration.month * integer,
-      week: duration.week * integer,
-      day: duration.day * integer,
-      hour: duration.hour * integer,
-      minute: duration.minute * integer,
-      second: duration.second * integer,
+      year: y * integer,
+      month: mo * integer,
+      week: w * integer,
+      day: d * integer,
+      hour: h * integer,
+      minute: mi * integer,
+      second: s * integer,
      microsecond: {ms * integer, p}
    }
  end
@@ -327,14 +339,16 @@ defmodule Duration do
  """
  @spec negate(t) :: t
  def negate(%Duration{microsecond: {ms, p}} = duration) do
+    %{year: y, month: mo, week: w, day: d, hour: h, minute: mi, second: s} = duration
+
    %Duration{
-      year: -duration.year,
-      month: -duration.month,
-      week: -duration.week,
-      day: -duration.day,
-      hour: -duration.hour,
-      minute: -duration.minute,
-      second: -duration.second,
+      year: -y,
+      month: -mo,
+      week: -w,
+      day: -d,
+      hour: -h,
+      minute: -mi,
+      second: -s,
      microsecond: {-ms, p}
    }
  end
@@ -33,8 +33,9 @@ defmodule Calendar.ISO do
  you to format datetimes however else you desire.

  Elixir does not support reduced accuracy formats (for example, a date without
-  the day component) nor decimal precisions in the lowest component (such as
-  `10:01:25,5`).
+  the day component) nor decimal precisions in components other than seconds
+  (such as `10:01,5`). Fractional seconds are supported and truncated to
+  microsecond precision.

  #### Examples

@@ -81,6 +82,11 @@ defmodule Calendar.ISO do
      iex> Calendar.ISO.parse_time("23")
      {:error, :invalid_format}

+  A decimal fraction is accepted on seconds, but not on any other component:
+
+      iex> Calendar.ISO.parse_time("10:01:25,5")
+      {:ok, {10, 1, 25, {500000, 1}}}
+
  ### Extensions

  The parser and formatter adopt one ISO 8601 extension: extended year notation.
@@ -417,16 +423,13 @@ defmodule Calendar.ISO do
  @spec parse_date(String.t(), format) ::
          {:ok, {year, month, day}}
          | {:error, atom}
-  def parse_date(string, format) when is_binary(string) and is_format(format),
-    do: parse_date_guarded(string, format)
-
-  defp parse_date_guarded("-" <> string, format),
+  def parse_date("-" <> string, format) when is_format(format),
    do: do_parse_date(string, -1, format)

-  defp parse_date_guarded("+" <> string, format),
+  def parse_date("+" <> string, format) when is_format(format),
    do: do_parse_date(string, 1, format)

-  defp parse_date_guarded(string, format),
+  def parse_date(string, format) when is_binary(string) and is_format(format),
    do: do_parse_date(string, 1, format)

  defp do_parse_date(unquote(match_basic_date), multiplier, :basic) when unquote(guard_date) do
@@ -502,16 +505,13 @@ defmodule Calendar.ISO do
  @spec parse_naive_datetime(String.t(), format) ::
          {:ok, {year, month, day, hour, minute, second, microsecond}}
          | {:error, atom}
-  def parse_naive_datetime(string, format) when is_binary(string) and is_format(format),
-    do: parse_naive_datetime_guarded(string, format)
-
-  defp parse_naive_datetime_guarded("-" <> string, format),
+  def parse_naive_datetime("-" <> string, format) when is_format(format),
    do: do_parse_naive_datetime(string, -1, format)

-  defp parse_naive_datetime_guarded("+" <> string, format),
+  def parse_naive_datetime("+" <> string, format) when is_format(format),
    do: do_parse_naive_datetime(string, 1, format)

-  defp parse_naive_datetime_guarded(string, format),
+  def parse_naive_datetime(string, format) when is_binary(string) and is_format(format),
    do: do_parse_naive_datetime(string, 1, format)

  defp do_parse_naive_datetime(
@@ -606,16 +606,13 @@ defmodule Calendar.ISO do
  @spec parse_utc_datetime(String.t(), format) ::
          {:ok, {year, month, day, hour, minute, second, microsecond}, utc_offset}
          | {:error, atom}
-  def parse_utc_datetime(string, format) when is_binary(string) and is_format(format),
-    do: parse_utc_datetime_guarded(string, format)
-
-  defp parse_utc_datetime_guarded("-" <> string, format),
+  def parse_utc_datetime("-" <> string, format) when is_format(format),
    do: do_parse_utc_datetime(string, -1, format)

-  defp parse_utc_datetime_guarded("+" <> string, format),
+  def parse_utc_datetime("+" <> string, format) when is_format(format),
    do: do_parse_utc_datetime(string, 1, format)

-  defp parse_utc_datetime_guarded(string, format),
+  def parse_utc_datetime(string, format) when is_binary(string) and is_format(format),
    do: do_parse_utc_datetime(string, 1, format)

  defp do_parse_utc_datetime(
@@ -666,12 +663,12 @@ defmodule Calendar.ISO do
          day_fraction = time_to_day_fraction(hour, minute, second, {0, 0})

          {{year, month, day}, {hour, minute, second, _}} =
-            case add_day_fraction_to_iso_days({0, day_fraction}, -offset, 86_400) do
+            case add_time_unit_to_iso_days({0, day_fraction}, -offset, :second) do
              {0, day_fraction} ->
                {{year, month, day}, time_from_day_fraction(day_fraction)}

              {extra_days, day_fraction} ->
-                base_days = date_to_iso_days(year, month, day)
+                base_days = valid_date_to_iso_days(year, month, day)
                {date_from_iso_days(base_days + extra_days), time_from_day_fraction(day_fraction)}
            end

@@ -683,75 +680,73 @@ defmodule Calendar.ISO do
  end

  @doc """
-  Parses an ISO 8601 formatted duration string to a list of `Duration` compabitble unit pairs.
+  Parses an ISO 8601 formatted duration string to a list of `Duration` compatible unit pairs.

  See `Duration.from_iso8601/1`.
  """
  @doc since: "1.17.0"
  @spec parse_duration(String.t()) :: {:ok, [Duration.unit_pair()]} | {:error, atom}
  def parse_duration("P" <> string) when byte_size(string) > 0 do
-    parse_duration_date(string, [], year: ?Y, month: ?M, week: ?W, day: ?D)
+    parse_duration_date(string, 1, [], 0)
  end

  def parse_duration("+P" <> string) when byte_size(string) > 0 do
-    parse_duration_date(string, [], year: ?Y, month: ?M, week: ?W, day: ?D)
+    parse_duration_date(string, 1, [], 0)
  end

  def parse_duration("-P" <> string) when byte_size(string) > 0 do
-    with {:ok, fields} <- parse_duration_date(string, [], year: ?Y, month: ?M, week: ?W, day: ?D) do
-      {:ok,
-       Enum.map(fields, fn
-         {:microsecond, {value, precision}} -> {:microsecond, {-value, precision}}
-         {unit, value} -> {unit, -value}
-       end)}
-    end
+    parse_duration_date(string, -1, [], 0)
  end

  def parse_duration(_) do
    {:error, :invalid_duration}
  end

-  defp parse_duration_date("", acc, _allowed), do: {:ok, acc}
+  defp parse_duration_date("", _sign, acc, _min_position), do: {:ok, acc}

-  defp parse_duration_date("T" <> string, acc, _allowed) when byte_size(string) > 0 do
-    parse_duration_time(string, acc, hour: ?H, minute: ?M, second: ?S)
+  defp parse_duration_date("T" <> string, sign, acc, _min_position)
+       when byte_size(string) > 0 do
+    parse_duration_time(string, sign, acc, 0)
  end

-  defp parse_duration_date(string, acc, allowed) do
-    with {integer, <<next, rest::binary>>} <- Integer.parse(string),
-         {key, allowed} <- find_unit(allowed, next) do
-      parse_duration_date(rest, [{key, integer} | acc], allowed)
+  defp parse_duration_date(string, sign, acc, min_position) do
+    with {integer, <<unit, rest::binary>>} <- Integer.parse(string),
+         {key, next_min_position} <- find_date_unit(min_position, unit) do
+      parse_duration_date(rest, sign, [{key, integer * sign} | acc], next_min_position)
    else
      _ -> {:error, :invalid_date_component}
    end
  end

-  defp parse_duration_time("", acc, _allowed), do: {:ok, acc}
+  defp parse_duration_time("", _sign, acc, _min_position), do: {:ok, acc}

-  defp parse_duration_time(string, acc, allowed) do
+  defp parse_duration_time(string, sign, acc, min_position) do
    case Integer.parse(string) do
      {second, <<delimiter, _::binary>> = rest} when delimiter in [?., ?,] ->
-        case parse_microsecond(rest) do
-          {{ms, precision}, "S"} ->
-            ms =
-              case string do
-                "-" <> _ ->
-                  -ms
+        with {:second, _next_min_position} <- find_time_unit(min_position, ?S),
+             {{ms, precision}, "S"} <- parse_microsecond(rest) do
+          ms =
+            case string do
+              "-" <> _ ->
+                -ms

-                _ ->
-                  ms
-              end
-
-            {:ok, [second: second, microsecond: {ms, precision}] ++ acc}
+              _ ->
+                ms
+            end

+          {:ok, [second: second * sign, microsecond: {ms * sign, precision}] ++ acc}
+        else
          _ ->
            {:error, :invalid_time_component}
        end

-      {integer, <<next, rest::binary>>} ->
-        case find_unit(allowed, next) do
-          {key, allowed} -> parse_duration_time(rest, [{key, integer} | acc], allowed)
-          false -> {:error, :invalid_time_component}
+      {integer, <<unit, rest::binary>>} ->
+        case find_time_unit(min_position, unit) do
+          {key, next_min_position} ->
+            parse_duration_time(rest, sign, [{key, integer * sign} | acc], next_min_position)
+
+          false ->
+            {:error, :invalid_time_component}
        end

      _ ->
@@ -759,9 +754,17 @@ defmodule Calendar.ISO do
    end
  end

-  defp find_unit([{key, unit} | rest], unit), do: {key, rest}
-  defp find_unit([_ | rest], unit), do: find_unit(rest, unit)
-  defp find_unit([], _unit), do: false
+  # The minimum position is the earliest unit still allowed, or one past the end.
+  defp find_date_unit(min_position, ?Y) when min_position <= 0, do: {:year, 1}
+  defp find_date_unit(min_position, ?M) when min_position <= 1, do: {:month, 2}
+  defp find_date_unit(min_position, ?W) when min_position <= 2, do: {:week, 3}
+  defp find_date_unit(min_position, ?D) when min_position <= 3, do: {:day, 4}
+  defp find_date_unit(_min_position, _unit), do: false
+
+  defp find_time_unit(min_position, ?H) when min_position <= 0, do: {:hour, 1}
+  defp find_time_unit(min_position, ?M) when min_position <= 1, do: {:minute, 2}
+  defp find_time_unit(min_position, ?S) when min_position <= 2, do: {:second, 3}
+  defp find_time_unit(_min_position, _unit), do: false

  @doc """
  Returns the `t:Calendar.iso_days/0` format of the specified date.
@@ -892,12 +895,15 @@ defmodule Calendar.ISO do

  # Converts year, month, day to count of days since 0000-01-01.
  @doc false
-  def date_to_iso_days(0, 1, 1), do: 0
-  def date_to_iso_days(1970, 1, 1), do: @unix_epoch_days
-
  def date_to_iso_days(year, month, day) do
    ensure_day_in_month!(year, month, day)
+    valid_date_to_iso_days(year, month, day)
+  end

+  defp valid_date_to_iso_days(0, 1, 1), do: 0
+  defp valid_date_to_iso_days(1970, 1, 1), do: @unix_epoch_days
+
+  defp valid_date_to_iso_days(year, month, day) do
    y = if month <= 2, do: year - 1, else: year
    era = if y >= 0, do: div(y, @years_per_era), else: div(y - 399, @years_per_era)
    year_of_era = y - era * @years_per_era
@@ -1131,6 +1137,11 @@ defmodule Calendar.ISO do

  It is an integer from 1 to 4.

+  In the ISO calendar, the quarter is determined solely by the month, so the `year`
+  and `day` arguments are ignored. Combination of `year`, `month`, and `day` is not
+  validated as a valid date, unlike in `day_of_year/3`. Use `valid_date?/3` when
+  full date validation is required.
+
  ## Examples

      iex> Calendar.ISO.quarter_of_year(2016, 1, 31)
@@ -1173,7 +1184,7 @@ defmodule Calendar.ISO do
  @doc since: "1.8.0"
  @spec year_of_era(year) :: {1..10_000, era}
  def year_of_era(year) when is_year_CE(year), do: {year, 1}
-  def year_of_era(year) when is_year_BCE(year), do: {abs(year) + 1, 0}
+  def year_of_era(year) when is_year_BCE(year), do: {1 - year, 0}

  @doc """
  Calendar callback to compute the year and era from the
@@ -1226,7 +1237,7 @@ defmodule Calendar.ISO do
  end

  def day_of_era(year, month, day) when is_year_BCE(year) do
-    day = abs(date_to_iso_days(year, month, day) - @iso_epoch)
+    day = @iso_epoch - date_to_iso_days(year, month, day)
    {day, 0}
  end

@@ -1453,7 +1464,7 @@ defmodule Calendar.ISO do
  @doc """
  Converts the given naive_datetime into a iodata.

-  See `naive_datetime_to_iodata/8` for more information.
+  See `naive_datetime_to_string/8` for more information.

  ## Examples

@@ -1578,7 +1589,7 @@ defmodule Calendar.ISO do
  @doc """
  Converts the given datetime into a iodata.

-  See `datetime_to_iodata/12` for more information.
+  See `datetime_to_string/12` for more information.

  ## Examples

@@ -1682,7 +1693,7 @@ defmodule Calendar.ISO do
  end

  @doc """
-  Determines if the date given is valid according to the proleptic Gregorian calendar.
+  Determines if the time given is valid.

  Leap seconds are not supported by the built-in Calendar.ISO.

@@ -1702,7 +1713,7 @@ defmodule Calendar.ISO do
          boolean
  def valid_time?(hour, minute, second, {ms_value, ms_precision} = _microsecond)
      when is_integer(hour) and is_integer(minute) and is_integer(second) and is_integer(ms_value) and
-             is_integer(ms_value) do
+             is_integer(ms_precision) do
    is_hour(hour) and is_minute(minute) and is_second(second) and
      is_microsecond(ms_value, ms_precision)
  end
@@ -1835,6 +1846,9 @@ defmodule Calendar.ISO do
    shift_options = shift_datetime_options(duration)

    Enum.reduce(shift_options, {year, month, day, hour, minute, second, microsecond}, fn
+      {:microsecond, {0, _}}, naive_datetime ->
+        naive_datetime
+
      {_, 0}, naive_datetime ->
        naive_datetime

@@ -1864,6 +1878,9 @@ defmodule Calendar.ISO do
    shift_options = shift_time_options(duration)

    Enum.reduce(shift_options, {hour, minute, second, microsecond}, fn
+      {:microsecond, {0, _}}, time ->
+        time
+
      {_, 0}, time ->
        time

@@ -1875,7 +1892,7 @@ defmodule Calendar.ISO do
  @doc false
  def shift_days({year, month, day}, days) do
    {year, month, day} =
-      date_to_iso_days(year, month, day)
+      valid_date_to_iso_days(year, month, day)
      |> Kernel.+(days)
      |> date_from_iso_days()

@@ -1926,8 +1943,7 @@ defmodule Calendar.ISO do

  def shift_time_unit({_days, _day_fraction} = iso_days, value, unit)
      when unit in [:second, :millisecond, :microsecond, :nanosecond] or is_integer(unit) do
-    ppd = System.convert_time_unit(86_400, :second, unit)
-    add_day_fraction_to_iso_days(iso_days, value, ppd)
+    add_time_unit_to_iso_days(iso_days, value, unit)
  end

  defp shift_time_unit_values({0, _}, {_, original_precision}) do
@@ -2008,8 +2024,8 @@ defmodule Calendar.ISO do
    total = System.convert_time_unit(integer, unit, :microsecond)

    if total in @unix_range_microseconds do
-      microseconds = Integer.mod(total, @microseconds_per_second)
-      seconds = @unix_epoch + floor_div_positive_divisor(total, @microseconds_per_second)
+      {seconds, microseconds} = div_rem(total, @microseconds_per_second)
+      seconds = @unix_epoch + seconds
      precision = precision_for_unit(unit)
      {date, time} = iso_seconds_to_datetime(seconds)
      {:ok, date, time, {microseconds, precision}}
@@ -2030,38 +2046,27 @@ defmodule Calendar.ISO do
    end
  end

-  defp parse_microsecond("." <> rest) do
-    case parse_microsecond(rest, 0, []) do
-      {[], 0, _} ->
-        :error
+  defp parse_microsecond("." <> rest), do: parse_microsecond(rest, rest, 0)
+  defp parse_microsecond("," <> rest), do: parse_microsecond(rest, rest, 0)
+  defp parse_microsecond(rest), do: {{0, 0}, rest}

-      {microsecond, precision, rest} ->
-        scale = scale_factor(precision)
-        {{:erlang.list_to_integer(microsecond) * scale, precision}, rest}
-    end
-  end
+  # Digits past the sixth are consumed but do not contribute to the value.
+  defp parse_microsecond(<<head, tail::binary>>, digits, 6) when head in ?0..?9,
+    do: parse_microsecond(tail, digits, 6)

-  defp parse_microsecond("," <> rest) do
-    parse_microsecond("." <> rest)
-  end
+  defp parse_microsecond(<<head, tail::binary>>, digits, precision) when head in ?0..?9,
+    do: parse_microsecond(tail, digits, precision + 1)

-  defp parse_microsecond(rest) do
-    {{0, 0}, rest}
-  end
+  defp parse_microsecond(_rest, _digits, 0), do: :error

-  defp parse_microsecond(<<head, tail::binary>>, 6, acc) when head in ?0..?9,
-    do: parse_microsecond(tail, 6, acc)
-
-  defp parse_microsecond(<<head, tail::binary>>, precision, acc) when head in ?0..?9,
-    do: parse_microsecond(tail, precision + 1, [head | acc])
-
-  defp parse_microsecond(rest, precision, acc) do
-    {:lists.reverse(acc), precision, rest}
+  defp parse_microsecond(rest, digits, precision) do
+    scale = scale_factor(precision)
+    microsecond = :erlang.binary_to_integer(:binary.part(digits, 0, precision)) * scale
+    {{microsecond, precision}, rest}
  end

  defp parse_offset(""), do: {nil, ""}
  defp parse_offset("Z"), do: {0, ""}
-  defp parse_offset("-00:00"), do: :error

  defp parse_offset(<<?+, h1, h2, ?:, m1, m2, rest::binary>>),
    do: parse_offset(1, h1, h2, m1, m2, rest)
@@ -2084,7 +2089,8 @@ defmodule Calendar.ISO do
         true <- m1 in ?0..?5 and m2 in ?0..?9,
         hour = (h1 - ?0) * 10 + h2 - ?0,
         min = (m1 - ?0) * 10 + m2 - ?0,
-         true <- hour < 24 do
+         true <- hour < 24,
+         true <- sign == 1 or hour != 0 or min != 0 do
      {(hour * 60 + min) * 60 * sign, rest}
    else
      _ -> :error
@@ -2100,10 +2106,43 @@ defmodule Calendar.ISO do
  end

  @doc false
-  def iso_days_to_unit({days, {parts, ppd}}, unit) do
-    day_microseconds = days * @parts_per_day
-    microseconds = divide_by_parts_per_day(parts, ppd)
-    System.convert_time_unit(day_microseconds + microseconds, :microsecond, unit)
+  def iso_days_to_unit(iso_days, :second) do
+    floor_div_positive_divisor(iso_days_to_microseconds(iso_days), @microseconds_per_second)
+  end
+
+  def iso_days_to_unit(iso_days, :millisecond) do
+    floor_div_positive_divisor(iso_days_to_microseconds(iso_days), 1_000)
+  end
+
+  def iso_days_to_unit(iso_days, :microsecond) do
+    iso_days_to_microseconds(iso_days)
+  end
+
+  def iso_days_to_unit(iso_days, unit) do
+    System.convert_time_unit(iso_days_to_microseconds(iso_days), :microsecond, unit)
+  end
+
+  defp iso_days_to_microseconds({days, {parts, ppd}}) do
+    days * @parts_per_day + divide_by_parts_per_day(parts, ppd)
+  end
+
+  @doc false
+  def add_time_unit_to_iso_days(iso_days, add, :second) do
+    add_day_fraction_to_iso_days(iso_days, add * @microseconds_per_second, @parts_per_day)
+  end
+
+  def add_time_unit_to_iso_days(iso_days, add, :millisecond) do
+    add_day_fraction_to_iso_days(iso_days, add * 1_000, @parts_per_day)
+  end
+
+  def add_time_unit_to_iso_days(iso_days, add, :microsecond) do
+    add_day_fraction_to_iso_days(iso_days, add, @parts_per_day)
+  end
+
+  def add_time_unit_to_iso_days(iso_days, add, unit)
+      when unit == :nanosecond or is_integer(unit) do
+    ppd = System.convert_time_unit(@seconds_per_day, :second, unit)
+    add_day_fraction_to_iso_days(iso_days, add, ppd)
  end

  @doc false
@@ -164,7 +164,7 @@ defmodule NaiveDateTime do
  Returns the "local time" for the machine the Elixir program is running on.

  WARNING: This function can cause insidious bugs. It depends on the time zone
-  configuration at run time. This can changed and be set to a time zone that has
+  configuration at run time. This can change and be set to a time zone that has
  daylight saving jumps (spring forward or fall back).

  This function can be used to display what the time is right now for the time
@@ -541,15 +541,15 @@ defmodule NaiveDateTime do
  def diff(naive_datetime1, naive_datetime2, unit \\ :second)

  def diff(naive_datetime1, naive_datetime2, :day) do
-    diff(naive_datetime1, naive_datetime2, :second) |> div(86400)
+    diff(naive_datetime1, naive_datetime2, :microsecond) |> div(86_400_000_000)
  end

  def diff(naive_datetime1, naive_datetime2, :hour) do
-    diff(naive_datetime1, naive_datetime2, :second) |> div(3600)
+    diff(naive_datetime1, naive_datetime2, :microsecond) |> div(3_600_000_000)
  end

  def diff(naive_datetime1, naive_datetime2, :minute) do
-    diff(naive_datetime1, naive_datetime2, :second) |> div(60)
+    diff(naive_datetime1, naive_datetime2, :microsecond) |> div(60_000_000)
  end

  def diff(
@@ -570,9 +570,11 @@ defmodule NaiveDateTime do
            "unsupported time unit. Expected :day, :hour, :minute, :second, :millisecond, :microsecond, :nanosecond, or a positive integer, got #{inspect(unit)}"
    end

-    units1 = naive_datetime1 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(unit)
-    units2 = naive_datetime2 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(unit)
-    units1 - units2
+    diff_microsecond =
+      (naive_datetime1 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(:microsecond)) -
+        (naive_datetime2 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(:microsecond))
+
+    System.convert_time_unit(diff_microsecond, :microsecond, unit)
  end

  @doc """
@@ -672,7 +674,7 @@ defmodule NaiveDateTime do

  """
  @doc since: "1.6.0"
-  @spec truncate(t(), :microsecond | :millisecond | :second) :: t()
+  @spec truncate(Calendar.naive_datetime(), :microsecond | :millisecond | :second) :: t()
  def truncate(%NaiveDateTime{microsecond: microsecond} = naive_datetime, precision) do
    %{naive_datetime | microsecond: Calendar.truncate(microsecond, precision)}
  end
@@ -715,16 +717,18 @@ defmodule NaiveDateTime do

  """
  @spec to_date(Calendar.naive_datetime()) :: Date.t()
-  def to_date(%{
-        year: year,
-        month: month,
-        day: day,
-        calendar: calendar,
-        hour: _,
-        minute: _,
-        second: _,
-        microsecond: _
-      }) do
+  def to_date(
+        %{
+          year: year,
+          month: month,
+          day: day,
+          calendar: calendar,
+          hour: _,
+          minute: _,
+          second: _,
+          microsecond: _
+        } = _naive_datetime
+      ) do
    %Date{year: year, month: month, day: day, calendar: calendar}
  end

@@ -741,16 +745,18 @@ defmodule NaiveDateTime do

  """
  @spec to_time(Calendar.naive_datetime()) :: Time.t()
-  def to_time(%{
-        year: _,
-        month: _,
-        day: _,
-        calendar: calendar,
-        hour: hour,
-        minute: minute,
-        second: second,
-        microsecond: microsecond
-      }) do
+  def to_time(
+        %{
+          year: _,
+          month: _,
+          day: _,
+          calendar: calendar,
+          hour: hour,
+          minute: minute,
+          second: second,
+          microsecond: microsecond
+        } = _naive_datetime
+      ) do
    %Time{
      hour: hour,
      minute: minute,
@@ -1146,16 +1152,18 @@ defmodule NaiveDateTime do
  """
  @doc since: "1.11.0"
  @spec to_gregorian_seconds(Calendar.naive_datetime()) :: {integer(), non_neg_integer()}
-  def to_gregorian_seconds(%{
-        calendar: calendar,
-        year: year,
-        month: month,
-        day: day,
-        hour: hour,
-        minute: minute,
-        second: second,
-        microsecond: {microsecond, precision}
-      }) do
+  def to_gregorian_seconds(
+        %{
+          calendar: calendar,
+          year: year,
+          month: month,
+          day: day,
+          hour: hour,
+          minute: minute,
+          second: second,
+          microsecond: {microsecond, precision}
+        } = _naive_datetime
+      ) do
    {days, day_fraction} =
      calendar.naive_datetime_to_iso_days(
        year,
@@ -62,7 +62,7 @@ defmodule Time do

  You can pass a time unit to automatically truncate the resulting time.

-  The default unit if none gets passed is `:native` which results on a default resolution of microseconds.
+  The default unit if none gets passed is `:native` which results in a default resolution of microseconds.

  ## Examples

@@ -146,8 +146,9 @@ defmodule Time do
      iex> Time.new(23, 59, 59, 1_000_000)
      {:error, :invalid_time}

-      # Invalid precision
-      Time.new(23, 59, 59, {999_999, 10})
+  Invalid precision:
+
+      iex> Time.new(23, 59, 59, {999_999, 10})
      {:error, :invalid_time}

  """
@@ -466,8 +467,12 @@ defmodule Time do
          Calendar.microsecond(),
          Calendar.calendar()
        ) :: t
-  def from_seconds_after_midnight(seconds, microsecond \\ {0, 0}, calendar \\ Calendar.ISO)
-      when is_integer(seconds) do
+  def from_seconds_after_midnight(
+        seconds,
+        {microsecond, precision} \\ {0, 0},
+        calendar \\ Calendar.ISO
+      )
+      when is_integer(seconds) and microsecond in 0..999_999 and precision in 0..6 do
    seconds_in_day = Integer.mod(seconds, @seconds_per_day)

    {hour, minute, second, {_, _}} =
@@ -478,7 +483,7 @@ defmodule Time do
      hour: hour,
      minute: minute,
      second: second,
-      microsecond: microsecond
+      microsecond: {microsecond, precision}
    }
  end

@@ -496,7 +501,7 @@ defmodule Time do

  """
  @doc since: "1.11.0"
-  @spec to_seconds_after_midnight(Calendar.time()) :: {integer(), non_neg_integer()}
+  @spec to_seconds_after_midnight(Calendar.time()) :: {non_neg_integer(), non_neg_integer()}
  def to_seconds_after_midnight(%{microsecond: {microsecond, _precision}} = time) do
    iso_days = {0, to_day_fraction(time)}
    {Calendar.ISO.iso_days_to_unit(iso_days, :second), microsecond}
@@ -689,7 +694,7 @@ defmodule Time do
  @doc """
  Compares two time structs.

-  Returns `:gt` if first time is later than the second
+  Returns `:gt` if the first time is later than the second
  and `:lt` for vice versa. If the two times are equal
  `:eq` is returned.

@@ -715,14 +720,32 @@ defmodule Time do
  """
  @doc since: "1.4.0"
  @spec compare(Calendar.time(), Calendar.time()) :: :lt | :eq | :gt
-  def compare(%{calendar: calendar} = time1, %{calendar: calendar} = time2) do
-    %{hour: hour1, minute: minute1, second: second1, microsecond: {microsecond1, _}} = time1
-    %{hour: hour2, minute: minute2, second: second2, microsecond: {microsecond2, _}} = time2
-
-    case {{hour1, minute1, second1, microsecond1}, {hour2, minute2, second2, microsecond2}} do
-      {first, second} when first > second -> :gt
-      {first, second} when first < second -> :lt
-      _ -> :eq
+  def compare(
+        %{
+          hour: hour1,
+          minute: minute1,
+          second: second1,
+          microsecond: {microsecond1, _},
+          calendar: calendar
+        },
+        %{
+          hour: hour2,
+          minute: minute2,
+          second: second2,
+          microsecond: {microsecond2, _},
+          calendar: calendar
+        }
+      ) do
+    cond do
+      hour1 > hour2 -> :gt
+      hour1 < hour2 -> :lt
+      minute1 > minute2 -> :gt
+      minute1 < minute2 -> :lt
+      second1 > second2 -> :gt
+      second1 < second2 -> :lt
+      microsecond1 > microsecond2 -> :gt
+      microsecond1 < microsecond2 -> :lt
+      true -> :eq
    end
  end

@@ -901,11 +924,11 @@ defmodule Time do
  def diff(time1, time2, unit \\ :second)

  def diff(time1, time2, :hour) do
-    diff(time1, time2, :second) |> div(3600)
+    diff(time1, time2, :microsecond) |> div(3_600_000_000)
  end

  def diff(time1, time2, :minute) do
-    diff(time1, time2, :second) |> div(60)
+    diff(time1, time2, :microsecond) |> div(60_000_000)
  end

  def diff(
@@ -50,7 +50,7 @@ defmodule Code do

  You can use `ensure_loaded/1` (as well as `ensure_loaded?/1` and
  `ensure_loaded!/1`) to check if a module is loaded before using it and
-  act.
+  act accordingly.

  ## `ensure_compiled/1` and `ensure_compiled!/1`

@@ -258,6 +258,7 @@ defmodule Code do
          | {:locals_without_parens, keyword()}
          | {:force_do_end_blocks, boolean()}
          | {:migrate, boolean()}
+          | {:migrate_atom_interpolations, boolean()}
          | {:migrate_bitstring_modifiers, boolean()}
          | {:migrate_call_parens_on_pipe, boolean()}
          | {:migrate_charlists_as_sigils, boolean()}
@@ -272,6 +273,7 @@ defmodule Code do
          | {:escape, boolean()}
          | {:locals_without_parens, keyword()}
          | {:comments, [term()]}
+          | {:syntax_colors, [{Inspect.Opts.color_key(), IO.ANSI.ansidata()}]}

  @typedoc """
  Options for parsing functions that convert strings to quoted expressions.
@@ -285,20 +287,26 @@ defmodule Code do
          unescape: boolean(),
          existing_atoms_only: boolean(),
          token_metadata: boolean(),
-          literal_encoder: (term(), Macro.metadata() -> term()),
-          static_atoms_encoder: (atom() -> term()),
+          literal_encoder: (term(), Macro.metadata() -> {:ok, Macro.t()} | {:error, binary()}),
+          static_atoms_encoder: (binary(), Macro.metadata() -> {:ok, term()} | {:error, binary()}),
          emit_warnings: boolean()
        ]

  @typedoc """
-  Options for environment evaluation functions like eval_string/3 and eval_quoted/3.
+  Options for evaluation environment, accepted by `env_for_eval/1`.
  """
-  @type env_eval_opts :: [
-          file: binary(),
-          line: pos_integer(),
-          module: module(),
-          prune_binding: boolean()
-        ]
+  @type env_eval_opt ::
+          {:file, binary()}
+          | {:line, pos_integer()}
+          | {:module, module()}
+
+  @typedoc """
+  Options for evaluation functions like `eval_string/3`, `eval_quoted/3`
+  and `eval_quoted_with_env/4`.
+  """
+  @type eval_opt ::
+          {:prune_binding, boolean()}
+          | {:dbg_callback, {module(), atom(), list()}}

  @boolean_compiler_options [
    :docs,
@@ -308,7 +316,7 @@ defmodule Code do
    :relative_paths
  ]

-  @list_compiler_options [:tracers, :parser_options]
+  @list_compiler_options [:tracers, :parser_options, :erlc_options]

  @available_compiler_options @boolean_compiler_options ++
                                @list_compiler_options ++
@@ -407,7 +415,7 @@ defmodule Code do
      operations.

  """
-  @spec append_path(Path.t(), cache: boolean()) :: true | false
+  @spec append_path(Path.t(), cache: boolean()) :: boolean()
  def append_path(path, opts \\ []) do
    apply(:code, :add_pathz, [to_charlist(Path.expand(path)) | cache(opts)]) == true
  end
@@ -546,8 +554,7 @@ defmodule Code do
  This is the list of directories the Erlang VM uses for finding
  module code. The list of files is managed per Erlang VM node.

-  The path is expanded with `Path.expand/1` before being deleted. If the
-  path does not exist, this function returns `false`.
+  All paths are expanded with `Path.expand/1` before being deleted.
  """
  @doc since: "1.15.0"
  @spec delete_paths([Path.t()]) :: :ok
@@ -573,9 +580,11 @@ defmodule Code do

  ## Options

-  It accepts the same options as `env_for_eval/1`. Additionally, you may
-  also pass an environment as second argument, so the evaluation happens
-  within that environment.
+  It accepts the same options as both `env_for_eval/1` and
+  `eval_quoted_with_env/4`. Additionally, you may also pass an environment
+  as third argument, so the evaluation happens within that environment.
+
+  ## Return

  Returns a tuple of the form `{value, binding}`, where `value` is the value
  returned from evaluating `string`. If an error occurs while evaluating
@@ -605,7 +614,7 @@ defmodule Code do
      iex> Enum.sort(binding)
      [a: 3, b: 2]

-  For convenience, you can pass `__ENV__/0` as the `opts` argument and
+  For convenience, you can pass `__ENV__/0` as the `opts_or_env` argument and
  all imports, requires and aliases defined in the current environment
  will be automatically carried over:

@@ -617,15 +626,16 @@ defmodule Code do
      [a: 1, b: 2]

  """
-  @spec eval_string(List.Chars.t(), binding, Macro.Env.t() | env_eval_opts) :: {term, binding}
-  def eval_string(string, binding \\ [], opts \\ [])
+  @spec eval_string(List.Chars.t(), binding, Macro.Env.t() | [eval_opt | env_eval_opt]) ::
+          {term, binding}
+  def eval_string(string, binding \\ [], opts_or_env \\ [])

  def eval_string(string, binding, %Macro.Env{} = env) do
-    validated_eval_string(string, validate_binding(binding), env)
+    validated_eval_string(string, validate_binding(binding), env_for_eval(env), [])
  end

  def eval_string(string, binding, opts) when is_list(opts) do
-    validated_eval_string(string, validate_binding(binding), opts)
+    validated_eval_string(string, validate_binding(binding), env_for_eval(opts), opts)
  end

  defp validate_binding(binding) when is_list(binding), do: binding
@@ -634,10 +644,10 @@ defmodule Code do
    raise ArgumentError, "binding must be a list, got: #{inspect(binding)}"
  end

-  defp validated_eval_string(string, binding, opts_or_env) do
-    %{line: line, file: file} = env = env_for_eval(opts_or_env)
+  defp validated_eval_string(string, binding, env, opts) do
+    %{line: line, file: file} = env
    forms = :elixir.string_to_quoted!(to_charlist(string), line, 1, file, [])
-    {value, binding, _env} = eval_verify(:eval_forms, [forms, binding, env])
+    {value, binding, _env} = eval_verify(:eval_forms, [forms, binding, env, opts])
    {value, binding}
  end

@@ -758,6 +768,12 @@ defmodule Code do
    * `:migrate` (since v1.18.0) - when `true`, sets all other migration options
      to `true` by default. Defaults to `false`.

+    * `:migrate_atom_interpolations` (since v1.21.0) - when `true`, rewrites
+      deprecated atom interpolations to explicit calls to `String.to_unsafe_atom/1`.
+      For example, `:"foo_#{bar}"` becomes `String.to_unsafe_atom("foo_#{bar}")`.
+      Interpolated keywords like `["foo_#{bar}": 1]` are **not** migrated.
+      Defaults to the value of the `:migrate` option. This option changes the AST.
+
    * `:migrate_bitstring_modifiers` (since v1.18.0) - when `true`,
      removes unnecessary parentheses in known bitstring
      [modifiers](`<<>>/1`), for example `<<foo::binary()>>`
@@ -1140,7 +1156,8 @@ defmodule Code do
  returned quoted expressions (instead of evaluated).

  See `eval_string/3` for a description of arguments and return types.
-  The options are described under `env_for_eval/1`.
+  It accepts the same options as both `env_for_eval/1` and
+  `eval_quoted_with_env/4`.

  ## Examples

@@ -1162,11 +1179,20 @@ defmodule Code do
      [a: 1, b: 2]

  """
-  @spec eval_quoted(Macro.t(), binding, Macro.Env.t() | env_eval_opts) :: {term, binding}
-  def eval_quoted(quoted, binding \\ [], env_or_opts \\ []) do
-    {value, binding, _env} =
-      eval_verify(:eval_quoted, [quoted, binding, env_for_eval(env_or_opts)])
+  @spec eval_quoted(Macro.t(), binding, Macro.Env.t() | [eval_opt | env_eval_opt]) ::
+          {term, binding}
+  def eval_quoted(quoted, binding \\ [], env_or_opts \\ [])

+  def eval_quoted(quoted, binding, %Macro.Env{} = env) do
+    eval_quoted(quoted, validate_binding(binding), env_for_eval(env), [])
+  end
+
+  def eval_quoted(quoted, binding, opts) when is_list(opts) do
+    eval_quoted(quoted, validate_binding(binding), env_for_eval(opts), opts)
+  end
+
+  defp eval_quoted(quoted, binding, env, opts) do
+    {value, binding, _env} = eval_verify(:eval_quoted, [quoted, binding, env, opts])
    {value, binding}
  end

@@ -1194,14 +1220,9 @@ defmodule Code do

    * `:module` - the module to run the environment on

-    * `:prune_binding` - (since v1.14.2) prune binding to keep only
-      variables read or written by the evaluated code. Note that
-      variables used by modules are always pruned, even if later used
-      by the modules. You can submit to the `:on_module` tracer event
-      and access the variables used by the module from its environment.
  """
  @doc since: "1.14.0"
-  @spec env_for_eval(Macro.Env.t() | env_eval_opts) :: Macro.Env.t()
+  @spec env_for_eval(Macro.Env.t() | [env_eval_opt]) :: Macro.Env.t()
  def env_for_eval(env_or_opts), do: :elixir.env_for_eval(env_or_opts)

  @doc """
@@ -1215,11 +1236,19 @@ defmodule Code do

  ## Options

-  It accepts the same options as `env_for_eval/1`.
+    * `:prune_binding` - (since v1.14.2) prune binding to keep only
+      variables read or written by the evaluated code. Note that
+      variables used by modules are always pruned, even if later used
+      by the modules. You can submit to the `:on_module` tracer event
+      and access the variables used by the module from its environment.
+
+    * `:dbg_callback` - (since v1.20.0) overrides the behaviour of `dbg/2`
+      used in the evaluated code. It must be a `{module, function, args}`
+      tuple, see `dbg/2` for more details.

  """
  @doc since: "1.14.0"
-  @spec eval_quoted_with_env(Macro.t(), binding, Macro.Env.t(), env_eval_opts) ::
+  @spec eval_quoted_with_env(Macro.t(), binding, Macro.Env.t(), [eval_opt]) ::
          {term, binding, Macro.Env.t()}
  def eval_quoted_with_env(quoted, binding, %Macro.Env{} = env, opts \\ [])
      when is_list(binding) do
@@ -1269,9 +1298,9 @@ defmodule Code do
    * `:literal_encoder` (since v1.10.0) - how to encode literals in the AST.
      It must be a function that receives two arguments, the literal and its
      metadata, and it must return `{:ok, ast :: Macro.t}` or
-      `{:error, reason :: binary}`. If you return anything than the literal
+      `{:error, reason :: binary}`. If you return anything other than the literal
      itself as the `term`, then the AST is no longer valid. This option
-      may still useful for textual analysis of the source code.
+      may still be useful for textual analysis of the source code.

    * `:static_atoms_encoder` - the static atom encoder function, see
      "The `:static_atoms_encoder` function" section below. Note this
@@ -1297,7 +1326,7 @@ defmodule Code do
  and keyword lists.

  The encoder function will receive the atom name (as a binary) and a
-  keyword list with the current file, line and column. It must return
+  keyword list with the current line and column. It must return
  `{:ok, token :: term} | {:error, reason :: binary}`.

  The encoder function is supposed to create an atom from the given
@@ -1365,12 +1394,15 @@ defmodule Code do
  while preserving information like comments and literals position.

  Returns `{:ok, quoted_form, comments}` if it succeeds,
-  `{:error, {line, error, token}}` otherwise.
+  `{:error, {location, error, token}}` otherwise, where `location`
+  is keyword metadata containing the line and column of the error.

  Comments are maps with the following fields:

    * `:line` - The line number of the source code

+    * `:column` - The column number of the source code
+
    * `:text` - The full text of the comment, including the leading `#`

    * `:previous_eol_count` - How many end of lines there are between the comment and the previous AST node or comment
@@ -1425,7 +1457,9 @@ defmodule Code do

  Returns the AST and a list of comments if it succeeds, raises an exception
  otherwise. The exception is a `TokenMissingError` in case a token is missing
-  (usually because the expression is incomplete), `SyntaxError` otherwise.
+  (usually because the expression is incomplete), `MismatchedDelimiterError`
+  (in case of mismatched opening and closing delimiters) and `SyntaxError`
+  otherwise.

  Check `string_to_quoted/2` for options information.
  """
@@ -1528,9 +1562,13 @@ defmodule Code do
      `string_to_quoted/2`, setting this option to `false` will prevent it from
      escaping the sequences twice. Defaults to `true`.

+    * `:syntax_colors` - a keyword list of colors the output is colorized.
+      See `Inspect.Opts` for more information.
+
  See `format_string!/2` for the full list of formatting options including
-  `:file`, `:line`, `:line_length`, `:locals_without_parens`, `:force_do_end_blocks`,
-  `:syntax_colors`, and all migration options like `:migrate_charlists_as_sigils`.
+  `:file`, `:line`, `:locals_without_parens`, `:force_do_end_blocks`, and all
+  migration options like `:migrate_charlists_as_sigils`. Note `:line_length`
+  does not apply here.
  """
  @doc since: "1.13.0"
  @spec quoted_to_algebra(Macro.t(), [format_opt() | quoted_to_algebra_opt()]) ::
@@ -1612,13 +1650,19 @@ defmodule Code do
        nil

      :proceed ->
-        loaded =
-          Module.ParallelChecker.verify(fn ->
-            :elixir_compiler.string(charlist, file, fn _, _ -> :ok end)
-          end)
+        try do
+          loaded =
+            Module.ParallelChecker.verify(fn ->
+              :elixir_compiler.string(charlist, file, fn _, _ -> :ok end)
+            end)

-        :elixir_code_server.cast({:required, file})
-        loaded
+          :elixir_code_server.cast({:required, file})
+          loaded
+        catch
+          kind, reason ->
+            :elixir_code_server.call({:release, file})
+            :erlang.raise(kind, reason, __STACKTRACE__)
+        end
    end
  end

@@ -1644,7 +1688,7 @@ defmodule Code do
  @doc """
  Stores all given compilation options.

-  Changing the compilation options affect all processes
+  Changing the compilation options affects all processes
  running in a given Erlang VM node. To store individual
  options and for a description of all options, see
  `put_compiler_option/2`.
@@ -1708,7 +1752,7 @@ defmodule Code do
  @doc """
  Stores a compilation option.

-  Changing the compilation options affect all processes running in a
+  Changing the compilation options affects all processes running in a
  given Erlang VM node.

  Available options are:
@@ -1724,13 +1768,17 @@ defmodule Code do
      remove the `:debug_info` while deploying, tools like `mix release`
      already do such by default.

-      Other environments, such as `mix test`, automatically disables this
+      Other environments, such as `mix test`, automatically disable this
      via the `:test_elixirc_options` project configuration, as there is
      typically no need to store debug chunks for test files.

    * `:docs` - when `true`, retains documentation in the compiled module.
      Defaults to `true`.

+    * `:erlc_options` (since v1.21.0) - a list of Erlang compiler options. For example,
+      `erlc_options: [:beam_debug_info, :beam_debug_stack]` emits Erlang/OTP
+      debug metadata for BEAM debuggers. Defaults to `[]`.
+
    * `:ignore_already_consolidated` (since v1.10.0) - when `true`, does not warn
      when a protocol has already been consolidated and a new implementation is added.
      Defaults to `false`.
@@ -1738,16 +1786,13 @@ defmodule Code do
    * `:ignore_module_conflict` - when `true`, does not warn when a module has
      already been defined. Defaults to `false`.

-    * `:infer_signatures` (since v1.18.0) - a list of applications of which modules
-      should be using during type inference. When `false`, it disables module-local
+    * `:infer_signatures` (since v1.18.0) - a list of applications whose modules
+      should be used during type inference. When `false`, it disables module-local
      signature inference used when type checking remote calls to the compiled
      module. Type checking will be executed regardless of the value of this option.
-      Defaults to `true`, which is equivalent to setting it to `[:elixir]` only.
-
-      When setting this option, we recommend running `mix clean` so the modules can be
-      recompiled with the new behaviour. `mix test` automatically disables this option
-      via the `:test_elixirc_options` project configuration, as there is typically no
-      need to infer signatures for test files.
+      Mix projects will set this option to your dependencies list in dev/prod, and
+      it will disable this option during test (as there is typically no need to infer
+      signatures for test files). Outside of Mix projects, it defaults to `[:elixir]`.

    * `:module_definition` (since v1.20.0) - stores if the module definition should
      be `:compiled` (the default) or `:interpreted`. Note this does not affect the
@@ -1770,7 +1815,7 @@ defmodule Code do

    * `:on_undefined_variable` (since v1.15.0) - either `:raise` or `:warn`.
      When `:raise` (the default), undefined variables will trigger a compilation
-      error. You may be set it to `:warn` if you want undefined variables to
+      error. You may set it to `:warn` if you want undefined variables to
      emit a warning and expand as to a local call to the zero-arity function
      of the same name (for example, `node` would be expanded as `node()`).
      This `:warn` behavior only exists for compatibility reasons when working
@@ -2105,7 +2150,7 @@ defmodule Code do
  If the module being checked is currently in a compiler deadlock,
  this function returns `{:error, :unavailable}`. Unavailable doesn't
  necessarily mean the module doesn't exist, just that it is not currently
-  available, but it (or may not) become available in the future.
+  available, but it may (or may not) become available in the future.

  Therefore, if you can only continue if the module is available, use
  `ensure_compiled!/1` instead. In particular, do not do this:
@@ -195,6 +195,7 @@ defmodule Code.Formatter do
    file = Keyword.get(opts, :file, nil)
    sigils = Keyword.get(opts, :sigils, [])
    migrate = Keyword.get(opts, :migrate, false)
+    migrate_atom_interpolations = Keyword.get(opts, :migrate_atom_interpolations, migrate)
    migrate_bitstring_modifiers = Keyword.get(opts, :migrate_bitstring_modifiers, migrate)
    migrate_call_parens_on_pipe = Keyword.get(opts, :migrate_call_parens_on_pipe, migrate)
    migrate_charlists_as_sigils = Keyword.get(opts, :migrate_charlists_as_sigils, migrate)
@@ -223,6 +224,7 @@ defmodule Code.Formatter do
      comments: comments,
      sigils: sigils,
      file: file,
+      migrate_atom_interpolations: migrate_atom_interpolations,
      migrate_bitstring_modifiers: migrate_bitstring_modifiers,
      migrate_call_parens_on_pipe: migrate_call_parens_on_pipe,
      migrate_charlists_as_sigils: migrate_charlists_as_sigils,
@@ -334,14 +336,20 @@ defmodule Code.Formatter do
  end

  defp quoted_to_algebra(
-         {{:., _, [:erlang, :binary_to_atom]}, _, [{:<<>>, _, entries}, :utf8]} = quoted,
+         {{:., _, [:erlang, :binary_to_atom]}, _, [{:<<>>, _, entries} = bitstring, :utf8]} =
+           quoted,
         context,
         state
       ) do
-    if interpolated?(entries) do
-      interpolation_to_algebra(entries, @double_quote, state, ":\"", @double_quote)
-    else
-      remote_to_algebra(quoted, context, state)
+    cond do
+      not interpolated?(entries) ->
+        remote_to_algebra(quoted, context, state)
+
+      state.migrate_atom_interpolations ->
+        quoted_to_algebra(quote(do: String.to_unsafe_atom(unquote(bitstring))), context, state)
+
+      true ->
+        interpolation_to_algebra(entries, @double_quote, state, ":\"", @double_quote)
    end
  end

@@ -1457,7 +1465,7 @@ defmodule Code.Formatter do
            metadata = [
              file: state.file,
              line: meta[:line],
-              sigil: String.to_atom(name),
+              sigil: String.to_unsafe_atom(name),
              modifiers: modifiers,
              opening_delimiter: opening_delimiter
            ]
@@ -1696,7 +1704,7 @@ defmodule Code.Formatter do
    iodata |> IO.iodata_to_binary() |> string() |> color_doc(:atom, inspect_opts)
  end

-  defp integer_to_algebra(text, inspect_otps) do
+  defp integer_to_algebra(text, inspect_opts) do
    case text do
      <<?0, ?x, rest::binary>> ->
        "0x" <> String.upcase(rest)
@@ -1710,15 +1718,15 @@ defmodule Code.Formatter do
      decimal ->
        insert_underscores(decimal)
    end
-    |> color_doc(:number, inspect_otps)
+    |> color_doc(:number, inspect_opts)
  end

-  defp float_to_algebra(text, inspect_otps) do
+  defp float_to_algebra(text, inspect_opts) do
    [int_part, decimal_part] = :binary.split(text, ".")
    decimal_part = String.downcase(decimal_part)

    string = insert_underscores(int_part) <> "." <> decimal_part
-    color_doc(string, :number, inspect_otps)
+    color_doc(string, :number, inspect_opts)
  end

  defp insert_underscores("-" <> digits) do
@@ -27,7 +27,8 @@ defmodule Code.Fragment do
          column: pos_integer(),
          columns: boolean(),
          token_metadata: boolean(),
-          literal_encoder: (term(), Macro.metadata() -> term()),
+          literal_encoder: (term(), Macro.metadata() -> {:ok, Macro.t()} | {:error, binary()}),
+          preserve_sigils: boolean(),
          trailing_fragment: String.t()
        ]

@@ -35,7 +36,7 @@ defmodule Code.Fragment do
  Returns the list of lines in the given string, preserving their line endings.

  Only the line endings recognized by the Elixir compiler are
-  considered, namely `\r\n` and `\n`. If you would like the retrieve
+  considered, namely `\r\n` and `\n`. If you would like to retrieve
  lines without their line endings, use `String.split(string, ["\r\n", "\n"])`.

  ## Examples
@@ -51,6 +52,7 @@ defmodule Code.Fragment do

  """
  @doc since: "1.19.0"
+  @spec lines(String.t()) :: [String.t()]
  def lines(string) do
    lines(string, <<>>)
  end
@@ -140,6 +142,9 @@ defmodule Code.Fragment do
    * `{:anonymous_call, inside_caller}` - the context is an anonymous
      call, such as `fun.(` and `@fun.(`.

+    * `{:capture_arg, charlist}` - the context is a capture argument,
+      such as `&1`
+
    * `{:module_attribute, charlist}` - the context is a module attribute,
      such as `@hello_wor`

@@ -157,8 +162,8 @@ defmodule Code.Fragment do
    * `:none` - no context possible

    * `{:sigil, charlist}` - the context is a sigil. It may be either the beginning
-      of a sigil, such as `~` or `~s`, or an operator starting with `~`, such as
-      `~>` and `~>>`
+      of a sigil, such as `~` or `~s`. Operators starting with `~`, such as
+      `~>` and `~>>`, are returned as :operator contexts

    * `{:struct, inside_struct}` - the context is a struct, such as `%`, `%UR` or `%URI`.
      `inside_struct` can either be a `charlist` in case of a static alias or an
@@ -204,6 +209,7 @@ defmodule Code.Fragment do
          | {:local_arity, charlist}
          | {:local_call, charlist}
          | {:anonymous_call, inside_caller}
+          | {:capture_arg, charlist}
          | {:module_attribute, charlist}
          | {:operator, charlist}
          | {:operator_arity, charlist}
@@ -656,7 +662,7 @@ defmodule Code.Fragment do
      iex> Code.Fragment.surround_context("foo", {1, 1})
      %{begin: {1, 1}, context: {:local_or_var, ~c"foo"}, end: {1, 4}}

-  ## Differences to `cursor_context/2`
+  ## Differences from `cursor_context/2`

  Because `surround_context/3` attempts to capture complex expressions,
  it has some differences to `cursor_context/2`:
@@ -670,7 +676,7 @@ defmodule Code.Fragment do
      be a local or variable

    * `@` when not followed by any identifier is returned as `{:operator, ~c"@"}`
-      (in contrast to `{:module_attribute, ~c""}` in `cursor_context/2`
+      (in contrast to `{:module_attribute, ~c""}` in `cursor_context/2`)

    * This function never returns empty sigils `{:sigil, ~c""}` or empty structs
      `{:struct, ~c""}` as context
@@ -1234,14 +1240,43 @@ defmodule Code.Fragment do
      the cursor. This is necessary to correctly complete anonymous functions
      and the left-hand side of `->`

+    * `:preserve_sigils` (since v1.20.0) - preserve sigil cursor location
+      (see "Tracking sigils" section below)
+
+  ## Tracking sigils
+
+  The `:preserve_sigils` option can be used to track cursor positions inside
+  a sigil.
+
+  If the sigil is terminated abruptly, the `sigil_*` call will have the cursor
+  as the second argument:
+
+      iex> Code.Fragment.container_cursor_to_quoted("~r/foo", preserve_sigils: true)
+      {:ok,
+      {:sigil_r, [delimiter: "/", line: 1],
+        [{:<<>>, [line: 1], ["foo"]}, {:__cursor__, [line: 1, column: 7], []}]}}
+
+  In case the sigil is completed and has zero or more modifiers, the cursor will
+  be nested in the list, with all previous delimiters specified:
+
+      iex> Code.Fragment.container_cursor_to_quoted("~r/foo/i", preserve_sigils: true)
+      {:ok,
+       {:sigil_r, [delimiter: "/", line: 1],
+        [{:<<>>, [line: 1], ["foo"]}, [105, {:__cursor__, [line: 1, column: 9], []}]]}}
+
+  If the cursor is after the sigil, then it is discarded as everything else:
+
+      iex> Code.Fragment.container_cursor_to_quoted("~r/foo/i ", preserve_sigils: true)
+      {:ok, {:__cursor__, [line: 1], []}}
  """
  @doc since: "1.13.0"
  @spec container_cursor_to_quoted(List.Chars.t(), container_cursor_to_quoted_opts()) ::
          {:ok, Macro.t()} | {:error, {location :: keyword, binary | {binary, binary}, binary}}
  def container_cursor_to_quoted(fragment, opts \\ []) do
    {trailing_fragment, opts} = Keyword.pop(opts, :trailing_fragment)
+    {preserve_sigils?, opts} = Keyword.pop(opts, :preserve_sigils, false)
    opts = Keyword.take(opts, [:columns, :token_metadata, :literal_encoder])
-    opts = [check_terminators: {:cursor, []}] ++ opts
+    opts = [check_terminators: {:cursor, preserve_sigils?, []}] ++ opts

    file = Keyword.get(opts, :file, "nofile")
    line = Keyword.get(opts, :line, 1)
@@ -1272,7 +1307,7 @@ defmodule Code.Fragment do
                 Enum.split_while(rev_terminators, &(elem(&1, 0) not in [:do, :fn])),
               true <- maybe_missing_stab?(rev_tokens, true),
               opts =
-                 Keyword.put(opts, :check_terminators, {:cursor, before_start}),
+                 Keyword.put(opts, :check_terminators, {:cursor, false, before_start}),
               {:error, {meta, _, ~c"end"}, _rest, _warnings, trailing_rev_tokens} <-
                 :elixir_tokenizer.tokenize(to_charlist(trailing_fragment), line, column, opts) do
            trailing_tokens =
@@ -65,7 +65,7 @@ defmodule Code.Identifier do
    with "-" <> rest <- Atom.to_string(atom),
         [trailing | reversed] = rest |> String.split("/") |> Enum.reverse(),
         [arity, _inner, _count, ""] <- String.split(trailing, "-") do
-      {reversed |> Enum.reverse() |> Enum.join("/") |> String.to_atom(), arity}
+      {reversed |> Enum.reverse() |> Enum.join("/") |> String.to_unsafe_atom(), arity}
    else
      _ -> :error
    end
@@ -150,20 +150,20 @@ defmodule Code.Identifier do
    <<acc::binary, char::utf8>>
  end

-  defp escape_char(char, acc) when char < 0x100 do
+  defp escape_char(char, acc) when char < 0x80 do
    <<a::4, b::4>> = <<char::8>>
    <<acc::binary, ?\\, ?x, to_hex(a), to_hex(b)>>
  end

  defp escape_char(char, acc) when char < 0x10000 do
    <<a::4, b::4, c::4, d::4>> = <<char::16>>
-    <<acc::binary, ?\\, ?x, ?{, to_hex(a), to_hex(b), to_hex(c), to_hex(d), ?}>>
+    <<acc::binary, ?\\, ?u, to_hex(a), to_hex(b), to_hex(c), to_hex(d)>>
  end

  defp escape_char(char, acc) when char < 0x1000000 do
    <<a::4, b::4, c::4, d::4, e::4, f::4>> = <<char::24>>

-    <<acc::binary, ?\\, ?x, ?{, to_hex(a), to_hex(b), to_hex(c), to_hex(d), to_hex(e), to_hex(f),
+    <<acc::binary, ?\\, ?u, ?{, to_hex(a), to_hex(b), to_hex(c), to_hex(d), to_hex(e), to_hex(f),
      ?}>>
  end

@@ -4,6 +4,8 @@
 defmodule Code.Normalizer do
  @moduledoc false

+  @do_end_keywords [:rescue, :catch, :else, :after]
+
  defguard is_literal(x)
           when is_integer(x) or
                  is_float(x) or
@@ -68,7 +70,7 @@ defmodule Code.Normalizer do

  # Bit containers
  defp do_normalize({:<<>>, _, args} = quoted, state) when is_list(args) do
-    normalize_bitstring(quoted, state)
+    normalize_bitstring(quoted, state, state.escape)
  end

  # Atoms with interpolations
@@ -89,13 +91,7 @@ defmodule Code.Normalizer do
        normalize_literal(:utf8, [], state)
      end

-    string =
-      if state.escape do
-        normalize_bitstring(string, state, true)
-      else
-        normalize_bitstring(string, state)
-      end
-
+    string = normalize_bitstring(string, state, state.escape)
    {{:., dot_meta, [:erlang, :binary_to_atom]}, call_meta, [string, utf8]}
  end

@@ -118,6 +114,7 @@ defmodule Code.Normalizer do
            end
        end)

+      parts = maybe_add_trailing_newline(call_meta, parts, state)
      {{:., dot_meta, [List, :to_charlist]}, call_meta, [parts]}
    else
      normalize_call(quoted, state)
@@ -187,7 +184,8 @@ defmodule Code.Normalizer do
        |> patch_meta_line(state.parent_meta)
        |> Keyword.put_new(:delimiter, "\"")

-      {sigil, meta, [do_normalize(string, %{state | parent_meta: meta}), modifiers]}
+      string = normalize_bitstring(string, %{state | parent_meta: meta}, false)
+      {sigil, meta, [string, modifiers]}
    else
      _ ->
        normalize_call(quoted, state)
@@ -269,7 +267,7 @@ defmodule Code.Normalizer do
          "Elixir." <> segments ->
            segments
            |> String.split(".")
-            |> Enum.map(&String.to_atom/1)
+            |> Enum.map(&String.to_unsafe_atom/1)
        end

      {:__aliases__, meta, segments}
@@ -351,18 +349,20 @@ defmodule Code.Normalizer do
        args = normalize_args(args, %{state | parent_meta: meta})
        {form, meta, args}

-      Keyword.has_key?(meta, :do) ->
+      Keyword.has_key?(meta, :do) and kw_blocks?(last) ->
        # def foo do :ok end
        # def foo, do: :ok
        normalize_kw_blocks(form, meta, args, state)

-      match?([{:do, _} | _], last) and Keyword.keyword?(last) ->
+      match?([{:do, _} | _], last) and kw_blocks?(last) ->
        # Non normalized kw blocks
        line = state.parent_meta[:line] || meta[:line]
        meta = meta ++ [do: [line: line], end: [line: line]]
        normalize_kw_blocks(form, meta, args, state)

      true ->
+        # The formatter renders do-end blocks from the meta alone
+        meta = Keyword.drop(meta, [:do, :end])
        args = normalize_args(args, %{state | parent_meta: meta})
        {last_arg, leading_args} = List.pop_at(args, -1, [])

@@ -401,11 +401,22 @@ defmodule Code.Normalizer do
  defp block_keyword?([]), do: true
  defp block_keyword?(_), do: false

+  # Anything after the do block that is not a block keyword makes it a keyword list
+  defp kw_blocks?([{:do, _} | rest] = kw) do
+    Keyword.keyword?(kw) and Enum.all?(rest, &match?({key, _} when key in @do_end_keywords, &1))
+  end
+
+  defp kw_blocks?([{{:__block__, _, [:do]}, _} | rest]) do
+    Enum.all?(rest, &match?({{:__block__, _, [key]}, _} when key in @do_end_keywords, &1))
+  end
+
+  defp kw_blocks?(_), do: false
+
  defp allow_keyword?(:when, 2), do: true
  defp allow_keyword?(:{}, _), do: false
  defp allow_keyword?(op, arity), do: not is_atom(op) or not Macro.operator?(op, arity)

-  defp normalize_bitstring({:<<>>, meta, parts}, state, escape_interpolation \\ false) do
+  defp normalize_bitstring({:<<>>, meta, parts}, state, escape_interpolation) do
    meta = patch_meta_line(meta, state.parent_meta)

    parts =
@@ -424,9 +435,21 @@ defmodule Code.Normalizer do
        end)
      end

+    parts = maybe_add_trailing_newline(meta, parts, state)
    {:<<>>, meta, parts}
  end

+  defp maybe_add_trailing_newline(meta, parts, state) do
+    with true <- state.escape and Keyword.get(meta, :delimiter) in ["\"\"\"", "'''"],
+         last = List.last(parts),
+         true <- is_binary(last) and not String.ends_with?(last, "\n") do
+      [_last | rest] = Enum.reverse(parts)
+      Enum.reverse([last <> "\n" | rest])
+    else
+      _ -> parts
+    end
+  end
+
  defp normalize_interpolation_parts(parts, state, escape_interpolation) do
    Enum.map(parts, fn
      {:"::", interpolation_meta,
@@ -552,7 +575,7 @@ defmodule Code.Normalizer do
    atom
    |> Atom.to_string()
    |> maybe_escape_literal(state)
-    |> String.to_atom()
+    |> String.to_unsafe_atom()
  end

  defp maybe_escape_literal(term, _) do
@@ -80,7 +80,8 @@ defmodule Code.Typespec do
  Returns all types available from the module's BEAM code.

  The result is returned as a list of tuples where the first
-  element is the type (`:typep`, `:type` and `:opaque`).
+  element is the type (`:typep`, `:type`, `:opaque` and, on Erlang/OTP 28+,
+  `:nominal`).

  The module must have a corresponding BEAM file which can be
  located by the runtime system. The types will be in the Erlang
@@ -95,9 +96,10 @@ defmodule Code.Typespec do

        types =
          for {:attribute, _, kind, {name, _, args} = type} <- abstract_code,
-              kind in [:opaque, :type] do
+              kind in [:opaque, :type, :nominal] do
            cond do
              kind == :opaque -> {:opaque, type}
+              kind == :nominal -> {:nominal, type}
              {name, length(args)} in exported_types -> {:type, type}
              true -> {:typep, type}
            end
@@ -117,7 +119,7 @@ defmodule Code.Typespec do
  element is spec name and arity and the second is the spec.

  The module must have a corresponding BEAM file which can be
-  located by the runtime system. The types will be in the Erlang
+  located by the runtime system. The specs will be in the Erlang
  Abstract Format.
  """
  @spec fetch_specs(module | binary) :: {:ok, [tuple]} | :error
@@ -135,10 +137,10 @@ defmodule Code.Typespec do
  Returns all callbacks available from the module's BEAM code.

  The result is returned as a list of tuples where the first
-  element is spec name and arity and the second is the spec.
+  element is the callback name and arity and the second is the callback.

  The module must have a corresponding BEAM file
-  which can be located by the runtime system. The types will be
+  which can be located by the runtime system. The callbacks will be
  in the Erlang Abstract Format.
  """
  @spec fetch_callbacks(module | binary) :: {:ok, [tuple]} | :error
@@ -191,8 +193,8 @@ defmodule Code.Typespec do

  ## To AST conversion

-  defp collect_vars({:ann_type, _anno, args}) when is_list(args) do
-    []
+  defp collect_vars({:ann_type, _anno, [_var, type]}) do
+    collect_vars(type)
  end

  defp collect_vars({:type, _anno, _kind, args}) when is_list(args) do
@@ -399,10 +401,10 @@ defmodule Code.Typespec do
  defp erl_to_ex_var(var) do
    case Atom.to_string(var) do
      <<"_", c::utf8, rest::binary>> ->
-        String.to_atom("_#{String.downcase(<<c::utf8>>)}#{rest}")
+        String.to_unsafe_atom("_#{String.downcase(<<c::utf8>>)}#{rest}")

      <<c::utf8, rest::binary>> ->
-        String.to_atom("#{String.downcase(<<c::utf8>>)}#{rest}")
+        String.to_unsafe_atom("#{String.downcase(<<c::utf8>>)}#{rest}")
    end
  end

@@ -69,6 +69,22 @@ defprotocol Collectable do
      iex> Enum.into([1, 2, 3], MapSet.new())
      MapSet.new([1, 2, 3])

+  ## Halting
+
+  The `:halt` flag will be given whenever the collection won't
+  terminate correctly and must be used to clean up existing resources
+  (such as sockets, file handles, etc).
+
+  Note it is not guaranteed that the accumulator given to halt will
+  be the latest version of the accumulator returned by a previous call
+  with `{:cont, elem}`. Therefore, you must track the collected results
+  within the resource you intend to halt.
+
+  This is by design: ensuring halt is always called with the latest
+  accumulator would make pure collectables (the ones that do not implement
+  halt) expensive. However, given the collectables that must implement halt
+  already need to track state, the burden of tracking the accumulator
+  across invocations is put on them.
  """

  @type command :: {:cont, term} | :done | :halt
@@ -152,6 +152,7 @@ defmodule Config do

  """
  @doc since: "1.9.0"
+  @spec config(atom(), keyword()) :: keyword()
  def config(root_key, opts) when is_atom(root_key) and is_list(opts) do
    if not Keyword.keyword?(opts) do
      raise ArgumentError, "config/2 expected a keyword list, got: #{inspect(opts)}"
@@ -198,6 +199,7 @@ defmodule Config do

  """
  @doc since: "1.9.0"
+  @spec config(atom(), atom(), term()) :: keyword()
  def config(root_key, key, opts) when is_atom(root_key) and is_atom(key) do
    get_config!()
    |> __merge__([{root_key, [{key, opts}]}])
@@ -225,6 +227,7 @@ defmodule Config do

  """
  @doc since: "1.18.0"
+  @spec read_config(atom()) :: keyword() | nil
  def read_config(root_key) when is_atom(root_key) do
    get_config!()[root_key]
  end
@@ -233,7 +236,7 @@ defmodule Config do
  Returns the environment this configuration file is executed on.

  In Mix projects this function returns the environment this configuration
-  file is executed on. 
+  file is executed on.
  In releases, returns the `MIX_ENV` specified when running `mix release`.

  This is most often used to execute conditional code:
@@ -284,8 +287,8 @@ defmodule Config do

  In case the file doesn't exist, an error is raised.

-  If file is a relative, it will be expanded relatively to the
-  directory the current configuration file is in.
+  If the file is relative, it will be expanded relative to the
+  directory of the current configuration file.

  ## Examples

@@ -377,21 +380,27 @@ defmodule Config do
    end
  end

-  defp validate!(config, file) do
-    Enum.all?(config, fn
+  defp validate!(config, file) when is_list(config) do
+    Enum.each(config, fn
      {app, value} when is_atom(app) ->
-        if Keyword.keyword?(value) do
-          true
-        else
+        if not Keyword.keyword?(value) do
          raise ArgumentError,
                "expected config for app #{inspect(app)} in #{Path.relative_to_cwd(file)} " <>
                  "to return keyword list, got: #{inspect(value)}"
        end

-      _ ->
-        false
+      other ->
+        raise ArgumentError,
+              "expected config in #{Path.relative_to_cwd(file)} to be a keyword list " <>
+                "of {atom, keyword} pairs, got entry: #{inspect(other)}"
    end)

    config
  end
+
+  defp validate!(config, file) do
+    raise ArgumentError,
+          "expected config in #{Path.relative_to_cwd(file)} to be a keyword list " <>
+            "of {atom, keyword} pairs, got: #{inspect(config)}"
+  end
 end
@@ -25,7 +25,7 @@ defmodule Config.Provider do
  For example, imagine you want to list some basic configuration
  on Mix's built-in `config/runtime.exs` file, but you also want
  to support additional configuration files. To do so, you can add
-  this inside the `def project` portion of  your `mix.exs`:
+  this inside the `def project` portion of your `mix.exs`:

      releases: [
        demo: [
@@ -16,7 +16,7 @@ defmodule Config.Reader do

  For example, if you expect the target system to have a config file
  in an absolute path, you can add this inside the `def project` portion
-  of  your `mix.exs`:
+  of your `mix.exs`:

      releases: [
        demo: [
@@ -618,7 +618,7 @@ defmodule Enum do
          acc,
          (element, acc -> {:cont, chunk, acc} | {:cont, acc} | {:halt, acc}),
          (acc -> {:cont, chunk, acc} | {:cont, acc})
-        ) :: Enumerable.t()
+        ) :: [chunk]
        when chunk: any
  def chunk_while(enumerable, acc, chunk_fun, after_fun) do
    {_, {res, acc}} =
@@ -666,7 +666,7 @@ defmodule Enum do
      [1, [2], 3, 4, 5, 6]

  """
-  @spec concat(t) :: t
+  @spec concat(Enumerable.t(Enumerable.t(elem))) :: [elem] when elem: term
  def concat(enumerables)

  def concat(list) when is_list(list) do
@@ -681,8 +681,8 @@ defmodule Enum do
  Concatenates the enumerable on the `right` with the enumerable on the
  `left`.

-  This function produces the same result as the `++/2` operator
-  for lists.
+  This function behaves similarly to the `++/2` operator with proper
+  lists, but applied to enumerables.

  ## Examples

@@ -693,7 +693,7 @@ defmodule Enum do
      [1, 2, 3, 4, 5, 6]

  """
-  @spec concat(t, t) :: t
+  @spec concat(Enumerable.t(elem), Enumerable.t(elem)) :: [elem] when elem: term
  def concat(left, right) when is_list(left) and is_list(right) do
    left ++ right
  end
@@ -829,7 +829,7 @@ defmodule Enum do
  """
  @spec dedup(t) :: list
  def dedup(enumerable) when is_list(enumerable) do
-    dedup_list(enumerable, []) |> :lists.reverse()
+    dedup_list(enumerable)
  end

  def dedup(enumerable) do
@@ -859,6 +859,10 @@ defmodule Enum do

  """
  @spec dedup_by(t, (element -> term)) :: list
+  def dedup_by([head | tail], fun) do
+    dedup_by_list(tail, fun, fun.(head), [head])
+  end
+
  def dedup_by(enumerable, fun) do
    {list, _} = reduce(enumerable, {[], []}, R.dedup(fun))
    :lists.reverse(list)
@@ -903,7 +907,7 @@ defmodule Enum do

  def drop(enumerable, amount) when is_integer(amount) and amount < 0 do
    {count, fun} = slice_count_and_fun(enumerable, 1)
-    amount = Kernel.min(amount + count, count)
+    amount = amount + count

    if amount > 0 do
      fun.(0, amount, 1)
@@ -1442,7 +1446,7 @@ defmodule Enum do
    )

    # Avoid warnings about Dict
-    dict_module = String.to_atom("Dict")
+    dict_module = String.to_unsafe_atom("Dict")

    reduce(reverse(enumerable), dict, fn entry, categories ->
      dict_module.update(categories, fun.(entry), [entry], &[entry | &1])
@@ -2023,11 +2027,16 @@ defmodule Enum do
  operators work by using this function.
  """
  @spec member?(t, element) :: boolean
-  def member?(enumerable, element) when is_list(enumerable) do
+  def member?(enumerable, element) do
+    __in__(element, enumerable)
+  end
+
+  @doc false
+  def __in__(element, enumerable) when is_list(enumerable) do
    :lists.member(element, enumerable)
  end

-  def member?(enumerable, element) do
+  def __in__(element, enumerable) do
    case Enumerable.member?(enumerable, element) do
      {:ok, element} when is_boolean(element) ->
        element
@@ -2224,14 +2233,19 @@ defmodule Enum do
      nil

  """
-  @spec min_max(t, (element, element -> boolean) | module()) :: {element, element}
-  @spec min_max(t, (-> empty_result)) :: {element, element} | empty_result when empty_result: any
+  @spec min_max(t, (element, element -> boolean) | module()) :: {min :: element, max :: element}
+  @spec min_max(t, (-> empty_result)) :: {min :: element, max :: element} | empty_result
+        when empty_result: any
  @spec min_max(t, (element, element -> boolean) | module(), (-> empty_result)) ::
-          {element, element} | empty_result
+          {min :: element, max :: element} | empty_result
        when empty_result: any

  def min_max(enumerable, sorter_or_empty_fallback \\ fn -> raise Enum.EmptyError end)

+  def min_max(list = [_ | _], empty_fallback) when is_function(empty_fallback, 0) do
+    min_max_list(list)
+  end
+
  def min_max(first..last//step = range, empty_fallback)
      when is_function(empty_fallback, 0) do
    case Range.size(range) do
@@ -2338,14 +2352,14 @@ defmodule Enum do

  """
  @spec min_max_by(t, (element -> any), (element, element -> boolean) | module()) ::
-          {element, element} | empty_result
+          {min :: element, max :: element} | empty_result
        when empty_result: any
  @spec min_max_by(
          t,
          (element -> any),
          (element, element -> boolean) | module(),
          (-> empty_result)
-        ) :: {element, element} | empty_result
+        ) :: {min :: element, max :: element} | empty_result
        when empty_result: any
  def min_max_by(
        enumerable,
@@ -2389,6 +2403,18 @@ defmodule Enum do

  defp min_max_sort_fun(module) when is_atom(module), do: &(module.compare(&1, &2) == :lt)

+  defp min_max_list([h | t]), do: min_max_list(t, h, h)
+
+  defp min_max_list([h | t], min, max) do
+    cond do
+      h < min -> min_max_list(t, h, max)
+      max < h -> min_max_list(t, min, h)
+      true -> min_max_list(t, min, max)
+    end
+  end
+
+  defp min_max_list([], min, max), do: {min, max}
+
  @doc """
  Splits the `enumerable` in two lists according to the given function `fun`.

@@ -2666,7 +2692,7 @@ defmodule Enum do
      5050

  """
-  @spec reduce_while(t, any, (element, any -> {:cont, any} | {:halt, any})) :: any
+  @spec reduce_while(t, acc, (element, acc -> {:cont, acc} | {:halt, acc})) :: acc
  def reduce_while(enumerable, acc, fun) do
    Enumerable.reduce(enumerable, {:cont, acc}, fun) |> elem(1)
  end
@@ -2909,8 +2935,7 @@ defmodule Enum do
  end

  defp slide_list_middle(list, 0, last, start_to_middle) do
-    {slid_range, tail} = slide_list_last(list, last + 1, [])
-    slid_range ++ :lists.reverse(start_to_middle, tail)
+    slide_list_last(list, last + 1, [], start_to_middle)
  end

  # You asked for a middle index off the end of the list... you get what we've got
@@ -2918,16 +2943,16 @@ defmodule Enum do
    :lists.reverse(acc)
  end

-  defp slide_list_last([h | t], last, acc) when last > 0 do
-    slide_list_last(t, last - 1, [h | acc])
+  defp slide_list_last([h | t], last, acc, start_to_middle) when last > 0 do
+    slide_list_last(t, last - 1, [h | acc], start_to_middle)
  end

-  defp slide_list_last(rest, 0, acc) do
-    {:lists.reverse(acc), rest}
+  defp slide_list_last(rest, 0, acc, start_to_middle) do
+    :lists.reverse(acc, :lists.reverse(start_to_middle, rest))
  end

-  defp slide_list_last([], _, acc) do
-    {:lists.reverse(acc), []}
+  defp slide_list_last([], _, acc, start_to_middle) do
+    :lists.reverse(acc, :lists.reverse(start_to_middle))
  end

  @doc """
@@ -4193,7 +4218,7 @@ defmodule Enum do

  `zip_with/2` can be used to transpose lists of lists:

-      iex> Enum.zip_with([[1, 2,], [3, 4]], & &1)
+      iex> Enum.zip_with([[1, 2], [3, 4]], & &1)
      [[1, 3], [2, 4]]

  """
@@ -4304,11 +4329,24 @@ defmodule Enum do
        empty.()

      _ ->
-        last = last - rem(last - first, step)
+        # The endpoint shortcut is only valid for sorters consistent with
+        # the natural integer order of the range elements, which is known
+        # to hold for the default sorters; any other sorter traverses the
+        # elements, seeded with the first one since the range is not empty
+        if fun == (&<=/2) or fun == (&>=/2) do
+          last = last - rem(last - first, step)

-        case fun.(first, last) do
-          true -> first
-          false -> last
+          case fun.(first, last) do
+            true -> first
+            false -> last
+          end
+        else
+          reduce_range(first + step, last, step, first, fn element, acc ->
+            case fun.(acc, element) do
+              true -> acc
+              false -> element
+            end
+          end)
        end
    end
  end
@@ -4411,10 +4449,9 @@ defmodule Enum do
  ## any?/2 all?/2

  defp predicate_list([h | t], initial, fun) do
-    if !!fun.(h) == initial do
-      predicate_list(t, initial, fun)
-    else
-      not initial
+    case !!fun.(h) do
+      ^initial -> predicate_list(t, initial, fun)
+      _ -> not initial
    end
  end

@@ -4425,10 +4462,9 @@ defmodule Enum do
  defp predicate_range(first, last, step, initial, fun)
       when step > 0 and first <= last
       when step < 0 and first >= last do
-    if !!fun.(first) == initial do
-      predicate_range(first + step, last, step, initial, fun)
-    else
-      not initial
+    case !!fun.(first) do
+      ^initial -> predicate_range(first + step, last, step, initial, fun)
+      _ -> not initial
    end
  end

@@ -4507,19 +4543,20 @@ defmodule Enum do

  # dedup

-  defp dedup_list([value | tail], acc) do
-    acc =
-      case acc do
-        [^value | _] -> acc
-        _ -> [value | acc]
-      end
+  defp dedup_list([value | [value | _] = tail]), do: dedup_list(tail)
+  defp dedup_list([value | tail]), do: [value | dedup_list(tail)]
+  defp dedup_list([]), do: []

-    dedup_list(tail, acc)
+  ## dedup_by
+
+  defp dedup_by_list([head | tail], fun, prev, acc) do
+    case fun.(head) do
+      ^prev -> dedup_by_list(tail, fun, prev, acc)
+      new_val -> dedup_by_list(tail, fun, new_val, [head | acc])
+    end
  end

-  defp dedup_list([], acc) do
-    acc
-  end
+  defp dedup_by_list([], _fun, _prev, acc), do: :lists.reverse(acc)

  ## drop

@@ -26,7 +26,7 @@ defmodule Exception do
  @typedoc "The exception type"
  @type t :: %{
          required(:__struct__) => module,
-          required(:__exception__) => true,
+          required(:__exception__) => term,
          optional(atom) => any
        }

@@ -77,7 +77,7 @@ defmodule Exception do
  @doc false
  @deprecated "Use Kernel.is_exception/1 instead"
  def exception?(term)
-  def exception?(%_{__exception__: true}), do: true
+  def exception?(%_{__exception__: _}), do: true
  def exception?(_), do: false

  @doc """
@@ -89,7 +89,7 @@ defmodule Exception do
  return a descriptive error message instead.
  """
  @spec message(t) :: String.t()
-  def message(%module{__exception__: true} = exception) do
+  def message(%module{__exception__: _} = exception) do
    try do
      module.message(exception)
    rescue
@@ -123,7 +123,7 @@ defmodule Exception do
  @spec normalize(:error, any, stacktrace) :: t
  @spec normalize(non_error_kind, payload, stacktrace) :: payload when payload: var
  def normalize(kind, payload, stacktrace \\ [])
-  def normalize(:error, %_{__exception__: true} = payload, _stacktrace), do: payload
+  def normalize(:error, %_{__exception__: _} = payload, _stacktrace), do: payload
  def normalize(:error, payload, stacktrace), do: ErlangError.normalize(payload, stacktrace)
  def normalize(_kind, payload, _stacktrace), do: payload

@@ -1046,7 +1046,7 @@ defmodule RuntimeError do
      iex> raise "oops!"
      ** (RuntimeError) oops!

-  You should use this exceptions sparingly, since most of the time it might be
+  You should use this exception sparingly, since most of the time it might be
  better to define your own exceptions specific to your application or library.
  Sometimes, however, there are situations in which you don't expect a condition to
  happen, but you want to give a meaningful error message if it does. In those cases,
@@ -1137,7 +1137,7 @@ defmodule SystemLimitError do

  For example, this can happen if you try to create an atom that is too large:

-      iex> String.to_atom(String.duplicate("a", 100_000))
+      iex> String.to_unsafe_atom(String.duplicate("a", 100_000))
      ** (SystemLimitError) a system limit has been reached
  """

@@ -1807,7 +1807,7 @@ defmodule UndefinedFunctionError do

  defp load_module({name, _path, _loaded?}) do
    name
-    |> List.to_atom()
+    |> List.to_unsafe_atom()
    |> Code.ensure_loaded()
  end

@@ -2376,7 +2376,7 @@ defmodule File.Error do
  @moduledoc """
  An exception that is raised when a file operation fails.

-  For example, this exception is raised, when trying to read a non existent file:
+  For example, this exception is raised, when trying to read a nonexistent file:

      iex> File.read!("nonexistent_file.txt")
      ** (File.Error) could not read file "nonexistent_file.txt": no such file or directory
@@ -2409,7 +2409,7 @@ defmodule File.CopyError do
  @moduledoc """
  An exception that is raised when copying a file fails.

-  For example, this exception is raised when trying to copy to file or directory that isn't present:
+  For example, this exception is raised when trying to copy to a file or directory that isn't present:

      iex> File.cp_r!("non_existent", "source_dir/subdir")
      ** (File.CopyError) could not copy recursively from "non_existent" to "source_dir/subdir". non_existent: no such file or directory
@@ -2477,7 +2477,7 @@ defmodule File.LinkError do
  @moduledoc """
  An exception that is raised when linking a file fails.

-  For example, this exception is raised when trying to link to file that isn't present:
+  For example, this exception is raised when trying to link to a file that isn't present:

      iex> File.ln!("existing.txt", "link.txt")
      ** (File.LinkError) could not create hard link from "link.txt" to "existing.txt": no such file or directory
@@ -400,6 +400,13 @@ defmodule File do

  You can use `:file.format_error/1` to get a descriptive string of the error.

+  ## Options (since v1.20)
+
+  The supported options are:
+
+    * `:raw` - a single atom to bypass the file server and only check
+      for the file locally
+
  ## Examples

      File.read("hello.txt")
@@ -408,15 +415,24 @@ defmodule File do
      File.read("non_existing.txt")
      #=> {:error, :enoent}
  """
-  @spec read(Path.t()) :: {:ok, binary} | {:error, posix | :badarg | :terminated | :system_limit}
-  def read(path) do
-    :file.read_file(IO.chardata_to_string(path))
+  @spec read(Path.t(), [exists_option]) ::
+          {:ok, binary} | {:error, posix | :badarg | :terminated | :system_limit}
+        when exists_option: :raw
+  def read(path, opts \\ []) do
+    :file.read_file(IO.chardata_to_string(path), opts)
  end

  @doc """
  Returns a binary with the contents of the given filename,
  or raises a `File.Error` exception if an error occurs.

+  ## Options (since v1.20)
+
+  The supported options are:
+
+    * `:raw` - a single atom to bypass the file server and only check
+      for the file locally
+
  ## Examples

      File.read!("hello.txt")
@@ -425,9 +441,9 @@ defmodule File do
      File.read!("non_existing.txt")
      ** (File.Error) could not read file "non_existing.txt": no such file or directory
  """
-  @spec read!(Path.t()) :: binary
-  def read!(path) do
-    case read(path) do
+  @spec read!(Path.t(), [exists_option]) :: binary when exists_option: :raw
+  def read!(path, opts \\ []) do
+    case read(path, opts) do
      {:ok, binary} ->
        binary

@@ -881,7 +897,7 @@ defmodule File do
      File.copy("non_existing.txt", "copy.txt")
      #=> {:error, :enoent}
  """
-  @spec copy(Path.t() | io_device, Path.t() | io_device, pos_integer | :infinity) ::
+  @spec copy(Path.t() | io_device, Path.t() | io_device, non_neg_integer | :infinity) ::
          {:ok, non_neg_integer} | {:error, posix | :badarg | :terminated}
  def copy(source, destination, bytes_count \\ :infinity) do
    source = normalize_path_or_io_device(source)
@@ -902,7 +918,7 @@ defmodule File do
      File.copy!("non_existing.txt", "copy.txt")
      ** (File.CopyError) could not copy from "non_existing.txt" to "copy.txt": no such file or directory
  """
-  @spec copy!(Path.t() | io_device, Path.t() | io_device, pos_integer | :infinity) ::
+  @spec copy!(Path.t() | io_device, Path.t() | io_device, non_neg_integer | :infinity) ::
          non_neg_integer
  def copy!(source, destination, bytes_count \\ :infinity) do
    case copy(source, destination, bytes_count) do
@@ -1087,7 +1103,7 @@ defmodule File do

  @doc ~S"""
  Copies the contents in `source` to `destination` recursively, maintaining the
-  source directory structure and modes.
+  source directory structure and regular file modes.

  If `source` is a file or a symbolic link to it, `destination` must be a path
  to an existent file, a symbolic link to one, or a path to a non-existent file.
@@ -1098,7 +1114,9 @@ defmodule File do
  If the source is a file, it copies `source` to `destination`. If the `source`
  is a directory, it copies the contents inside source into the `destination` directory.

-  If a file already exists in the destination, it invokes the optional `on_conflict`
+  For regular files, their respective file modes are preserved in the destination.
+  Directory modes are preserved only when `:preserve_directory_permissions` is `true`.
+  If a file already exists in the destination, it invokes the optional `:on_conflict`
  callback given as an option. See "Options" for more information.

  This function may fail while copying files, in such cases, it will leave the
@@ -1135,6 +1153,11 @@ defmodule File do
      dereferenced and have their contents copied instead when set to `true`. If the dereferenced
      files do not exist, than the operation fails. The default is `false`.

+    * `:preserve_directory_permissions` - (since v1.20.0) when `true`, the permissions of
+      source directories are copied to the destination directories after their contents are
+      written. This is useful when source directories are read-only or have restricted
+      permissions that must be preserved. The default is `false`.
+
  ## Examples

      # Copies file "a.txt" to "b.txt"
@@ -1160,7 +1183,8 @@ defmodule File do
  """
  @spec cp_r(Path.t(), Path.t(),
          on_conflict: on_conflict_callback,
-          dereference_symlinks: boolean()
+          dereference_symlinks: boolean(),
+          preserve_directory_permissions: boolean()
        ) ::
          {:ok, [binary]} | {:error, posix | :badarg | :terminated, binary}

@@ -1182,6 +1206,7 @@ defmodule File do
  def cp_r(source, destination, options) when is_list(options) do
    on_conflict = Keyword.get(options, :on_conflict, fn _, _ -> true end)
    dereference? = Keyword.get(options, :dereference_symlinks, false)
+    preserve_directory_permissions? = Keyword.get(options, :preserve_directory_permissions, false)

    source =
      source
@@ -1201,7 +1226,14 @@ defmodule File do
    else
      dereference = if dereference?, do: MapSet.new(), else: nil

-      case do_cp_r(source, destination, on_conflict, dereference, []) do
+      case do_cp_r(
+             source,
+             destination,
+             on_conflict,
+             dereference,
+             preserve_directory_permissions?,
+             []
+           ) do
        {:error, _, _} = error -> error
        res -> {:ok, res}
      end
@@ -1225,7 +1257,8 @@ defmodule File do
  """
  @spec cp_r!(Path.t(), Path.t(),
          on_conflict: on_conflict_callback,
-          dereference_symlinks: boolean()
+          dereference_symlinks: boolean(),
+          preserve_directory_permissions: boolean()
        ) :: [binary]
  def cp_r!(source, destination, options \\ []) do
    case cp_r(source, destination, options) do
@@ -1242,7 +1275,7 @@ defmodule File do
    end
  end

-  defp do_cp_r(src, dest, on_conflict, dereference, acc) when is_list(acc) do
+  defp do_cp_r(src, dest, on_conflict, dereference, preserve_dir_perms?, acc) when is_list(acc) do
    case :elixir_utils.read_link_type(src) do
      {:ok, :regular} ->
        case do_cp_file(src, dest, on_conflict, acc) do
@@ -1262,7 +1295,7 @@ defmodule File do
              {:error, :eloop, src}
            else
              dereference = MapSet.put(dereference, resolved)
-              do_cp_r(resolved, dest, on_conflict, dereference, acc)
+              do_cp_r(resolved, dest, on_conflict, dereference, preserve_dir_perms?, acc)
            end

          {:ok, link} ->
@@ -1277,23 +1310,34 @@ defmodule File do
          {:ok, files} ->
            case mkdir(dest) do
              success when success in [:ok, {:error, :eexist}] ->
-                case copy_file_mode(src, dest) do
-                  :ok ->
-                    Enum.reduce_while(files, [dest | acc], fn x, acc ->
-                      case do_cp_r(
-                             Path.join(src, x),
-                             Path.join(dest, x),
-                             on_conflict,
-                             dereference,
-                             acc
-                           ) do
-                        {:error, _, _} = error -> {:halt, error}
-                        acc -> {:cont, acc}
-                      end
-                    end)
+                files
+                |> Enum.reduce_while([dest | acc], fn x, acc ->
+                  case do_cp_r(
+                         Path.join(src, x),
+                         Path.join(dest, x),
+                         on_conflict,
+                         dereference,
+                         preserve_dir_perms?,
+                         acc
+                       ) do
+                    {:error, _, _} = error -> {:halt, error}
+                    acc -> {:cont, acc}
+                  end
+                end)
+                |> case do
+                  {:error, _, _} = error ->
+                    error

-                  {:error, reason} ->
-                    {:error, reason, src}
+                  files when preserve_dir_perms? ->
+                    # Change the directory after writing files in case
+                    # it was originally read only
+                    case copy_file_mode(src, dest) do
+                      :ok -> files
+                      {:error, reason} -> {:error, reason, src}
+                    end
+
+                  files ->
+                    files
                end

              {:error, reason} ->
@@ -1313,14 +1357,13 @@ defmodule File do
  end

  # If we reach this clause, there was an error while processing a file.
-  defp do_cp_r(_, _, _, _, acc) do
+  defp do_cp_r(_, _, _, _, _, acc) do
    acc
  end

  defp copy_file_mode(src, dest) do
-    with {:ok, dest_fileinfo} <- stat(dest),
-         {:ok, src_fileinfo} <- stat(src) do
-      write_stat(dest, %{dest_fileinfo | mode: src_fileinfo.mode})
+    with {:ok, src_fileinfo} <- stat(src) do
+      chmod(dest, src_fileinfo.mode)
    end
  end

@@ -2177,7 +2220,7 @@ defmodule File do
  type. If you pass, for example, `[encoding: :utf8]` or
  `[encoding: {:utf16, :little}]` in the modes parameter, the underlying stream
  will use `IO.write/2` and the `String.Chars` protocol to convert the data.
-  See `IO.binwrite/2` and `IO.write/2` .
+  See `IO.binwrite/2` and `IO.write/2`.

  One may also consider passing the `:delayed_write` option if the stream
  is meant to be written to under a tight loop.
@@ -18,7 +18,13 @@ defmodule File.Stream do

  defstruct path: nil, modes: [], line_or_bytes: :line, raw: true, node: nil

-  @type t :: %__MODULE__{}
+  @type t :: %__MODULE__{
+          path: Path.t(),
+          modes: [term()],
+          line_or_bytes: :line | pos_integer(),
+          raw: boolean(),
+          node: node()
+        }

  @doc false
  def __build__(path, line_or_bytes, modes) do
@@ -25,7 +25,7 @@ defmodule Float do
  and arithmetic due to the fact most decimal fractions cannot be
  represented by a floating-point binary and most operations are not exact,
  but operate on approximations. Those issues are not specific
-  to Elixir, they are a property of floating point representation itself.
+  to Elixir, they are a property of floating-point representation itself.

  For example, the numbers 0.1 and 0.01 are two of them, what means the result
  of squaring 0.1 does not give 0.01 neither the closest representable. Here is
@@ -167,40 +167,73 @@ defmodule Float do
    parse_unsigned(binary)
  end

-  defp parse_unsigned(<<digit, rest::binary>>) when digit in ?0..?9,
-    do: parse_unsigned(rest, false, false, [digit])
+  defp parse_unsigned(<<digit, rest::binary>> = binary) when digit in ?0..?9,
+    do: parse_mantissa(binary, rest, false)

  defp parse_unsigned(binary) when is_binary(binary), do: :error

-  defp parse_unsigned(<<digit, rest::binary>>, dot?, e?, acc) when digit in ?0..?9,
-    do: parse_unsigned(rest, dot?, e?, [digit | acc])
+  defp parse_mantissa(binary, <<digit, rest::binary>>, dot?) when digit in ?0..?9,
+    do: parse_mantissa(binary, rest, dot?)

-  defp parse_unsigned(<<?., digit, rest::binary>>, false, false, acc) when digit in ?0..?9,
-    do: parse_unsigned(rest, true, false, [digit, ?. | acc])
+  defp parse_mantissa(binary, <<?., digit, rest::binary>>, false) when digit in ?0..?9,
+    do: parse_mantissa(binary, rest, true)

-  defp parse_unsigned(<<exp_marker, digit, rest::binary>>, dot?, false, acc)
+  defp parse_mantissa(binary, <<exp_marker, digit, rest::binary>> = tail, dot?)
       when exp_marker in ~c"eE" and digit in ?0..?9,
-       do: parse_unsigned(rest, true, true, [digit, ?e | add_dot(acc, dot?)])
+       do: parse_exponent(binary, byte_size(binary) - byte_size(tail), rest, dot?)

-  defp parse_unsigned(<<exp_marker, sign, digit, rest::binary>>, dot?, false, acc)
+  defp parse_mantissa(binary, <<exp_marker, sign, digit, rest::binary>> = tail, dot?)
       when exp_marker in ~c"eE" and sign in ~c"-+" and digit in ?0..?9,
-       do: parse_unsigned(rest, true, true, [digit, sign, ?e | add_dot(acc, dot?)])
+       do: parse_exponent(binary, byte_size(binary) - byte_size(tail), rest, dot?)

-  # :erlang.binary_to_float/1 can raise an ArgumentError if the e exponent is too big. For example,
-  # "1.0e400". Because of this, we rescue the ArgumentError here and return an error.
-  defp parse_unsigned(rest, dot?, _, acc) do
-    acc
-    |> add_dot(dot?)
-    |> :lists.reverse()
-    |> :erlang.list_to_float()
+  defp parse_mantissa(binary, rest, dot?), do: finish_mantissa(binary, rest, dot?)
+
+  defp parse_exponent(binary, exp_pos, <<digit, rest::binary>>, dot?) when digit in ?0..?9,
+    do: parse_exponent(binary, exp_pos, rest, dot?)
+
+  defp parse_exponent(binary, exp_pos, rest, dot?),
+    do: finish_exponent(binary, exp_pos, rest, dot?)
+
+  defp finish_mantissa(binary, rest, _dot? = true) do
+    {:erlang.binary_to_float(consumed(binary, rest)), rest}
  rescue
    ArgumentError -> :error
-  else
-    float -> {float, rest}
  end

-  defp add_dot(acc, true), do: acc
-  defp add_dot(acc, false), do: [?0, ?. | acc]
+  # Bare integer: * 1.0 casts to the nearest float without building a new binary,
+  # and raises ArithmeticError on overflow (for example a 400-digit integer).
+  defp finish_mantissa(binary, rest, _dot? = false) do
+    {:erlang.binary_to_integer(consumed(binary, rest)) * 1.0, rest}
+  rescue
+    ArithmeticError -> :error
+  end
+
+  # binary_to_float/1 raises ArgumentError when the exponent is too big, e.g. "1.0e400".
+  defp finish_exponent(binary, _exp_pos, rest, _dot? = true) do
+    {:erlang.binary_to_float(consumed(binary, rest)), rest}
+  rescue
+    ArgumentError -> :error
+  end
+
+  # No decimal point, so ".0" is spliced in before the exponent (at exp_pos) to
+  # form a valid float literal.
+  defp finish_exponent(binary, exp_pos, rest, _dot? = false) do
+    len = byte_size(binary) - byte_size(rest)
+
+    literal =
+      IO.iodata_to_binary([
+        :binary.part(binary, 0, exp_pos),
+        ".0",
+        :binary.part(binary, exp_pos, len - exp_pos)
+      ])
+
+    {:erlang.binary_to_float(literal), rest}
+  rescue
+    ArgumentError -> :error
+  end
+
+  defp consumed(binary, ""), do: binary
+  defp consumed(binary, rest), do: :binary.part(binary, 0, byte_size(binary) - byte_size(rest))

  @doc """
  Rounds a float to the largest float less than or equal to `number`.
@@ -253,7 +286,7 @@ defmodule Float do
  @doc """
  Rounds a float to the smallest float greater than or equal to `number`.

-  `ceil/2` also accepts a precision to round a floating-point value down
+  `ceil/2` also accepts a precision to round a floating-point value up
  to an arbitrary number of fractional digits (between 0 and 15).

  The operation is performed on the binary floating point, without a
@@ -322,7 +355,7 @@ defmodule Float do
  and therefore the number above is internally represented as 5.567499999,
  which explains the behavior above. If you want exact rounding for decimals,
  you must use a decimal library. The behavior above is also in accordance
-  to reference implementations, such as "Correctly Rounded Binary-Decimal and
+  with reference implementations, such as "Correctly Rounded Binary-Decimal and
  Decimal-Binary Conversions" by David M. Gay.

  ## Examples
@@ -344,15 +377,12 @@ defmodule Float do

  """
  @spec round(float, precision_range) :: float
-  # This implementation is slow since it relies on big integers.
-  # Faster implementations are available on more recent papers
-  # and could be implemented in the future.
  def round(float, precision \\ 0)

-  def round(float, 0) when float == 0.0, do: float
+  def round(float, 0) when float === 0.0 or float === -0.0, do: float

  def round(float, 0) when is_float(float) do
-    case float |> :erlang.round() |> :erlang.float() do
+    case :erlang.round(float) * 1.0 do
      zero when zero == 0.0 and float < 0.0 -> -0.0
      rounded -> rounded
    end
@@ -366,140 +396,170 @@ defmodule Float do
    raise ArgumentError, invalid_precision_message(precision)
  end

+  # Decimal-place rounding via exact rational scaling. This is the bignum
+  # core used by reference implementations like David M. Gay's "Correctly
+  # Rounded Binary-Decimal and Decimal-Binary Conversions" (cited in the
+  # @doc above), Python's round(), and Java's BigDecimal.setScale.
+  #
+  # 1. Decompose float exactly: |float| = mantissa / 2^shift.
+  # 2. Scale exactly: |float| * 10^precision = mantissa * 10^precision / 2^shift.
+  #    Because precision is bounded to 0..15, the product fits in ~103 bits
+  #    (53-bit mantissa + ~50-bit power of ten) and BEAM bignums handle it
+  #    directly without approximation.
+  # 3. Round the exact rational to an integer per the requested mode
+  #    (half_up / floor / ceil) using quotient and remainder.
+  # 4. Emit the float closest to rounded_int / 10^precision:
+  #    - fast path: when rounded_int < 2^53, both operands are exactly
+  #      representable as floats and IEEE division is correctly rounded.
+  #    - slow path: bignum alignment + manual mantissa extraction with
+  #      round-to-nearest-even for the trailing bit.
+  #
+  # The integer-rounding decision (step 3) and the binary-emission decision
+  # (step 4) are deliberately independent: step 3 picks the exact rational
+  # the user asked for, step 4 picks the closest float to that rational.
+  # Conflating them is the classic source of double-rounding bugs.
+  #
+  # Faster algorithms exist (Cox 2026's table-based uscale; Ryū / Schubfach
+  # for round-trip printing) but target different problems or assume
+  # fixed-width machine arithmetic that BEAM doesn't expose efficiently.
+  # At precision <= 15, the exact path is small, easy to audit, and fast
+  # enough that a more complex algorithm has not been justified by benchmarks.
  defp round(num, _precision, _rounding) when is_float(num) and num == 0.0, do: num

-  defp round(float, precision, rounding) do
-    <<sign::1, exp::11, significant::52-bitstring>> = <<float::float>>
-    {num, count} = decompose(significant, 1)
-    count = count - exp + 1023
+  defp round(float, precision, mode) do
+    <<sign::1, exp::11, mantissa::52>> = <<float::float>>

    cond do
-      # Precision beyond 15 digits
-      count >= 104 ->
-        case rounding do
-          :ceil when sign === 0 -> 1 / power_of_10(precision)
-          :floor when sign === 1 -> -1 / power_of_10(precision)
-          :ceil when sign === 1 -> minus_zero()
-          :half_up when sign === 1 -> minus_zero()
-          _ -> 0.0
-        end
+      # Subnormal — tiny but non-zero; treat per-mode (ceil(+) and floor(-) bump
+      # to 10^-precision; everything else rounds to signed zero).
+      exp == 0 ->
+        tiny_round(sign, precision, mode)

-      # We are asking more precision than we have
-      count <= precision ->
+      # |float| >= 2^52 — has no fractional bits, return unchanged.
+      exp - 1075 >= 0 ->
        float

      true ->
-        # Difference in precision between float and asked precision
-        # We subtract 1 because we need to calculate the remainder too
-        diff = count - precision - 1
-
-        # Get up to latest so we calculate the remainder
-        power_of_10 = power_of_10(diff)
-
-        # Convert the numerand to decimal base
-        num = num * power_of_5(count)
-
-        # Move to the given precision - 1
-        num = div(num, power_of_10)
-        div = div(num, 10)
-        num = rounding(rounding, sign, num, div)
-
-        # Convert back to float without loss
-        # https://www.exploringbinary.com/correct-decimal-to-floating-point-using-big-integers/
-        den = power_of_10(precision)
-        boundary = den <<< 52
-
-        cond do
-          num == 0 and sign == 1 ->
-            minus_zero()
-
-          num == 0 ->
-            0.0
-
-          num >= boundary ->
-            {den, exp} = scale_down(num, boundary, 52)
-            decimal_to_float(sign, num, den, exp)
-
-          true ->
-            {num, exp} = scale_up(num, boundary, 52)
-            decimal_to_float(sign, num, den, exp)
-        end
+        mantissa = @power_of_2_to_52 ||| mantissa
+        shift = 1075 - exp
+        do_round(sign, mantissa, shift, precision, mode)
    end
  end

-  # TODO remove once we require Erlang/OTP 27+
-  # This function tricks the compiler to avoid this bug in previous versions:
-  # https://github.com/elixir-lang/elixir/blob/main/lib/elixir/lib/float.ex#L408-L412
-  defp minus_zero, do: -0.0
-
-  defp decompose(significant, initial) do
-    decompose(significant, 1, 0, initial)
+  # |float * 10^precision| < 0.5 — integer round is 0; ceil/floor still bump per sign.
+  defp do_round(sign, _mantissa, shift, precision, mode) when shift >= 104 do
+    tiny_round(sign, precision, mode)
  end

-  defp decompose(<<1::1, bits::bitstring>>, count, last_count, acc) do
-    decompose(bits, count + 1, count, (acc <<< (count - last_count)) + 1)
-  end
+  defp do_round(sign, mantissa, shift, precision, mode) do
+    power = power_of_10(precision)
+    product = mantissa * power
+    half = 1 <<< (shift - 1)
+    quotient = product >>> shift
+    remainder = product - (quotient <<< shift)
+    rounded_int = round_step(mode, sign, quotient, remainder, half)

-  defp decompose(<<0::1, bits::bitstring>>, count, last_count, acc) do
-    decompose(bits, count + 1, last_count, acc)
-  end
+    cond do
+      rounded_int == 0 ->
+        signed_zero(sign)

-  defp decompose(<<>>, _count, last_count, acc) do
-    {acc, last_count}
-  end
+      rounded_int < @power_of_2_to_52 <<< 1 ->
+        # Both rounded_int and power fit in 53 bits, so IEEE float division
+        # is correctly rounded.
+        result = rounded_int / power
+        if sign == 1, do: -result, else: result

-  defp scale_up(num, boundary, exp) when num >= boundary, do: {num, exp}
-  defp scale_up(num, boundary, exp), do: scale_up(num <<< 1, boundary, exp - 1)
-
-  defp scale_down(num, den, exp) do
-    new_den = den <<< 1
-
-    if num < new_den do
-      {den >>> 52, exp}
-    else
-      scale_down(num, new_den, exp + 1)
+      true ->
+        bignum_to_float(sign, rounded_int, power)
    end
  end

-  defp decimal_to_float(sign, num, den, exp) do
-    quo = div(num, den)
-    rem = num - quo * den
+  defp round_step(:half_up, _sign, quotient, remainder, half) do
+    if remainder >= half, do: quotient + 1, else: quotient
+  end

-    tmp =
-      case den >>> 1 do
-        den when rem > den -> quo + 1
-        den when rem < den -> quo
-        _ when (quo &&& 1) === 1 -> quo + 1
-        _ -> quo
+  defp round_step(:floor, 0, quotient, _remainder, _half), do: quotient
+  defp round_step(:floor, 1, quotient, remainder, _half) when remainder > 0, do: quotient + 1
+  defp round_step(:floor, 1, quotient, _remainder, _half), do: quotient
+
+  defp round_step(:ceil, 0, quotient, remainder, _half) when remainder > 0, do: quotient + 1
+  defp round_step(:ceil, 0, quotient, _remainder, _half), do: quotient
+  defp round_step(:ceil, 1, quotient, _remainder, _half), do: quotient
+
+  defp signed_zero(0), do: 0.0
+  defp signed_zero(1), do: -0.0
+
+  # Result of rounding a non-zero float whose |float * 10^precision| < 0.5.
+  # ceil(+) → +10^-precision, floor(-) → -10^-precision, others → signed 0.
+  defp tiny_round(0, precision, :ceil), do: 1.0 / power_of_10(precision)
+  defp tiny_round(1, precision, :floor), do: -1.0 / power_of_10(precision)
+  defp tiny_round(sign, _precision, _mode), do: signed_zero(sign)
+
+  # Slow path: emit float closest to `sign * rounded_int / power` when
+  # rounded_int >= 2^53. The binary emission step is always IEEE
+  # round-to-nearest-even, regardless of the integer-rounding mode.
+  defp bignum_to_float(sign, rounded_int, power) do
+    shift_adjust = bit_length(rounded_int) - bit_length(power) - 53
+    {numerator, denominator, exp} = align(rounded_int, power, shift_adjust)
+
+    quotient = div(numerator, denominator)
+    remainder = numerator - quotient * denominator
+    half = denominator >>> 1
+
+    mantissa =
+      cond do
+        remainder > half -> quotient + 1
+        remainder < half -> quotient
+        (quotient &&& 1) === 1 -> quotient + 1
+        true -> quotient
      end

-    tmp = tmp - @power_of_2_to_52
-    <<tmp::float>> = <<sign::1, exp + 1023::11, tmp::52>>
-    tmp
+    # Carry-bit normalization: `mantissa` lives in [2^52, 2^53]. The upper
+    # bound `2^53` is reachable when `align/3` returns an upper-bound quotient
+    # or when rounding carries. Rebalance into the canonical [2^52, 2^53)
+    # range so the 52-bit packing below doesn't silently truncate.
+    {mantissa, exp} =
+      if mantissa == @power_of_2_to_52 <<< 1,
+        do: {@power_of_2_to_52, exp + 1},
+        else: {mantissa, exp}
+
+    <<result::float>> = <<sign::1, exp + 1023::11, mantissa - @power_of_2_to_52::52>>
+    result
  end

-  defp rounding(:floor, 1, _num, div), do: div + 1
-  defp rounding(:ceil, 0, _num, div), do: div + 1
+  # Pick (numerator, denominator, exp) so that numerator/denominator ∈ [2^52, 2^53)
+  # and the resulting float = numerator/denominator * 2^(exp-52).
+  defp align(rounded_int, power, shift_adjust) when shift_adjust >= 0 do
+    new_power = power <<< shift_adjust

-  defp rounding(:half_up, _sign, num, div) do
-    case rem(num, 10) do
-      rem when rem < 5 -> div
-      rem when rem >= 5 -> div + 1
+    if rounded_int < new_power <<< 53,
+      do: {rounded_int, new_power, 52 + shift_adjust},
+      else: {rounded_int, new_power <<< 1, 53 + shift_adjust}
+  end
+
+  defp align(rounded_int, power, shift_adjust) do
+    shifted = rounded_int <<< -shift_adjust
+
+    cond do
+      shifted >= power <<< 53 -> {shifted, power <<< 1, 53 + shift_adjust}
+      shifted >= power <<< 52 -> {shifted, power, 52 + shift_adjust}
+      true -> {shifted <<< 1, power, 51 + shift_adjust}
    end
  end

-  defp rounding(_, _, _, div), do: div
+  defp bit_length(0), do: 0
+  defp bit_length(integer) when integer > 0, do: bit_length(integer, 0)
+  defp bit_length(integer, acc) when integer >= 1 <<< 64, do: bit_length(integer >>> 64, acc + 64)
+  defp bit_length(integer, acc) when integer >= 1 <<< 16, do: bit_length(integer >>> 16, acc + 16)
+  defp bit_length(integer, acc) when integer >= 1 <<< 4, do: bit_length(integer >>> 4, acc + 4)
+  defp bit_length(integer, acc) when integer >= 1, do: bit_length(integer >>> 1, acc + 1)
+  defp bit_length(_integer, acc), do: acc

-  Enum.reduce(0..104, 1, fn x, acc ->
-    defp power_of_10(unquote(x)), do: unquote(acc)
+  Enum.reduce(0..15, 1, fn exponent, acc ->
+    defp power_of_10(unquote(exponent)), do: unquote(acc)
    acc * 10
  end)

-  Enum.reduce(0..104, 1, fn x, acc ->
-    defp power_of_5(unquote(x)), do: unquote(acc)
-    acc * 5
-  end)
-
  @doc """
  Returns a pair of integers whose ratio is exactly equal
  to the original float and with a positive denominator.
@@ -645,7 +705,7 @@ defmodule Float do
  end

  defp invalid_precision_message(precision) do
-    "precision #{precision} is out of valid range of #{inspect(@precision_range)}"
+    "precision #{inspect(precision)} is out of valid range of #{inspect(@precision_range)}"
  end

  defp expand_compact([{:compact, false} | t]), do: expand_compact(t)
@@ -69,7 +69,6 @@ defmodule Function do
          | :name
          | :new_index
          | :new_uniq
-          | :pid
          | :type
          | :uniq

@@ -112,8 +111,6 @@ defmodule Function do
  When `fun` is an anonymous function (that is, the type is `:local`), the following
  additional keys are returned:

-    * `:pid` - PID of the process that originally created the function.
-
    * `:index` - (integer) an index into the module function table.

    * `:new_index` - (integer) an index into the module function table.
@@ -159,7 +156,7 @@ defmodule Function do
  `:module`, `:name`, `:arity`, `:env`, or `:type`.

  For anonymous functions, there is also information about any of the
-  atoms `:index`, `:new_index`, `:new_uniq`, `:uniq`, and `:pid`.
+  atoms `:index`, `:new_index`, `:new_uniq`, and `:uniq`.
  For a named function, the value of any of these items is always the
  atom `:undefined`.

@@ -179,8 +176,6 @@ defmodule Function do
      iex> fun = &String.length/1
      iex> Function.info(fun, :name)
      {:name, :length}
-      iex> Function.info(fun, :pid)
-      {:pid, :undefined}

  """
  @doc since: "1.7.0"
@@ -36,7 +36,7 @@ defmodule GenEvent do
  alternative. GenStage is an external Elixir library maintained by the Elixir
  team; it provides a tool to implement systems that exchange events in a
  demand-driven way with built-in support for back-pressure. See the [GenStage
-  documentation](https://hexdocs.pm/gen_stage) for more information.
+  documentation](https://gen-stage.hexdocs.pm) for more information.

  ### `:gen_event`

@@ -232,6 +232,8 @@ defmodule GenServer do
  a name on start via the `:name` option. Registered names are also
  automatically cleaned up on termination. The supported values are:

+    * `nil` (default) - the GenServer is not registered with a name.
+
    * an atom - the GenServer is registered locally (to the current node)
      with the given name using `Process.register/2`.

@@ -565,9 +567,12 @@ defmodule GenServer do
     `Supervisor`. Likely this approach involves calling `Supervisor.restart_child/2`
      after a delay to attempt a restart.

-  Returning `{:stop, reason}` will cause `start_link/3` to return
-  `{:error, reason}` and the process to exit with reason `reason` without
-  entering the loop or calling `c:terminate/2`.
+  Returning `{:error, reason}` will cause `start_link/3` to return
+  `{:error, reason}`.
+
+  Returning `{:stop, reason}` will the process to exit with reason `reason`,
+  without entering the loop or calling `c:terminate/2`. `start_link/3` will
+  return `{:error, reason}`, but only if the caller is trapping exits.
  """
  @callback init(init_arg :: term) ::
              {:ok, state}
@@ -858,7 +863,7 @@ defmodule GenServer do
  @type on_start :: {:ok, pid} | :ignore | {:error, {:already_started, pid} | term}

  @typedoc "The GenServer name"
-  @type name :: atom | {:global, term} | {:via, module, term}
+  @type name :: nil | atom | {:global, term} | {:via, module, term}

  @typedoc "Options used by the `start*` functions"
  @type options :: [option]
@@ -1153,7 +1158,7 @@ defmodule GenServer do

  ## Timeouts

-  `timeout` is an integer greater than zero which specifies how many
+  `timeout` is a non-negative integer which specifies how many
  milliseconds to wait for a reply, or the atom `:infinity` to wait
  indefinitely. The default value is `5000`. If no reply is received within
  the specified time, the function call fails and the caller exits. If the
@@ -265,13 +265,13 @@ defimpl Enumerable, for: HashDict do

  def reduce(dict, acc, fun) do
    # Avoid warnings about HashDict being deprecated.
-    module = String.to_atom("HashDict")
+    module = String.to_unsafe_atom("HashDict")
    module.reduce(dict, acc, fun)
  end

  def member?(dict, {key, value}) do
    # Avoid warnings about HashDict being deprecated.
-    module = String.to_atom("HashDict")
+    module = String.to_unsafe_atom("HashDict")
    {:ok, match?({:ok, ^value}, module.fetch(dict, key))}
  end

@@ -281,7 +281,7 @@ defimpl Enumerable, for: HashDict do

  def count(dict) do
    # Avoid warnings about HashDict being deprecated.
-    module = String.to_atom("HashDict")
+    module = String.to_unsafe_atom("HashDict")
    {:ok, module.size(dict)}
  end

@@ -296,7 +296,7 @@ defimpl Collectable, for: HashDict do

  def into(original) do
    # Avoid warnings about HashDict being deprecated.
-    module = String.to_atom("HashDict")
+    module = String.to_unsafe_atom("HashDict")

    collector_fun = fn
      dict, {:cont, {key, value}} -> module.put(dict, key, value)
@@ -315,7 +315,7 @@ defimpl Inspect, for: HashDict do

  def inspect(dict, opts) do
    # Avoid warnings about HashDict being deprecated.
-    module = String.to_atom("HashDict")
+    module = String.to_unsafe_atom("HashDict")
    concat(["#HashDict<", Inspect.List.inspect(module.to_list(dict), opts), ">"])
  end
 end
@@ -279,19 +279,19 @@ defimpl Enumerable, for: HashSet do

  def reduce(set, acc, fun) do
    # Avoid warnings about HashSet being deprecated.
-    module = String.to_atom("HashSet")
+    module = String.to_unsafe_atom("HashSet")
    module.reduce(set, acc, fun)
  end

  def member?(set, term) do
    # Avoid warnings about HashSet being deprecated.
-    module = String.to_atom("HashSet")
+    module = String.to_unsafe_atom("HashSet")
    {:ok, module.member?(set, term)}
  end

  def count(set) do
    # Avoid warnings about HashSet being deprecated.
-    module = String.to_atom("HashSet")
+    module = String.to_unsafe_atom("HashSet")
    {:ok, module.size(set)}
  end

@@ -306,7 +306,7 @@ defimpl Collectable, for: HashSet do

  def into(original) do
    # Avoid warnings about HashSet being deprecated.
-    module = String.to_atom("HashSet")
+    module = String.to_unsafe_atom("HashSet")

    collector_fun = fn
      set, {:cont, term} -> module.put(set, term)
@@ -325,7 +325,7 @@ defimpl Inspect, for: HashSet do

  def inspect(set, opts) do
    # Avoid warnings about HashSet being deprecated.
-    module = String.to_atom("HashSet")
+    module = String.to_unsafe_atom("HashSet")
    concat(["#HashSet<", Inspect.List.inspect(module.to_list(set), opts), ">"])
  end
 end
@@ -92,7 +92,7 @@ defprotocol Inspect do
      end

      inspect(%Point{x: 1})
-      %Point{x: 1, y: 0}
+      #=> %Point{x: 1, y: 0}

  ## Custom implementation

@@ -387,13 +387,20 @@ defimpl Inspect, for: List do
    close = color_doc("]", :list, opts)

    cond do
-      lists == :as_charlists or (lists == :infer and List.ascii_printable?(term, printable_limit)) ->
-        inspected =
-          case Identifier.escape(IO.chardata_to_string(term), ?", printable_limit) do
-            {escaped, ""} -> [?~, ?c, ?", escaped, ?"]
-            {escaped, _} -> [?~, ?c, ?", escaped, ?", " ++ ..."]
+      (lists == :as_charlists and unicode_list?(term, printable_limit)) or
+          (lists == :infer and List.ascii_printable?(term, printable_limit)) ->
+        {split, tail} =
+          if is_integer(printable_limit) do
+            case Enum.split(term, printable_limit) do
+              {split, []} -> {split, []}
+              {split, _} -> {split, " ++ ..."}
+            end
+          else
+            {term, []}
          end

+        {escaped, _} = Identifier.escape(IO.chardata_to_string(split), ?")
+        inspected = [?~, ?c, ?", escaped, ?" | tail]
        color_doc(IO.iodata_to_binary(inspected), :charlist, opts)

      keyword?(term) ->
@@ -407,6 +414,19 @@ defimpl Inspect, for: List do
    end
  end

+  defp unicode_list?(_, 0), do: true
+
+  defp unicode_list?([char | rest], counter)
+       when char in 0..0xD7FF or char in 0xE000..0x10FFFF,
+       do: unicode_list?(rest, decrement(counter))
+
+  defp unicode_list?([], _counter), do: true
+  defp unicode_list?(_, _counter), do: false
+
+  @compile {:inline, decrement: 1}
+  defp decrement(:infinity), do: :infinity
+  defp decrement(counter), do: counter - 1
+
  @doc false
  def keyword({key, value}, opts) do
    key = color_doc(Macro.inspect_atom(:key, key), :atom, opts)
@@ -416,8 +436,8 @@ defimpl Inspect, for: List do

  @doc false
  def keyword?([{key, _value} | rest]) when is_atom(key) do
-    case Atom.to_charlist(key) do
-      [?E, ?l, ?i, ?x, ?i, ?r, ?.] ++ _ -> false
+    case Atom.to_string(key) do
+      "Elixir." <> _ -> false
      _ -> keyword?(rest)
    end
  end
@@ -669,6 +689,11 @@ defimpl Inspect, for: Any do
    Inspect.Map.inspect_as_struct(struct, Macro.inspect_atom(:literal, module), info, opts)
  end

+  # A temporary clause to deal with native records until they are officially supported
+  def inspect(native_record, _opts) do
+    :io_lib.format("~p", [native_record]) |> IO.iodata_to_binary()
+  end
+
  def inspect_as_struct(map, name, infos, opts) do
    open = color_doc("#" <> name <> "<", :map, opts)
    sep = color_doc(",", :map, opts)
@@ -21,8 +21,8 @@ defmodule Inspect.Opts do
      is `:decimal` and if it is printable, otherwise in bit syntax. See
      `String.printable?/1` to learn when a string is printable.

-    * `:charlists` - when `:as_charlists` all lists will be printed as charlists,
-      non-printable elements will be escaped.
+    * `:charlists` - when `:as_charlists` all charlists will be printed as charlists,
+      non-printable code points will be escaped. Other lists will be printed as lists.

      When `:as_lists` all lists will be printed as lists.

@@ -46,8 +46,8 @@ defmodule Inspect.Opts do
    * `:limit` - limits the number of items that are inspected for tuples,
      bitstrings, maps, lists and any other collection of items, with the exception of
      printable strings and printable charlists which use the `:printable_limit` option.
-      It accepts a positive integer or `:infinity`. It defaults to `100` since
-      `Elixir v1.19.0`, as it has better defaults to deal with nested collections.
+      It accepts a positive integer or `:infinity`. It defaults to `200` since
+      `Elixir v1.20.0`, as it has better defaults to deal with nested collections.

    * `:pretty` - if set to `true` enables pretty printing. Defaults to `false`.

@@ -90,7 +90,7 @@ defmodule Inspect.Opts do
            charlists: :infer,
            custom_options: [],
            inspect_fun: &Inspect.inspect/2,
-            limit: 100,
+            limit: 200,
            pretty: false,
            printable_limit: 4096,
            safe: true,
@@ -108,7 +108,7 @@ defmodule Integer do
  defguard is_even(integer) when is_integer(integer) and (integer &&& 1) == 0

  @doc """
-  Computes `base` raised to power of `exponent`.
+  Computes `base` raised to the power of `exponent`.

  Both `base` and `exponent` must be integers.
  The exponent must be zero or positive.
@@ -297,8 +297,9 @@ defmodule Integer do

  defp undigits([], _base, acc), do: acc

-  defp undigits([digit | _], base, _) when is_integer(digit) and digit >= base,
-    do: raise(ArgumentError, "invalid digit #{digit} in base #{base}")
+  defp undigits([digit | _], base, _)
+       when is_integer(digit) and (digit >= base or digit <= -base),
+       do: raise(ArgumentError, "invalid digit #{digit} in base #{base}")

  defp undigits([digit | tail], base, acc) when is_integer(digit),
    do: undigits(tail, base, acc * base + digit)
@@ -531,8 +532,12 @@ defmodule Integer do

      iex> Integer.extended_gcd(10, 0)
      {10, 1, 0}
+      iex> Integer.extended_gcd(-10, 0)
+      {10, -1, 0}
      iex> Integer.extended_gcd(0, 10)
      {10, 0, 1}
+      iex> Integer.extended_gcd(0, -10)
+      {10, 0, -1}
      iex> Integer.extended_gcd(0, 0)
      {0, 0, 0}

@@ -540,8 +545,10 @@ defmodule Integer do
  @doc since: "1.12.0"
  @spec extended_gcd(integer, integer) :: {non_neg_integer, integer, integer}
  def extended_gcd(0, 0), do: {0, 0, 0}
-  def extended_gcd(0, b), do: {b, 0, 1}
-  def extended_gcd(a, 0), do: {a, 1, 0}
+  def extended_gcd(0, b) when is_integer(b) and b > 0, do: {b, 0, 1}
+  def extended_gcd(0, b) when is_integer(b) and b < 0, do: {-b, 0, -1}
+  def extended_gcd(a, 0) when is_integer(a) and a > 0, do: {a, 1, 0}
+  def extended_gcd(a, 0) when is_integer(a) and a < 0, do: {-a, -1, 0}

  def extended_gcd(integer1, integer2) when is_integer(integer1) and is_integer(integer2) do
    extended_gcd(integer2, integer1, 0, 1, 1, 0)
@@ -128,7 +128,7 @@ defmodule IO do
  @type nodata :: {:error, term} | :eof
  @type chardata :: String.t() | maybe_improper_list(char | chardata, String.t() | [])

-  @type inspect_opts :: [Inspect.Opts.new_opt() | {:label, term}]
+  @type inspect_opts :: [Inspect.Opts.new_opt() | {:label, String.Chars.t()}]

  @typedoc """
  Stacktrace information as keyword options for `warn/2`.
@@ -152,9 +152,9 @@ defmodule IO do

  The `device` is iterated as specified by the `line_or_chars` argument:

-    * if `line_or_chars` is an integer, it represents a number of bytes. The device is
-      iterated by that number of bytes. This should be the preferred mode for reading
-      non-textual inputs.
+    * if `line_or_chars` is an integer, it is the number of Unicode
+      code points to be retrieved for devices open in Unicode/utf8 mode.
+      Otherwise, it is the number of raw bytes to be retrieved.

    * if `line_or_chars` is `:line`, the device is iterated line by line.
      CRLF newlines  ("\r\n") are automatically normalized to "\n".
@@ -521,7 +521,7 @@ defmodule IO do
  end

  @doc """
-  Gets a number of bytes from IO device `:stdio`.
+  Gets a number of characters from IO device `:stdio`.

  If `:stdio` is a Unicode device, `count` implies
  the number of Unicode code points to be retrieved.
@@ -549,7 +549,7 @@ defmodule IO do
  end

  @doc """
-  Gets a number of bytes from the IO `device`.
+  Gets a number of characters from the IO `device`.

  If the IO `device` is a Unicode device, `count` implies
  the number of Unicode code points to be retrieved.
@@ -111,13 +111,15 @@ defmodule IO.ANSI do
  end

  defsequence = fn name, code, terminator ->
+    sequence = "\e[#{code}#{terminator}"
+
    @spec unquote(name)() :: String.t()
    def unquote(name)() do
-      "\e[#{unquote(code)}#{unquote(terminator)}"
+      unquote(sequence)
    end

    defp format_sequence(unquote(name)) do
-      unquote(name)()
+      unquote(sequence)
    end
  end

@@ -159,7 +161,7 @@ defmodule IO.ANSI do

  for font_n <- [1, 2, 3, 4, 5, 6, 7, 8, 9] do
    @doc "Sets alternative font #{font_n}."
-    defsequence.(:"font_#{font_n}", font_n + 10, "m")
+    defsequence.(String.to_unsafe_atom("font_#{font_n}"), font_n + 10, "m")
  end

  @doc "Normal color or intensity."
@@ -193,13 +195,13 @@ defmodule IO.ANSI do
    defsequence.(color, code + 30, "m")

    @doc "Sets foreground color to light #{color}."
-    defsequence.(:"light_#{color}", code + 90, "m")
+    defsequence.(String.to_unsafe_atom("light_#{color}"), code + 90, "m")

    @doc "Sets background color to #{color}."
-    defsequence.(:"#{color}_background", code + 40, "m")
+    defsequence.(String.to_unsafe_atom("#{color}_background"), code + 40, "m")

    @doc "Sets background color to light #{color}."
-    defsequence.(:"light_#{color}_background", code + 100, "m")
+    defsequence.(String.to_unsafe_atom("light_#{color}_background"), code + 100, "m")
  end

  @doc "Default text color."
@@ -332,7 +334,7 @@ defmodule IO.ANSI do
  end

  defp do_format([], [], acc, true, true) do
-    [acc | IO.ANSI.reset()]
+    [acc | reset()]
  end

  defp do_format([], [], acc, _emit?, _append_reset) do
@@ -673,7 +673,7 @@ defmodule IO.ANSI.Docs do

  # Characters that can mark the beginning or the end of a word.
  # Only support the most common ones at this moment.
-  @delimiters [?\s, ?', ?", ?!, ?@, ?#, ?$, ?%, ?^, ?&] ++
+  @delimiters [?\s, ?', ?", ?!, ??, ?,, ?:, ?;, ?/, ?@, ?#, ?$, ?%, ?^, ?&] ++
                [?-, ?+, ?(, ?), ?[, ?], ?{, ?}, ?<, ?>, ?.]

  ### Inline start
@@ -31,7 +31,7 @@ defmodule IO.Stream do
  @type t :: %__MODULE__{
          device: IO.device(),
          raw: boolean(),
-          line_or_bytes: :line | non_neg_integer()
+          line_or_bytes: :line | pos_integer()
        }

  @doc false
@@ -24,7 +24,7 @@ defprotocol JSON.Encoder do
  > #### Leaking Private Information {: .error}
  >
  > Prefer using `:only` to avoid accidentally leaking private information when
-  > new fields are added. Other approaches should be used with auction.
+  > new fields are added. Other approaches should be used with caution.

  You can also use `Protocol.derive/3` if you don't own the struct that you want
  to encode to JSON:
@@ -67,7 +67,7 @@ defprotocol JSON.Encoder do

    {io, _prefix} =
      Enum.flat_map_reduce(kv, ?{, fn {field, value}, prefix ->
-        key = IO.iodata_to_binary([prefix, :elixir_json.encode_binary(Atom.to_string(field)), ?:])
+        key = IO.iodata_to_binary([prefix, :json.encode_binary(Atom.to_string(field)), ?:])
        {[key, quote(do: encoder.(unquote(value), encoder))], ?,}
      end)

@@ -130,25 +130,25 @@ end

 defimpl JSON.Encoder, for: BitString do
  def encode(value, _encoder) do
-    :elixir_json.encode_binary(value)
+    :json.encode_binary(value)
  end
 end

 defimpl JSON.Encoder, for: List do
  def encode(value, encoder) do
-    :elixir_json.encode_list(value, encoder)
+    :json.encode_list(value, encoder)
  end
 end

 defimpl JSON.Encoder, for: Integer do
  def encode(value, _encoder) do
-    :elixir_json.encode_integer(value)
+    :json.encode_integer(value)
  end
 end

 defimpl JSON.Encoder, for: Float do
  def encode(value, _encoder) do
-    :elixir_json.encode_float(value)
+    :json.encode_float(value)
  end
 end

@@ -175,6 +175,7 @@ defimpl JSON.Encoder, for: Map do

  # Erlang supports only numbers, binaries, and atoms as keys,
  # we support anything that implements the String.Chars protocol.
+  @compile inline: [key: 2]
  defp key(key, encoder) when is_atom(key), do: encoder.(Atom.to_string(key), encoder)
  defp key(key, encoder) when is_binary(key), do: encoder.(key, encoder)
  defp key(key, encoder), do: encoder.(String.Chars.to_string(key), encoder)
@@ -408,7 +409,7 @@ defmodule JSON do
    decoders = Keyword.put_new(decoders, :null, nil)

    try do
-      :elixir_json.decode(binary, acc, Map.new(decoders))
+      :json.decode(binary, acc, Map.new(decoders))
    catch
      :error, :unexpected_end ->
        {:error, {:unexpected_end, byte_size(binary)}}
@@ -528,16 +529,16 @@ defmodule JSON do
  end

  def protocol_encode(value, _encoder) when is_binary(value),
-    do: :elixir_json.encode_binary(value)
+    do: :json.encode_binary(value)

  def protocol_encode(value, _encoder) when is_integer(value),
-    do: :elixir_json.encode_integer(value)
+    do: :json.encode_integer(value)

  def protocol_encode(value, _encoder) when is_float(value),
-    do: :elixir_json.encode_float(value)
+    do: :json.encode_float(value)

  def protocol_encode(value, encoder) when is_list(value),
-    do: :elixir_json.encode_list(value, encoder)
+    do: :json.encode_list(value, encoder)

  def protocol_encode(%{} = value, encoder) when not is_map_key(value, :__struct__),
    do: JSON.Encoder.Map.encode(value, encoder)
@@ -142,7 +142,7 @@ defmodule Kernel do
    * [Patterns and guards](patterns-and-guards.md) - an introduction to patterns,
      guards, and extensions
    * [Syntax reference](syntax-reference.md) - the language syntax reference
-    * [Typespecs reference](typespecs.md)- types and function specifications, including list of types
+    * [Typespecs reference](typespecs.md) - types and function specifications, including list of types
    * [Unicode syntax](unicode-syntax.md) - outlines Elixir support for Unicode

  ## Guards
@@ -2109,7 +2109,7 @@ defmodule Kernel do
    assert_no_match_or_guard_scope(__CALLER__.context, "!")

    annotate_case(
-      [optimize_boolean: true, type_check: {:case, :!}],
+      [optimize_boolean: true, type_check: {:case, :"!!"}],
      quote do
        case unquote(value) do
          x when unquote(x_is_false_or_nil()) -> false
@@ -2752,7 +2752,7 @@ defmodule Kernel do
      nil ->
        quote do
          case unquote(term) do
-            %_{__exception__: true} -> true
+            %_{__exception__: _} -> true
            _ -> false
          end
        end
@@ -2764,8 +2764,7 @@ defmodule Kernel do
        quote do
          is_map(unquote(term)) and :erlang.is_map_key(:__struct__, unquote(term)) and
            is_atom(:erlang.map_get(:__struct__, unquote(term))) and
-            :erlang.is_map_key(:__exception__, unquote(term)) and
-            :erlang.map_get(:__exception__, unquote(term)) == true
+            :erlang.is_map_key(:__exception__, unquote(term))
        end
    end
  end
@@ -2792,7 +2791,7 @@ defmodule Kernel do
          case unquote(name) do
            name when is_atom(name) ->
              case unquote(term) do
-                %{__struct__: ^name, __exception__: true} -> true
+                %{__struct__: ^name, __exception__: _} -> true
                _ -> false
              end

@@ -2810,8 +2809,7 @@ defmodule Kernel do
            (is_atom(unquote(name)) or :fail) and
            :erlang.is_map_key(:__struct__, unquote(term)) and
            :erlang.map_get(:__struct__, unquote(term)) == unquote(name) and
-            :erlang.is_map_key(:__exception__, unquote(term)) and
-            :erlang.map_get(:__exception__, unquote(term)) == true
+            :erlang.is_map_key(:__exception__, unquote(term))
        end
    end
  end
@@ -3090,7 +3088,7 @@ defmodule Kernel do
  @doc """
  Pops a key from the given nested structure.

-  Uses the `Access` protocol to traverse the structures
+  Uses the `Access` behaviour to traverse the structures
  according to the given `keys`, unless the `key` is a
  function. If the key is a function, it will be invoked
  as specified in `get_and_update_in/3`.
@@ -4705,10 +4703,8 @@ defmodule Kernel do
        false

      [] ->
-        quote do
-          _ = unquote(left)
-          false
-        end
+        # inlined as false in erlang pass
+        quote(do: :lists.member(unquote(left), []))

      [head | tail] = list ->
        case in_body? do
@@ -4725,7 +4721,7 @@ defmodule Kernel do
            in_var(in_body?, left, &in_range(&1, expand.(first), expand.(last), expand.(step)))

          _ when in_body? ->
-            quote(do: Elixir.Enum.member?(unquote(right), unquote(left)))
+            quote(do: Elixir.Enum.__in__(unquote(left), unquote(right)))

          _ ->
            raise_on_invalid_args_in_2(right)
@@ -4964,6 +4960,7 @@ defmodule Kernel do

  """
  @doc since: "1.14.0"
+  @spec binary_slice(binary, integer, non_neg_integer) :: binary
  def binary_slice(binary, start, size)
      when is_binary(binary) and is_integer(start) and is_integer(size) and size >= 0 do
    total = byte_size(binary)
@@ -5037,6 +5034,7 @@ defmodule Kernel do

  """
  @doc since: "1.14.0"
+  @spec binary_slice(binary, Range.t()) :: binary
  def binary_slice(binary, first..last//step)
      when is_binary(binary) and step > 0 do
    total = byte_size(binary)
@@ -5151,7 +5149,7 @@ defmodule Kernel do
  warning saying that a module has been redefined.

  There are some modules that Elixir does not currently implement but it
-  may be implement in the future. Those modules are reserved and defining
+  may implement in the future. Those modules are reserved and defining
  them will result in a compilation error:

      defmodule Any do
@@ -5252,12 +5250,12 @@ defmodule Kernel do
  # defmodule Alias nested
  defp alias_defmodule({:__aliases__, _, [h | t]}, _module, env) when is_atom(h) do
    module = :elixir_aliases.concat([env.module, h])
-    alias = String.to_atom("Elixir." <> Atom.to_string(h))
+    alias = String.to_unsafe_atom("Elixir." <> Atom.to_string(h))
    opts = [as: alias, warn: false]

    case t do
      [] -> {module, module, opts}
-      _ -> {String.to_atom(Enum.join([module | t], ".")), module, opts}
+      _ -> {String.to_unsafe_atom(Enum.join([module | t], ".")), module, opts}
    end
  end

@@ -5564,7 +5562,7 @@ defmodule Kernel do
  when the struct is printed:

      defmodule User do
-        @derive {Inspect, only: :name}
+        @derive {Inspect, only: [:name]}
        defstruct name: nil, age: nil
      end

@@ -5591,6 +5589,9 @@ defmodule Kernel do
        defstruct name: nil, age: 10 + 11
      end

+  `@enforce_keys` must be set to an atom or a list of unique atoms,
+  all of which must name fields defined by `defstruct/1`
+
  Now trying to build a struct without the name key will fail:

      %User{age: 21}
@@ -5875,11 +5876,15 @@ defmodule Kernel do
  end

  @doc """
-  Defines a macro suitable for use in guard expressions.
+  Defines a custom guard with the given name.

-  It raises at compile time if the `guard` uses expressions that aren't
-  allowed in [guard clauses](patterns-and-guards.html#guards),
-  and otherwise creates a macro that can be used both inside or outside guards.
+  Once defined, custom guards can be invoked within regular code or in
+  guards. The module that contains the custom guard must be required before usage.
+
+  Custom guards are defined by providing a valid guard expression to
+  the right-hand side of `when`. `defguard` will then expand and validate
+  the expressions as guards. `defguard` will raise at compile time if the
+  guard uses expressions that aren't allowed in [guard clauses](patterns-and-guards.html#guards).

  When defining your own guards, consider the
  [naming conventions](naming-conventions.html#is_-prefix-is_foo)
@@ -5887,31 +5892,30 @@ defmodule Kernel do

  ## Example

+  For example, to define a guard similar to `Integer.is_even/1`, you can write:
+
      defmodule Integer.Guards do
        defguard is_even(value) when is_integer(value) and rem(value, 2) == 0
      end

-      defmodule Collatz do
-        @moduledoc "Tools for working with the Collatz sequence."
-        import Integer.Guards
+  which can then be used as:

-        @doc "Determines the number of steps `n` takes to reach `1`."
-        # If this function never converges, please let me know what `n` you used.
-        def converge(n) when n > 0, do: step(n, 0)
+      require Integer.Guards
+      Integer.Guards.is_even(3)
+      #=> false

-        defp step(1, step_count) do
-          step_count
-        end
+  ## Implementation details

-        defp step(n, step_count) when is_even(n) do
-          step(div(n, 2), step_count + 1)
-        end
+  Behind the scenes, `defguard` will generate a macro which can be used
+  inside and outside of guards, preserving their respective semantics.

-        defp step(n, step_count) do
-          step(3 * n + 1, step_count + 1)
-        end
-      end
+  When invoked inside a guard, it behaves as if the right-hand side of
+  `when` is injected as part of the guard, replacing the custom guard
+  arguments by the expressions given as inputs.

+  When invoked outside of a guard, it preserves regular function calling
+  semantics with one caveat: all arguments are evaluated before invocation,
+  except arguments which are unused, which are then never evaluated.
  """
  @doc since: "1.6.0"
  @spec defguard(Macro.t()) :: Macro.t()
@@ -6279,7 +6283,7 @@ defmodule Kernel do
  step through the code it sees). For general stepping, you can set breakpoints
  using `IEx.break!/4`.

-  For more information, [see IEx documentation](https://hexdocs.pm/iex/IEx.html#module-dbg-and-breakpoints).
+  For more information, [see IEx documentation](https://iex.hexdocs.pm/IEx.html#module-dbg-and-breakpoints).

  ## Configuring the debug function

@@ -6326,7 +6330,15 @@ defmodule Kernel do
  """
  @doc since: "1.14.0"
  defmacro dbg(code \\ quote(do: binding()), options \\ []) do
-    {mod, fun, args} = Application.compile_env!(__CALLER__, :elixir, :dbg_callback)
+    # The compiling process may override the callback by putting it in
+    # the process dictionary.
+    dbg_callback =
+      case :erlang.get({:elixir, :dbg_callback}) do
+        :undefined -> Application.compile_env!(__CALLER__, :elixir, :dbg_callback)
+        value -> value
+      end
+
+    {mod, fun, args} = dbg_callback
    Macro.compile_apply(mod, fun, [code, options, __CALLER__ | args], __CALLER__)
  end

@@ -6370,7 +6382,7 @@ defmodule Kernel do

  ### Passing timeouts

-  You can also pass timeouts directly to this functions, that is, milliseconds or
+  You can also pass timeouts directly to this function, that is, milliseconds or
  the atom `:infinity`. In this case, this function just returns the given argument.

  ## Examples
@@ -6415,12 +6427,20 @@ defmodule Kernel do
        {microsecond, _precision} = duration.microsecond
        millisecond = :erlang.convert_time_unit(microsecond, :microsecond, :millisecond)

-        duration.week * unquote(week_in_ms) +
-          duration.day * unquote(day_in_ms) +
-          duration.hour * unquote(hour_in_ms) +
-          duration.minute * 60_000 +
-          duration.second * 1000 +
-          millisecond
+        total =
+          duration.week * unquote(week_in_ms) +
+            duration.day * unquote(day_in_ms) +
+            duration.hour * unquote(hour_in_ms) +
+            duration.minute * 60_000 +
+            duration.second * 1000 +
+            millisecond
+
+        if total < 0 do
+          raise ArgumentError,
+                "duration must be positive, got: #{inspect(duration)}"
+        end
+
+        total
    end
  end

@@ -6882,7 +6902,7 @@ defmodule Kernel do
  defp maybe_atomize_calendar(<<alias, _::binary>> = last_part, string)
       when alias >= ?A and alias <= ?Z do
    string = binary_part(string, 0, byte_size(string) - byte_size(last_part) - 1)
-    {String.to_atom("Elixir." <> last_part), string}
+    {String.to_unsafe_atom("Elixir." <> last_part), string}
  end

  defp maybe_atomize_calendar(_last_part, string) do
@@ -6993,7 +7013,7 @@ defmodule Kernel do

        case mod do
          ?s -> parts
-          ?a -> :lists.map(&String.to_atom/1, parts)
+          ?a -> :lists.map(&String.to_unsafe_atom/1, parts)
          ?c -> :lists.map(&String.to_charlist/1, parts)
        end

@@ -7002,7 +7022,7 @@ defmodule Kernel do

        case mod do
          ?s -> parts
-          ?a -> quote(do: :lists.map(&String.to_atom/1, unquote(parts)))
+          ?a -> quote(do: :lists.map(&String.to_unsafe_atom/1, unquote(parts)))
          ?c -> quote(do: :lists.map(&String.to_charlist/1, unquote(parts)))
        end
    end
@@ -7036,7 +7056,7 @@ defmodule Kernel do
      :guard ->
        raise ArgumentError,
              "invalid expression in guard, #{exp} is not allowed in guards. " <>
-                "To learn more about guards, visit: https://hexdocs.pm/elixir/patterns-and-guards.html"
+                "To learn more about guards, visit: https://elixir.hexdocs.pm/patterns-and-guards.html"

      _ ->
        :ok
@@ -49,7 +49,7 @@ defmodule Kernel.CLI do

  @doc """
  Runs the given function by catching any failure
-  and printing them to stdout. `at_exit` hooks are
+  and printing them to stderr. `at_exit` hooks are
  also invoked before exiting.

  This function is used by Elixir's CLI and also
@@ -99,9 +99,14 @@ defmodule Kernel.CLI do
  Shared helper for error formatting on CLI tools.
  """
  def format_error(kind, reason, stacktrace) do
+    {banner, rest} = format_error_parts(kind, reason, stacktrace)
+    [banner, rest]
+  end
+
+  defp format_error_parts(kind, reason, stacktrace) do
    {blamed, stacktrace} = Exception.blame(kind, reason, stacktrace)

-    iodata =
+    banner =
      case blamed do
        %FunctionClauseError{} ->
          formatted = Exception.format_banner(kind, reason, stacktrace)
@@ -112,7 +117,7 @@ defmodule Kernel.CLI do
          Exception.format_banner(kind, blamed, stacktrace)
      end

-    [iodata, ?\n, Exception.format_stacktrace(prune_stacktrace(stacktrace))]
+    {banner, [?\n, Exception.format_stacktrace(prune_stacktrace(stacktrace))]}
  end

  @doc """
@@ -179,7 +184,8 @@ defmodule Kernel.CLI do
  ## Error handling

  defp print_error(kind, reason, stacktrace) do
-    IO.write(:stderr, format_error(kind, reason, stacktrace))
+    {banner, rest} = format_error_parts(kind, reason, stacktrace)
+    IO.write(:stderr, [IO.ANSI.format([:red, banner]), rest])
  end

  defp blame_match(%{match?: true, node: node}), do: blame_ansi(:normal, "+", node)
@@ -200,7 +206,7 @@ defmodule Kernel.CLI do
  end

  @elixir_internals [:elixir, :elixir_aliases, :elixir_clauses, :elixir_compiler, :elixir_def] ++
-                      [:elixir_def, :elixir_dispatch, :elixir_expand, :elixir_lexical] ++
+                      [:elixir_dispatch, :elixir_expand, :elixir_lexical] ++
                      [:elixir_map, :elixir_module] ++
                      [:elixir_erl, :elixir_erl_clauses, :elixir_erl_compiler, :elixir_erl_pass] ++
                      [Kernel.ErrorHandler, Module.ParallelChecker]
@@ -338,7 +344,7 @@ defmodule Kernel.CLI do
    parse_argv(t, %{config | verbose_compile: true})
  end

-  defp parse_argv([~c"--profile", "time" | t], %{mode: :elixirc} = config) do
+  defp parse_argv([~c"--profile", ~c"time" | t], %{mode: :elixirc} = config) do
    parse_argv(t, %{config | profile: :time})
  end

@@ -427,7 +433,7 @@ defmodule Kernel.CLI do
  end

  defp process_command({:rpc_eval, node, expr}, _config) when is_list(expr) do
-    node = List.to_atom(node)
+    node = List.to_unsafe_atom(node)

    # Explicitly connect the node in case the rpc node was started with --sname/--name undefined.
    _ = :net_kernel.connect_node(node)
@@ -23,7 +23,7 @@ defmodule Kernel.LexicalTracker do

  @doc """
  Invoked during module expansion to annotate a require
-  must be warned if unused.
+  that must be warned if unused.
  """
  def warn_require(pid, meta, module, alias) do
    :gen_server.cast(pid, {:warn_require, module, meta, alias})
@@ -32,7 +32,7 @@ defmodule Kernel.LexicalTracker do

  @doc """
  Invoked during module expansion to annotate an alias
-  must be warned if unused.
+  that must be warned if unused.
  """
  def warn_alias(pid, meta, alias, module) do
    :gen_server.cast(pid, {:warn_alias, alias, meta})
@@ -41,7 +41,7 @@ defmodule Kernel.LexicalTracker do

  @doc """
  Invoked during module expansion to annotate an import
-  must be warned if unused.
+  that must be warned if unused.
  """
  def warn_import(pid, module) do
    :gen_server.cast(pid, {:warn_import, module})
@@ -25,9 +25,8 @@ defmodule Kernel.ParallelCompiler do
          each_long_compilation: (Path.t() -> term()) | (Path.t(), pid() -> term()),
          each_long_verification: (module() -> term()) | (module(), pid() -> term()),
          each_module: (Path.t(), module(), binary() -> term()),
-          each_cycle: ([module()], [Code.diagnostic(:warning)] ->
-                         {:compile, [module()], [Code.diagnostic(:warning)]}
-                         | {:runtime, [module()], [Code.diagnostic(:warning)]}),
+          each_cycle: (-> {:compile, [Path.t()], [Code.diagnostic(:warning)]}
+                          | {:runtime, [{module(), Path.t()}], [Code.diagnostic(:warning)]}),
          long_compilation_threshold: pos_integer(),
          long_verification_threshold: pos_integer(),
          verification: boolean(),
@@ -101,6 +100,8 @@ defmodule Kernel.ParallelCompiler do
  This function allows a developer to perform such tasks.
  """
  @doc since: "1.16.0"
+  @spec pmap(Enumerable.t(input), (input -> output)) :: [output]
+        when input: term, output: term
  def pmap(collection, fun) when is_function(fun, 1) do
    ref = make_ref()

@@ -172,25 +173,25 @@ defmodule Kernel.ParallelCompiler do

    * `:each_long_verification` (since v1.19.0) - for each file that takes more
      than a given timeout (see the `:long_verification_threshold` option) to
-      compile, invoke this callback passing the module as its argument (and
+      verify, invoke this callback passing the module as its argument (and
      optionally the PID of the process verifying the module)

    * `:each_module` - for each module compiled, invokes the callback passing
      the file, module and the module bytecode

-    * `:each_cycle` - after the given files are compiled, invokes this function
-      that should return the following values:
+    * `:each_cycle` - invoked after each compilation cycle and should return one
+      of the following values:
      * `{:compile, modules, warnings}` - to continue compilation with a list of
-        further modules to compile
+        further module files to compile
      * `{:runtime, modules, warnings}` - to stop compilation and verify the list
-        of modules because dependent modules have changed
+        of `{module, path}` pairs because dependent modules have changed

    * `:long_compilation_threshold` - the timeout (in seconds) to check for files
      taking too long to compile. For each file that exceeds the threshold, the
      `:each_long_compilation` callback is invoked. Defaults to `10` seconds.

    * `:long_verification_threshold` (since v1.19.0) - the timeout (in seconds) to
-      check for modules taking too long to compile. For each module that exceeds the
+      check for modules taking too long to verify. For each module that exceeds the
      threshold, the `:each_long_verification` callback is invoked. Defaults to
      `10` seconds.

@@ -198,8 +199,8 @@ defmodule Kernel.ParallelCompiler do
      deprecation warnings, and type checking should run. Defaults to `true`.
      We recommend disabling it only for debugging purposes.

-    * `:profile` - if set to `:time` measure the compilation time of each compilation cycle
-       and group pass checker
+    * `:profile` - if set to `:time`, measure the compilation time of each compilation cycle,
+      each module type check, and group pass checker

    * `:purge_compiler_modules` - if set to `true`, automatically purge compilation modules
      after compilation (see `Code.purge_compiler_modules/0`)
@@ -213,7 +214,7 @@ defmodule Kernel.ParallelCompiler do

    * `:return_diagnostics` (since v1.15.0) - returns maps with information instead of
      a list of warnings and returns diagnostics as maps instead of tuples.
-      This option must be set to true, except for backwards compatibibility reasons.
+      This option must be set to true, except for backward compatibility reasons.

    * `:max_concurrency` - the maximum number of files to compile in parallel.
      Setting this option to 1 will compile files sequentially.
@@ -270,7 +271,7 @@ defmodule Kernel.ParallelCompiler do

    * `:return_diagnostics` (since v1.15.0) - returns maps with information instead of
      a list of warnings and returns diagnostics as maps instead of tuples.
-      This option must be set to true, except for backwards compatibibility reasons.
+      This option must be set to true, except for backward compatibility reasons.

  """
  @doc since: "1.6.0"
@@ -340,7 +341,7 @@ defmodule Kernel.ParallelCompiler do

  defp spawn_workers(schedulers, checker, files, output, options) do
    threshold = Keyword.get(options, :long_compilation_threshold, 10) * 1000
-    timer_ref = Process.send_after(self(), :threshold_check, threshold)
+    timer_ref = :erlang.send_after(threshold, self(), :threshold_check)

    purge_compiler_modules =
      if Keyword.get(options, :purge_compiler_modules, false) do
@@ -841,7 +842,7 @@ defmodule Kernel.ParallelCompiler do
            end
          end

-        timer_ref = Process.send_after(self(), :threshold_check, state.long_compilation_threshold)
+        timer_ref = :erlang.send_after(state.long_compilation_threshold, self(), :threshold_check)
        state = %{state | timer_ref: timer_ref}
        spawn_workers(queue, spawned, waiting, files, result, warnings, errors, state)

@@ -768,8 +768,8 @@ defmodule Kernel.SpecialForms do
  It is used in typespecs to specify the type of a variable,
  function or of a type itself:

-      @type number :: integer | float
-      @spec add(number, number) :: number
+      @type num :: integer | float
+      @spec add(num, num) :: num

  It may also be used in bit strings to specify the type
  of a given bit segment:
@@ -1333,9 +1333,9 @@ defmodule Kernel.SpecialForms do
        sum(1, value, 3)
      end

-
-  Which the argument for the `:sum` function call is not the
-  expected result:
+  However, the code above does not work as expected, because this
+  injects the representation of the `value` variable,
+  not its contents:

      {:sum, [], [1, {:value, [], Elixir}, 3]}

@@ -2077,9 +2077,9 @@ defmodule Kernel.SpecialForms do
      iex> try do
      ...>   1 / 0
      ...> rescue
-      ...>   x in [ArithmeticError] -> [:rescued, is_exception(x)]
+      ...>   x in [ArithmeticError] -> {:rescued, Exception.message(x)}
      ...> end
-      [:rescued, true]
+      {:rescued, "bad argument in arithmetic expression"}

  Rescue different errors with separate clauses:

@@ -610,8 +610,7 @@ defmodule Kernel.Typespec do
        types =
          :lists.map(
            fn %{field: field} ->
-              default_type = if field == :__exception__, do: true, else: quote(do: term())
-              {field, Keyword.get(fields, field, default_type)}
+              {field, Keyword.get(fields, field, quote(do: term()))}
            end,
            struct_info
          )
@@ -687,6 +686,13 @@ defmodule Kernel.Typespec do
    {{:type, location(meta), :range, [left, right]}, state}
  end

+  defp typespec({:..//, _meta, [_first, _last, _step]} = range, _vars, caller, _state) do
+    compile_error(
+      caller,
+      "ranges with steps are not supported in typespecs, got: #{Macro.to_string(range)}"
+    )
+  end
+
  # Handle special forms
  defp typespec({:__MODULE__, _, atom}, vars, caller, state) when is_atom(atom) do
    typespec(caller.module, vars, caller, state)
@@ -861,29 +867,29 @@ defmodule Kernel.Typespec do
    {{:type, location(meta), :nonempty_string, args}, state}
  end

-  defp typespec({type, _meta, []}, vars, caller, state) when type in [:charlist, :char_list] do
+  defp typespec({type, meta, []}, vars, caller, state) when type in [:charlist, :char_list] do
    if type == :char_list do
      warning = "the char_list() type is deprecated, use charlist()"
      IO.warn(warning, caller)
    end

-    typespec(quote(do: :elixir.charlist()), vars, caller, state)
+    remote_typespec(:charlist, meta, [], vars, caller, state)
  end

-  defp typespec({:nonempty_charlist, _meta, []}, vars, caller, state) do
-    typespec(quote(do: :elixir.nonempty_charlist()), vars, caller, state)
+  defp typespec({:nonempty_charlist, meta, []}, vars, caller, state) do
+    remote_typespec(:nonempty_charlist, meta, [], vars, caller, state)
  end

-  defp typespec({:struct, _meta, []}, vars, caller, state) do
-    typespec(quote(do: :elixir.struct()), vars, caller, state)
+  defp typespec({:struct, meta, []}, vars, caller, state) do
+    remote_typespec(:struct, meta, [], vars, caller, state)
  end

-  defp typespec({:as_boolean, _meta, [arg]}, vars, caller, state) do
-    typespec(quote(do: :elixir.as_boolean(unquote(arg))), vars, caller, state)
+  defp typespec({:as_boolean, meta, [arg]}, vars, caller, state) do
+    remote_typespec(:as_boolean, meta, [arg], vars, caller, state)
  end

-  defp typespec({:keyword, _meta, args}, vars, caller, state) when length(args) <= 1 do
-    typespec(quote(do: :elixir.keyword(unquote_splicing(args))), vars, caller, state)
+  defp typespec({:keyword, meta, args}, vars, caller, state) when length(args) <= 1 do
+    remote_typespec(:keyword, meta, args, vars, caller, state)
  end

  defp typespec({:fun, meta, args}, vars, caller, state) do
@@ -1008,6 +1014,10 @@ defmodule Kernel.Typespec do
    {{:remote_type, location(meta), [remote, name, args]}, state}
  end

+  defp remote_typespec(name, meta, args, vars, caller, state) do
+    typespec({{:., meta, [:elixir, name]}, meta, args}, vars, caller, state)
+  end
+
  defp collect_union({:|, _, [a, b]}), do: [a | collect_union(b)]
  defp collect_union(v), do: [v]

@@ -162,16 +162,8 @@ defmodule Kernel.Utils do
    # TODO: Make it raise on v2.0
    warn_on_duplicate_struct_key(:lists.keysort(1, fields), env)

-    foreach = fn
-      key when is_atom(key) ->
-        :ok
-
-      key ->
-        raise ArgumentError, "keys given to @enforce_keys must be atoms, got: #{inspect(key)}"
-    end
-
-    :lists.foreach(foreach, enforce_keys)
-    struct = :maps.from_list([__struct__: module] ++ fields)
+    field_map = :maps.from_list(fields)
+    struct = :maps.put(:__struct__, module, field_map)
    escaped_struct = :elixir_quote.escape(struct, {:struct, module}, false)

    body =
@@ -215,7 +207,7 @@ defmodule Kernel.Utils do
          end
      end

-    case enforce_keys -- :maps.keys(struct) do
+    case enforce_keys -- :maps.keys(field_map) do
      [] ->
        mapper = fn {key, val} ->
          %{field: key, default: val, required: :lists.member(key, enforce_keys)}
@@ -225,10 +217,9 @@ defmodule Kernel.Utils do
        derive = :lists.map(fn {_, value} -> value end, :ets.take(bag, {:accumulate, :derive}))
        {struct, :lists.reverse(derive), escaped_struct, quote(do: kv), body}

-      error_keys ->
+      invalid_keys ->
        raise ArgumentError,
-              "@enforce_keys required keys (#{inspect(error_keys)}) that are not defined in defstruct: " <>
-                "#{inspect(fields)}"
+              "unknown or duplicate keys given to @enforce_keys, got: #{inspect(invalid_keys)}"
    end
  end

@@ -266,7 +257,7 @@ defmodule Kernel.Utils do
    module.exception([])
  end

-  def raise(%_{__exception__: true} = exception) do
+  def raise(%_{__exception__: _} = exception) do
    exception
  end

@@ -329,21 +320,27 @@ defmodule Kernel.Utils do

  @spec defguard([Macro.t()], Macro.t(), Macro.Env.t()) :: Macro.t()
  def defguard(args, expr, env) do
-    {^args, vars} = extract_refs_from_args(args)
-    env = :elixir_env.with_vars(%{env | context: :guard}, vars)
-    {expr, _, _} = :elixir_expand.expand(expr, :elixir_env.env_to_ex(env), env)
+    {_, vars} = extract_refs_from_args(args)
+    guard_expr = expand_defguard(expr, %{env | context: :guard}, vars)
+    body_expr = expand_defguard(expr, %{env | context: nil}, vars)

    quote do
      case Macro.Env.in_guard?(__CALLER__) do
        true ->
-          unquote(literal_quote(unquote_every_ref(expr, vars), []))
+          unquote(literal_quote(unquote_every_ref(guard_expr, vars), []))

        false ->
-          unquote(literal_quote(unquote_refs_once(expr, vars, env.module), generated: true))
+          unquote(literal_quote(unquote_refs_once(body_expr, vars, env), generated: true))
      end
    end
  end

+  defp expand_defguard(expr, env, vars) do
+    env = :elixir_env.with_vars(env, vars)
+    {expr, _, _} = :elixir_expand.expand(expr, :elixir_env.env_to_ex(env), env)
+    expr
+  end
+
  defp extract_refs_from_args(args) do
    Macro.postwalk(args, [], fn
      {ref, meta, context} = var, acc when is_atom(ref) and is_atom(context) ->
@@ -369,7 +366,7 @@ defmodule Kernel.Utils do
  end

  # Prefaces `guard` with unquoted versions of `refs`.
-  defp unquote_refs_once(guard, refs, module) do
+  defp unquote_refs_once(guard, refs, %{module: module}) do
    {guard, used_refs} =
      Macro.postwalk(guard, %{}, fn
        {ref, meta, context} = var, acc when is_atom(ref) and is_atom(context) ->
@@ -382,7 +379,7 @@ defmodule Kernel.Utils do
                  {new_var, acc}

                %{} ->
-                  generated = String.to_atom("arg" <> Integer.to_string(map_size(acc) + 1))
+                  generated = String.to_unsafe_atom("arg" <> Integer.to_string(map_size(acc) + 1))
                  new_var = Macro.unique_var(generated, module)
                  {new_var, Map.put(acc, pair, {new_var, var})}
              end
@@ -38,7 +38,7 @@ defmodule Keyword do

  ## Duplicate keys and ordering

-  A keyword may have duplicate keys so it is not strictly a key-value
+  A keyword list may have duplicate keys so it is not strictly a key-value
  data type. However, most of the functions in this module work on a
  key-value structure and behave similar to the functions you would
  find in the `Map` module. For example, `Keyword.get/3` will get the first
@@ -119,14 +119,23 @@ defmodule Keyword do

      iex> Keyword.from_keys([:foo, :bar, :baz], :atom)
      [foo: :atom, bar: :atom, baz: :atom]
+
      iex> Keyword.from_keys([], :atom)
      []

+      iex> Keyword.from_keys(["foo"], :bar)
+      ** (ArgumentError) expected a list of atoms as keys, got: "foo"
  """
  @doc since: "1.14.0"
  @spec from_keys([key], value) :: t(value)
  def from_keys(keys, value) when is_list(keys) do
-    :lists.map(&{&1, value}, keys)
+    :lists.map(
+      fn
+        key when is_atom(key) -> {key, value}
+        other -> raise ArgumentError, "expected a list of atoms as keys, got: #{inspect(other)}"
+      end,
+      keys
+    )
  end

  @doc """
@@ -260,38 +269,60 @@ defmodule Keyword do
  @doc since: "1.13.0"
  @spec validate(keyword(), values :: [atom() | {atom(), term()}]) ::
          {:ok, keyword()} | {:error, [atom]}
+  def validate([], values) when is_list(values), do: {:ok, move_pairs!(values, [])}
+
  def validate(keyword, values) when is_list(keyword) and is_list(values) do
-    validate(keyword, values, [], [], [])
+    validate_merge(keyword, values, [], keyword)
  end

-  defp validate([{key, _} = pair | keyword], values1, values2, acc, bad_keys) when is_atom(key) do
-    case find_key!(key, values1, values2) do
-      {values1, values2} ->
-        validate(keyword, values1, values2, [pair | acc], bad_keys)
+  defp validate_merge([], values, values_pre, original),
+    do: {:ok, move_pairs!(values, move_pairs!(values_pre, original))}

-      :error ->
-        case find_key!(key, values2, values1) do
-          {values1, values2} ->
-            validate(keyword, values1, values2, [pair | acc], bad_keys)
-
-          :error ->
-            validate(keyword, values1, values2, acc, [key | bad_keys])
-        end
+  defp validate_merge([{key, _} = pair | keyword], [head | tail], values_pre, original)
+       when is_atom(key) do
+    case head do
+      ^key -> validate_merge(keyword, tail, values_pre, original)
+      {^key, _} -> validate_merge(keyword, tail, values_pre, original)
+      _ -> validate_merge([pair | keyword], tail, [head | values_pre], original)
    end
  end

-  defp validate([], values1, values2, acc, []) do
-    {:ok, move_pairs!(values1, move_pairs!(values2, acc))}
+  defp validate_merge([{key, _} | keyword], [], values_pre, original) when is_atom(key) do
+    case find_key!(key, values_pre, []) do
+      {new_values, new_values_pre} ->
+        validate_merge(keyword, new_values, new_values_pre, original)
+
+      :error ->
+        validate_fallback(keyword, values_pre, [key])
+    end
  end

-  defp validate([], _values1, _values2, _acc, bad_keys) do
-    {:error, bad_keys}
+  defp validate_merge([pair | _], _, _, _),
+    do:
+      raise(
+        ArgumentError,
+        "expected a keyword list as first argument, got invalid entry: #{inspect(pair)}"
+      )
+
+  defp validate_fallback([{key, _} | keyword], values, bad_keys)
+       when is_atom(key) do
+    case find_key!(key, values, []) do
+      {rest, acc} ->
+        validate_fallback(keyword, rest ++ acc, bad_keys)
+
+      :error ->
+        validate_fallback(keyword, values, [key | bad_keys])
+    end
  end

-  defp validate([pair | _], _values1, _values2, _acc, []) do
-    raise ArgumentError,
-          "expected a keyword list as first argument, got invalid entry: #{inspect(pair)}"
-  end
+  defp validate_fallback([], _, bad), do: {:error, bad}
+
+  defp validate_fallback([p | _], _, _),
+    do:
+      raise(
+        ArgumentError,
+        "expected a keyword list as first argument, got invalid entry: #{inspect(p)}"
+      )

  defp find_key!(key, [key | rest], acc), do: {rest, acc}
  defp find_key!(key, [{key, _} | rest], acc), do: {rest, acc}
@@ -486,7 +517,7 @@ defmodule Keyword do
        {get, :lists.reverse(acc, [{key, value} | delete(t, key)])}

      :pop ->
-        {current, :lists.reverse(acc, t)}
+        {current, :lists.reverse(acc, delete(t, key))}

      other ->
        raise "the given function must return a two-element tuple or :pop, got: #{inspect(other)}"
@@ -552,7 +583,7 @@ defmodule Keyword do
        {get, :lists.reverse(acc, [{key, value} | delete(t, key)])}

      :pop ->
-        {value, :lists.reverse(acc, t)}
+        {value, :lists.reverse(acc, delete(t, key))}

      other ->
        raise "the given function must return a two-element tuple or :pop, got: #{inspect(other)}"
@@ -956,7 +987,7 @@ defmodule Keyword do
      iex> Keyword.equal?([a: 1, b: 2, a: 3], [b: 2, a: 3, a: 1])
      true

-  Comparison between values is done with `===/3`,
+  Comparison between values is done with `===/2`,
  which means integers are not equivalent to floats:

      iex> Keyword.equal?([a: 1.0], [a: 1])
@@ -1292,7 +1323,8 @@ defmodule Keyword do
  @doc """
  Drops the given `keys` from the keyword list.

-  Removes duplicate keys from the new keyword list.
+  If a key occurs multiple times, all of its entries are removed when that key
+  is included in `keys`.

  ## Examples

@@ -1333,9 +1365,9 @@ defmodule Keyword do
  """
  @spec pop(t, key, default) :: {value | default, t}
  def pop(keywords, key, default \\ nil) when is_list(keywords) and is_atom(key) do
-    case fetch(keywords, key) do
-      {:ok, value} -> {value, delete(keywords, key)}
-      :error -> {default, keywords}
+    case :lists.keyfind(key, 1, keywords) do
+      {^key, value} -> {value, delete_key(keywords, key)}
+      false -> {default, keywords}
    end
  end

@@ -1360,9 +1392,9 @@ defmodule Keyword do
  @doc since: "1.10.0"
  @spec pop!(t, key) :: {value, t}
  def pop!(keywords, key) when is_list(keywords) and is_atom(key) do
-    case fetch(keywords, key) do
-      {:ok, value} -> {value, delete(keywords, key)}
-      :error -> raise KeyError, key: key, term: keywords
+    case :lists.keyfind(key, 1, keywords) do
+      {^key, value} -> {value, delete_key(keywords, key)}
+      false -> raise KeyError, key: key, term: keywords
    end
  end

@@ -1402,7 +1434,7 @@ defmodule Keyword do
    do: {values, acc}

  @doc """
-  Lazily returns and removes all values associated with `key` in the keyword list.
+  Lazily returns the first value for `key` and removes all associated entries in the keyword list.

  This is useful if the default value is very expensive to calculate or
  generally difficult to set up and tear down again.
@@ -1425,9 +1457,9 @@ defmodule Keyword do
  @spec pop_lazy(t, key, (-> value)) :: {value, t}
  def pop_lazy(keywords, key, fun)
      when is_list(keywords) and is_atom(key) and is_function(fun, 0) do
-    case fetch(keywords, key) do
-      {:ok, value} -> {value, delete(keywords, key)}
-      :error -> {fun.(), keywords}
+    case :lists.keyfind(key, 1, keywords) do
+      {^key, value} -> {value, delete_key(keywords, key)}
+      false -> {fun.(), keywords}
    end
  end

@@ -102,7 +102,7 @@ defmodule List do
  Even though the representation changed, the raw data does remain a list of
  integers, which can be handled as such:

-      iex> inspect(~c"abc", charlists: :as_list)
+      iex> inspect(~c"abc", charlists: :as_lists)
      "[97, 98, 99]"
      iex> Enum.map(~c"abc", fn num -> 1000 + num end)
      [1097, 1098, 1099]
@@ -311,9 +311,6 @@ defmodule List do
      iex> List.first!([1, 2, 3])
      1

-      iex> List.first!([])
-      ** (ArgumentError) attempted to get the first element of an empty list
-
  """
  @doc since: "1.20.0"
  @spec first!([elem, ...]) :: elem when elem: var
@@ -366,9 +363,6 @@ defmodule List do
      iex> List.last!([1, 2, 3])
      3

-      iex> List.last!([])
-      ** (ArgumentError) attempted to get the last element of an empty list
-
  """
  @doc since: "1.20.0"
  @spec last!([elem, ...]) :: elem when elem: var
@@ -435,7 +429,7 @@ defmodule List do

  """
  @doc since: "1.13.0"
-  @spec keyfind!([tuple], any, non_neg_integer) :: any
+  @spec keyfind!([tuple], any, non_neg_integer) :: tuple
  def keyfind!(list, key, position) when is_integer(position) do
    :lists.keyfind(key, position + 1, list) ||
      raise KeyError,
@@ -520,8 +514,8 @@ defmodule List do

  As in `Enum.sort/2`, avoid using the default sorting function to sort
  structs, as by default it performs structural comparison instead of a
-  semantic one. In such cases, you shall pass a sorting function as third
-  element or any module that implements a `compare/2` function. For example,
+  semantic one. In such cases, you shall pass a sorting function as the third
+  argument or any module that implements a `compare/2` function. For example,
  if you have tuples with user names and their birthday, and you want to
  sort on their birthday, in both ascending and descending order, you should
  do:
@@ -661,7 +655,7 @@ defmodule List do
  end

  @doc """
-  Wraps `term` in a list if this is not list.
+  Wraps `term` in a list if it is not a list.

  If `term` is already a list, it returns the list.
  If `term` is `nil`, it returns an empty list.
@@ -960,9 +954,12 @@ defmodule List do
  @spec pop_at(list, integer, any) :: {any, list}
  def pop_at(list, index, default \\ nil) when is_integer(index) do
    if index < 0 do
-      do_pop_at(list, length(list) + index, default, [])
+      case length(list) + index do
+        index when index < 0 -> {default, list}
+        index -> do_pop_at(list, index, default, [], list)
+      end
    else
-      do_pop_at(list, index, default, [])
+      do_pop_at(list, index, default, [], list)
    end
  end

@@ -1024,31 +1021,51 @@ defmodule List do
    :lists.suffix(suffix, list)
  end

-  @doc """
-  Converts a charlist to an atom.
-
-  Elixir supports conversions from charlists which contain any Unicode
-  code point.
-
-  Inlined by the compiler.
-
-  ## Examples
-
-      iex> List.to_atom(~c"Elixir")
-      :Elixir
-
-      iex> List.to_atom(~c"🌢 Elixir")
-      :"🌢 Elixir"
-
-  """
-  @spec to_atom(charlist) :: atom
+  @doc deprecated: "Use to_existing_atom/1 or to_unsafe_atom/1 instead"
  def to_atom(charlist) do
    :erlang.list_to_atom(charlist)
  end

+  @doc """
+  Converts a charlist to an existing atom or creates a new one.
+
+  Elixir supports conversions from charlists which contain any Unicode
+  code point.
+
+  > #### Dynamic Atom Creation {: .warning}
+  >
+  > This function creates atoms dynamically and atoms are
+  > not garbage-collected. Therefore, `charlist` should not be an
+  > untrusted value, such as input received from a socket or during
+  > a web request. Consider using `to_existing_atom/1` instead.
+
+  By default, the maximum number of atoms is `1_048_576`. This limit
+  can be raised or lowered using the VM option `+t`.
+
+  The maximum atom size is 255 Unicode code points.
+
+  Inlined by the compiler.
+
+  ## Examples
+
+      iex> List.to_unsafe_atom(~c"Elixir")
+      :Elixir
+
+      iex> List.to_unsafe_atom(~c"🌢 Elixir")
+      :"🌢 Elixir"
+
+  """
+  @doc since: "1.21.0"
+  @spec to_unsafe_atom(charlist) :: atom
+  def to_unsafe_atom(charlist) do
+    :erlang.list_to_atom(charlist)
+  end
+
  @doc """
  Converts a charlist to an existing atom.

+  If the list of expected atoms is known upfront, prefer `to_existing_atom/2`.
+
  Elixir supports conversions from charlists which contain any Unicode
  code point. Raises an `ArgumentError` if the atom does not exist.

@@ -1079,6 +1096,47 @@ defmodule List do
    :erlang.list_to_existing_atom(charlist)
  end

+  @doc """
+  Converts a charlist to one of the `allowed_atoms` or raises.
+
+  Raises an `ArgumentError` if the atom either does not exist or is not within
+  the existing list.
+
+  This should be preferred to `to_existing_atom/1` if the list is known upfront,
+  since there is no risk that the atom has not been loaded.
+
+  ## Examples
+
+      iex> List.to_existing_atom(~c"foo", [:foo, :bar])
+      :foo
+
+      iex> List.to_existing_atom(~c"unknown", [:foo, :bar])
+      ** (ArgumentError) unexpected value: ~c\"unknown\", the allowed atoms are: [:foo, :bar]
+
+  """
+  @doc since: "1.21.0"
+  @spec to_existing_atom(charlist, nonempty_list(a)) :: a when a: atom()
+  def to_existing_atom(charlist, [_ | _] = allowed_atoms) when is_list(charlist) do
+    atom = :erlang.list_to_existing_atom(charlist)
+
+    if atom not in allowed_atoms do
+      to_existing_atom_unexpected(charlist, allowed_atoms)
+    end
+
+    atom
+  end
+
+  # used just to have a less cryptic stacktrace and consistent error
+  @doc false
+  def __to_existing_atom__(charlist, allowed_atoms) do
+    to_existing_atom_unexpected(charlist, allowed_atoms)
+  end
+
+  defp to_existing_atom_unexpected(charlist, allowed_atoms) do
+    raise ArgumentError,
+          "unexpected value: #{inspect(charlist)}, the allowed atoms are: #{inspect(allowed_atoms)}"
+  end
+
  @doc """
  Returns the float whose text representation is `charlist`.

@@ -1090,7 +1148,7 @@ defmodule List do
      2.2017764

  """
-  @spec to_float(charlist) :: float
+  @spec to_float(nonempty_charlist) :: float
  def to_float(charlist) do
    :erlang.list_to_float(charlist)
  end
@@ -1106,7 +1164,7 @@ defmodule List do
      123

  """
-  @spec to_integer(charlist) :: integer
+  @spec to_integer(nonempty_charlist) :: integer
  def to_integer(charlist) do
    :erlang.list_to_integer(charlist)
  end
@@ -1124,7 +1182,7 @@ defmodule List do
      1023

  """
-  @spec to_integer(charlist, 2..36) :: integer
+  @spec to_integer(nonempty_charlist, 2..36) :: integer
  def to_integer(charlist, base) do
    :erlang.list_to_integer(charlist, base)
  end
@@ -1193,7 +1251,7 @@ defmodule List do

        Please check the given list or call inspect/1 to get the list representation, got:

-        #{inspect(list)}
+        #{inspect(list, charlists: :as_lists)}
        """
    else
      result when is_binary(result) ->
@@ -1245,7 +1303,7 @@ defmodule List do

        Please check the given list or call inspect/1 to get the list representation, got:

-        #{inspect(list)}
+        #{inspect(list, charlists: :as_lists)}
        """
    else
      result when is_list(result) ->
@@ -1310,13 +1368,13 @@ defmodule List do

  defp myers_difference_with_diff_script(list1, list2, diff_script) do
    path = {0, list1, list2, []}
-    find_script(0, length(list1) + length(list2), [path], diff_script)
+    find_script(0, [path], diff_script)
  end

-  defp find_script(envelope, max, paths, diff_script) do
+  defp find_script(envelope, paths, diff_script) do
    case each_diagonal(-envelope, envelope, paths, [], diff_script) do
      {:done, edits} -> compact_reverse(edits, [])
-      {:next, paths} -> find_script(envelope + 1, max, paths, diff_script)
+      {:next, paths} -> find_script(envelope + 1, paths, diff_script)
    end
  end

@@ -1461,15 +1519,16 @@ defmodule List do

  # pop_at

-  defp do_pop_at([], _index, default, acc) do
-    {default, :lists.reverse(acc)}
+  # The original list is returned when the index is out of bounds
+  defp do_pop_at([], _index, default, _acc, original) do
+    {default, original}
  end

-  defp do_pop_at([head | tail], 0, _default, acc) do
+  defp do_pop_at([head | tail], 0, _default, acc, _original) do
    {head, :lists.reverse(acc, tail)}
  end

-  defp do_pop_at([head | tail], index, default, acc) do
-    do_pop_at(tail, index - 1, default, [head | acc])
+  defp do_pop_at([head | tail], index, default, acc, original) do
+    do_pop_at(tail, index - 1, default, [head | acc], original)
  end
 end
@@ -166,7 +166,7 @@ defmodule Macro do
      of a `__block__` or the right side of `->`. The last expression of the
      block does not have metadata if it is not followed by an end of line
      character (either a newline or `;`). This entry may appear multiple times
-      in the same metadata if the expression is surround by parens
+      in the same metadata if the expression is surrounded by parens

    * `:format` - set to `:keyword` when an atom is defined as a keyword.
      It may also be set to `:atom` to distinguish `nil`, `false`, and `true`
@@ -235,7 +235,7 @@ defmodule Macro do
  `div/2` function, so that the AST for that function will become `{:div, [],
  [100, 5]}` (`div(100, 5)`).
  """
-  @spec unpipe(t()) :: [t()]
+  @spec unpipe(t()) :: [{t(), non_neg_integer}]
  def unpipe(expr) do
    :lists.reverse(unpipe(expr, []))
  end
@@ -518,7 +518,7 @@ defmodule Macro do
  @doc since: "1.11.3"
  @spec generate_unique_arguments(0, context :: atom) :: []
  @spec generate_unique_arguments(pos_integer, context) ::
-          [{atom, [counter: integer], context}, ...]
+          [{atom, metadata(), context}, ...]
        when context: atom
  def generate_unique_arguments(amount, context),
    do: generate_arguments(amount, context, &unique_var/2)
@@ -527,7 +527,7 @@ defmodule Macro do

  defp generate_arguments(amount, context, fun)
       when is_integer(amount) and amount > 0 and is_atom(context) do
-    for id <- 1..amount, do: fun.(String.to_atom("arg" <> Integer.to_string(id)), context)
+    for id <- 1..amount, do: fun.(String.to_unsafe_atom("arg" <> Integer.to_string(id)), context)
  end

  @doc """
@@ -576,7 +576,7 @@ defmodule Macro do

  """
  @doc since: "1.11.3"
-  @spec unique_var(var, context) :: {var, [counter: integer], context}
+  @spec unique_var(var, context) :: {var, metadata(), context}
        when var: atom, context: atom
  def unique_var(var, context) when is_atom(var) and is_atom(context) do
    {var, [counter: :elixir_module.next_counter(context)], context}
@@ -690,7 +690,39 @@ defmodule Macro do
  """
  @spec prewalk(t, (t -> t)) :: t
  def prewalk(ast, fun) when is_function(fun, 1) do
-    elem(prewalk(ast, nil, fn x, nil -> {fun.(x), nil} end), 0)
+    do_prewalk(fun.(ast), fun)
+  end
+
+  # Mirrors do_traverse/4 with an always-pre fun and no accumulator,
+  # avoiding the wrapper closures and tuple threading of traverse/4.
+  # Each clause dispatches on the already-transformed node, so fun's
+  # rewrites are descended into, exactly as in traverse/4.
+  defp do_prewalk({form, meta, args}, fun) when is_atom(form) do
+    {form, meta, do_prewalk_args(args, fun)}
+  end
+
+  defp do_prewalk({form, meta, args}, fun) do
+    form = do_prewalk(fun.(form), fun)
+    {form, meta, do_prewalk_args(args, fun)}
+  end
+
+  defp do_prewalk({left, right}, fun) do
+    left = do_prewalk(fun.(left), fun)
+    {left, do_prewalk(fun.(right), fun)}
+  end
+
+  defp do_prewalk(list, fun) when is_list(list) do
+    do_prewalk_args(list, fun)
+  end
+
+  defp do_prewalk(x, _fun) do
+    x
+  end
+
+  defp do_prewalk_args(args, _fun) when is_atom(args), do: args
+
+  defp do_prewalk_args(args, fun) when is_list(args) do
+    :lists.map(fn x -> do_prewalk(fun.(x), fun) end, args)
  end

  @doc """
@@ -728,11 +760,41 @@ defmodule Macro do
  """
  @spec postwalk(t, (t -> t)) :: t
  def postwalk(ast, fun) when is_function(fun, 1) do
-    elem(postwalk(ast, nil, fn x, nil -> {fun.(x), nil} end), 0)
+    do_postwalk(ast, fun)
+  end
+
+  # Mirrors do_traverse/4 with an always-post fun and no accumulator,
+  # avoiding the wrapper closures and tuple threading of traverse/4
+  defp do_postwalk({form, meta, args}, fun) when is_atom(form) do
+    fun.({form, meta, do_postwalk_args(args, fun)})
+  end
+
+  defp do_postwalk({form, meta, args}, fun) do
+    form = do_postwalk(form, fun)
+    fun.({form, meta, do_postwalk_args(args, fun)})
+  end
+
+  defp do_postwalk({left, right}, fun) do
+    left = do_postwalk(left, fun)
+    fun.({left, do_postwalk(right, fun)})
+  end
+
+  defp do_postwalk(list, fun) when is_list(list) do
+    fun.(do_postwalk_args(list, fun))
+  end
+
+  defp do_postwalk(x, fun) do
+    fun.(x)
+  end
+
+  defp do_postwalk_args(args, _fun) when is_atom(args), do: args
+
+  defp do_postwalk_args(args, fun) when is_list(args) do
+    :lists.map(fn x -> do_postwalk(x, fun) end, args)
  end

  @doc """
-  This functions behaves like `prewalk/3`, but performs a depth-first,
+  This function behaves like `prewalk/3`, but performs a depth-first,
  post-order traversal of quoted expressions using an accumulator.
  """
  @spec postwalk(t, any, (t, any -> {t, any})) :: {t, any}
@@ -940,7 +1002,7 @@ defmodule Macro do

  This is useful when a struct needs to be expanded at
  compilation time and the struct being expanded may or may
-  not have been compiled (including structs in the defined
+  not have been compiled (including structs defined
  under the module being compiled). For compiled modules,
  it will invoke `module.__info__(:struct)`.

@@ -973,7 +1035,7 @@ defmodule Macro do
  def struct_info!(module, env) when is_atom(module) do
    meta = [line: env.line]

-    case :elixir_map.maybe_load_struct_info(meta, module, env) do
+    case :elixir_map.maybe_load_struct_info(meta, module, :hard, env) do
      {:ok, info} ->
        :elixir_env.trace({:struct_expansion, meta, module, []}, env)
        info
@@ -1034,7 +1096,7 @@ defmodule Macro do
  defp find_invalid(other), do: {:error, other}

  @doc """
-  Returns an enumerable that traverses the  `ast` in depth-first,
+  Returns an enumerable that traverses the `ast` in depth-first,
  pre-order traversal.

  ## Examples
@@ -1092,7 +1154,7 @@ defmodule Macro do
  end

  @doc """
-  Returns an enumerable that traverses the  `ast` in depth-first,
+  Returns an enumerable that traverses the `ast` in depth-first,
  post-order traversal.

  ## Examples
@@ -1195,14 +1257,15 @@ defmodule Macro do
  The mapping function receives an integer representing the code point
  of the character it wants to unescape. There are also the special atoms
  `:newline`, `:unicode`, and `:hex`, which control newline, unicode,
-  and escaping respectively.
+  and escaping respectively, and for which the mapping function must return
+  a boolean.

  Here is the default mapping function implemented by Elixir:

      def unescape_map(:newline), do: true
      def unescape_map(:unicode), do: true
      def unescape_map(:hex), do: true
-      def unescape_map(?0), do: ?0
+      def unescape_map(?0), do: 0
      def unescape_map(?a), do: ?\a
      def unescape_map(?b), do: ?\b
      def unescape_map(?d), do: ?\d
@@ -1225,7 +1288,11 @@ defmodule Macro do
      Macro.unescape_string("example\\n", &unescape_map(&1))

  """
-  @spec unescape_string(String.t(), (non_neg_integer -> non_neg_integer | false)) :: String.t()
+  @spec unescape_string(
+          String.t(),
+          (non_neg_integer | :newline | :unicode | :hex ->
+             non_neg_integer | boolean)
+        ) :: String.t()
  def unescape_string(string, map) do
    :elixir_interpolation.unescape_string(string, map)
  end
@@ -1740,9 +1807,9 @@ defmodule Macro do

  defp kw_blocks_to_string(kw, fun) do
    Enum.reduce(unquote(kw_keywords), " ", fn x, acc ->
-      case Keyword.has_key?(kw, x) do
-        true -> acc <> kw_block_to_string(x, Keyword.get(kw, x), fun)
-        false -> acc
+      case Keyword.fetch(kw, x) do
+        {:ok, value} -> acc <> kw_block_to_string(x, value, fun)
+        :error -> acc
      end
    end) <> "end"
  end
@@ -1856,6 +1923,7 @@ defmodule Macro do
  definition compile-time, and you can use `Macro.expand/2`.
  """
  @doc since: "1.16.0"
+  @spec compile_apply(module(), atom(), [term()], Macro.Env.t()) :: term()
  def compile_apply(mod, fun, args, caller) do
    :elixir_env.trace({:remote_function, [], mod, fun, length(args)}, %{caller | function: nil})
    Kernel.apply(mod, fun, args)
@@ -1980,10 +2048,9 @@ defmodule Macro do

  defp do_expand_once({{:., _, [{:__ENV__, _, atom}, field]}, _, []} = original, env)
       when is_atom(atom) and is_atom(field) and env.context != :match do
-    if Map.has_key?(env, field) do
-      {maybe_escape_map(Map.get(env, field)), true}
-    else
-      {original, false}
+    case Map.fetch(env, field) do
+      {:ok, value} -> {maybe_escape_map(value), true}
+      :error -> {original, false}
    end
  end

@@ -2121,6 +2188,9 @@ defmodule Macro do
  def quoted_literal?({:__aliases__, _, args}),
    do: quoted_literal?(args)

+  def quoted_literal?({:__block__, _, [wrapped]}),
+    do: quoted_literal?(wrapped)
+
  def quoted_literal?({:%, _, [left, right]}),
    do: quoted_literal?(left) and quoted_literal?(right)

@@ -2196,7 +2266,7 @@ defmodule Macro do
  Please check `expand_literals/2` for use cases and pitfalls.
  """
  @doc since: "1.14.1"
-  @spec expand_literals(t(), acc, (t(), acc -> {t(), acc})) :: t() when acc: term()
+  @spec expand_literals(t(), acc, (t(), acc -> {t(), acc})) :: {t(), acc} when acc: term()
  def expand_literals(ast, acc, fun)

  def expand_literals({:__aliases__, meta, args}, acc, fun) do
@@ -2500,7 +2570,7 @@ defmodule Macro do

  ### As a remote call

-  Inspect an atom the function name of a remote call.
+  Inspect an atom as the function name of a remote call.

      iex> Macro.inspect_atom(:remote_call, :foo)
      "foo"
@@ -2621,55 +2691,77 @@ defmodule Macro do
        :unquoted_operator

      true ->
-        charlist = Atom.to_charlist(atom)
-
-        if valid_alias?(charlist) do
-          :alias
-        else
-          case :elixir_config.identifier_tokenizer().tokenize(charlist) do
-            {kind, _acc, [], _, _, special} ->
-              cond do
-                kind != :identifier or :lists.member(:at, special) ->
-                  :not_callable
-
-                # identifier_tokenizer used to return errors for non-nfc, but
-                # now it nfc-normalizes everything. However, lack of nfc is
-                # still a good reason to quote an atom when printing.
-                :lists.member(:nfkc, special) ->
-                  :other
-
-                true ->
-                  :identifier
-              end
-
-            _ ->
-              :other
-          end
-        end
+        classify_binary(Atom.to_string(atom), atom)
    end
  end

-  defp valid_alias?([?E, ?l, ?i, ?x, ?i, ?r] ++ rest), do: valid_alias_piece?(rest)
-  defp valid_alias?(_other), do: false
+  # ASCII identifiers and aliases are recognized on the binary to avoid building a
+  # charlist and running the (unicode aware) tokenizer, which dominates the cost of
+  # classifying keyword list, map and struct keys.
+  defp classify_binary(<<char, rest::binary>>, atom)
+       when char >= ?a and char <= ?z
+       when char == ?_ do
+    if valid_identifier_rest?(rest), do: :identifier, else: classify_with_tokenizer(atom)
+  end

-  defp valid_alias_piece?([?., char | rest]) when char >= ?A and char <= ?Z,
-    do: valid_alias_piece?(trim_leading_while_valid_identifier(rest))
+  defp classify_binary("Elixir" <> rest, atom) do
+    if valid_alias_piece?(rest), do: :alias, else: classify_with_tokenizer(atom)
+  end

-  defp valid_alias_piece?([]), do: true
-  defp valid_alias_piece?(_other), do: false
+  defp classify_binary(_binary, atom), do: classify_with_tokenizer(atom)

-  defp trim_leading_while_valid_identifier([char | rest])
+  defp classify_with_tokenizer(atom) do
+    case :elixir_config.identifier_tokenizer().tokenize(Atom.to_charlist(atom)) do
+      {kind, _acc, [], _, _, special} ->
+        cond do
+          kind != :identifier or :lists.member(:at, special) ->
+            :not_callable
+
+          # identifier_tokenizer used to return errors for non-nfc, but
+          # now it nfc-normalizes everything. However, lack of nfc is
+          # still a good reason to quote an atom when printing.
+          :lists.member(:nfkc, special) ->
+            :other
+
+          true ->
+            :identifier
+        end
+
+      _ ->
+        :other
+    end
+  end
+
+  defp valid_identifier_rest?(<<char, rest::binary>>)
       when char >= ?a and char <= ?z
       when char >= ?A and char <= ?Z
       when char >= ?0 and char <= ?9
       when char == ?_ do
-    trim_leading_while_valid_identifier(rest)
+    valid_identifier_rest?(rest)
  end

-  defp trim_leading_while_valid_identifier(other) do
-    other
+  defp valid_identifier_rest?(<<char>>) when char == ?? when char == ?!, do: true
+  defp valid_identifier_rest?(<<>>), do: true
+  defp valid_identifier_rest?(_other), do: false
+
+  defp valid_alias_piece?(<<?., char, rest::binary>>) when char >= ?A and char <= ?Z,
+    do: valid_alias_piece_rest?(rest)
+
+  defp valid_alias_piece?(<<>>), do: true
+  defp valid_alias_piece?(_other), do: false
+
+  # A helper returning the rest of the binary would build a sub binary per piece,
+  # so branch back into valid_alias_piece?/1 to keep the match context.
+  defp valid_alias_piece_rest?(<<char, rest::binary>>)
+       when char >= ?a and char <= ?z
+       when char >= ?A and char <= ?Z
+       when char >= ?0 and char <= ?9
+       when char == ?_ do
+    valid_alias_piece_rest?(rest)
  end

+  defp valid_alias_piece_rest?(other), do: valid_alias_piece?(other)
+
  @doc """
  Default backend for `Kernel.dbg/2`.

@@ -2699,7 +2791,7 @@ defmodule Macro do
      :guard ->
        raise ArgumentError,
              "invalid expression in guard, dbg is not allowed in guards. " <>
-                "To learn more about guards, visit: https://hexdocs.pm/elixir/patterns-and-guards.html"
+                "To learn more about guards, visit: https://elixir.hexdocs.pm/patterns-and-guards.html"

      _ ->
        :ok
@@ -2954,6 +3046,8 @@ defmodule Macro do
  @doc false
  def __dbg__(to_debug, header, options) do
    {print_location?, options} = Keyword.pop(options, :print_location, true)
+    syntax_colors = if IO.ANSI.enabled?(), do: IO.ANSI.syntax_colors(), else: []
+    options = Keyword.merge([width: 80, pretty: true, syntax_colors: syntax_colors], options)
    ansi_enabled? = options[:syntax_colors] != []

    if print_location? and is_binary(header) do
@@ -2961,8 +3055,6 @@ defmodule Macro do
      :ok = IO.write(IO.ANSI.format(formatted, ansi_enabled?))
    end

-    syntax_colors = if IO.ANSI.enabled?(), do: IO.ANSI.syntax_colors(), else: []
-    options = Keyword.merge([width: 80, pretty: true, syntax_colors: syntax_colors], options)
    {formatted, result} = dbg_format_ast_to_debug(to_debug, options)
    :ok = IO.write(IO.ANSI.format([formatted, ?\n], ansi_enabled?))
    result
@@ -203,23 +203,23 @@ defmodule Macro.Env do
  Returns a keyword list containing the file and line
  information as keys.
  """
-  @spec location(t) :: keyword
+  @spec location(t) :: [file: file, line: line]
  def location(env)

  def location(%{__struct__: Macro.Env, file: file, line: line}) do
    [file: file, line: line]
  end

-  # TODO: Deprecate on Elixir 1.21 in favor of expand_alias/4
  @doc false
+  @deprecated "Use Macro.Env.expand_alias/4 instead"
  def fetch_alias(%{__struct__: Macro.Env, aliases: aliases}, atom) when is_atom(atom),
-    do: Keyword.fetch(aliases, :"Elixir.#{atom}")
+    do: Keyword.fetch(aliases, String.to_unsafe_atom("Elixir.#{atom}"))

-  # TODO: Deprecate on Elixir 1.21 in favor of expand_alias/4
  @doc false
+  @deprecated "Use Macro.Env.expand_alias/4 instead"
  def fetch_macro_alias(%{__struct__: Macro.Env, macro_aliases: aliases}, atom)
      when is_atom(atom),
-      do: Keyword.fetch(aliases, :"Elixir.#{atom}")
+      do: Keyword.fetch(aliases, String.to_unsafe_atom("Elixir.#{atom}"))

  @doc """
  Returns the modules from which the given `{name, arity}` was
@@ -234,7 +234,7 @@ defmodule Macro.Env do
  > This function does not emit compiler tracing events,
  > which may block the compiler from correctly tracking
  > dependencies. Use this function for reflection purposes
-  > but to do not use it to expand imports into qualified
+  > but do not use it to expand imports into qualified
  > calls. Instead, use `expand_import/5`.

  ## Examples
@@ -345,7 +345,7 @@ defmodule Macro.Env do

  ## Additional options

-  It accepts the same options as `Kernel.SpecialForm.require/2` plus:
+  It accepts the same options as `Kernel.SpecialForms.require/2` plus:

    * #{trace_option}

@@ -367,7 +367,8 @@ defmodule Macro.Env do

  """
  @doc since: "1.17.0"
-  @spec define_require(t, Macro.metadata(), module, define_require_opts) :: {:ok, t}
+  @spec define_require(t, Macro.metadata(), module, define_require_opts) ::
+          {:ok, t} | {:error, String.t()}
  def define_require(env, meta, module, opts \\ [])
      when is_list(meta) and is_atom(module) and is_list(opts) do
    {trace, opts} = Keyword.pop(opts, :trace, true)
@@ -385,7 +386,7 @@ defmodule Macro.Env do

  ## Additional options

-  It accepts the same options as `Kernel.SpecialForm.import/2` plus:
+  It accepts the same options as `Kernel.SpecialForms.import/2` plus:

    * `:emit_warnings` - emit warnings found when defining imports

@@ -393,7 +394,7 @@ defmodule Macro.Env do

    * `:info_callback` - a function to use instead of `c:Module.__info__/1`.
      The function will be invoked with `:functions` or `:macros` argument.
-      It has to return a list of `{function, arity}` key value pairs.
+      It has to return a list of `{function, arity}` key-value pairs.
      If it fails, it defaults to using module metadata based on `module_info/1`.

  ## Examples
@@ -405,7 +406,7 @@ defmodule Macro.Env do
      iex> Macro.Env.lookup_import(env, {:flatten, 1})
      [{:function, List}]

-  It accepts the same options as `Kernel.SpecialForm.import/2`:
+  It accepts the same options as `Kernel.SpecialForms.import/2`:

      iex> env = __ENV__
      iex> Macro.Env.lookup_import(env, {:is_odd, 1})
@@ -447,7 +448,7 @@ defmodule Macro.Env do

  ## Additional options

-  It accepts the same options as `Kernel.SpecialForm.alias/2` plus:
+  It accepts the same options as `Kernel.SpecialForms.alias/2` plus:

    * #{trace_option}

@@ -699,7 +700,7 @@ defmodule Macro.Env do
  @doc """
  Returns the environment stacktrace.
  """
-  @spec stacktrace(t) :: list
+  @spec stacktrace(t) :: [{module, atom, arity, keyword}]
  def stacktrace(%{__struct__: Macro.Env} = env) do
    cond do
      is_nil(env.module) ->
@@ -201,7 +201,7 @@ defmodule Map do
      %{}

  """
-  @spec new :: map
+  @spec new :: %{}
  def new, do: %{}

  @doc """
@@ -1109,7 +1109,7 @@ defmodule Map do
      iex> Map.equal?(%{a: 1, b: 2}, %{b: 1, a: 2})
      false

-  Comparison between keys and values is done with `===/3`,
+  Comparison between keys and values is done with `===/2`,
  which means integers are not equivalent to floats:

      iex> Map.equal?(%{a: 1.0}, %{a: 1})
@@ -1138,9 +1138,9 @@ defmodule Map do
  > #### Performance considerations {: .tip}
  >
  > If you find yourself doing multiple calls to `Map.filter/2`
-  > and `Map.reject/2` in a pipeline, it is likely more efficient
-  > to use `Enum.map/2` and `Enum.filter/2` instead and convert to
-  > a map at the end using `Map.new/1`.
+  > and/or `Map.reject/2` in a pipeline, it is likely more efficient
+  > to use `Enum.filter/2` and `Enum.reject/2` instead and convert to
+  > a map at the end using `Map.new/1` or `Map.new/2`.

  ## Examples

@@ -1172,6 +1172,13 @@ defmodule Map do

  See also `filter/2`.

+  > #### Performance considerations {: .tip}
+  >
+  > If you find yourself doing multiple calls to `Map.filter/2`
+  > and/or `Map.reject/2` in a pipeline, it is likely more efficient
+  > to use `Enum.filter/2` and `Enum.reject/2` instead and convert to
+  > a map at the end using `Map.new/1` or `Map.new/2`.
+
  ## Examples

      iex> Map.reject(%{one: 1, two: 2, three: 3}, fn {_key, val} -> rem(val, 2) == 1 end)
@@ -51,13 +51,23 @@ defmodule MapSet do
  that they share many properties, including logarithmic time complexity. Erlang
  `:sets` (version 2) are implemented on top of maps, so see the documentation
  for `Map` for more information on its execution time complexity.
+
+  > #### Dialyzer opaqueness warnings {: .warning}
+  >
+  > `MapSet` internally relies on the `:sets` module which uses
+  > opaque types. This might cause Dialyzer to report opaqueness violations.
+  > These can be silenced by setting the following module attribute: 
+  >   
+  >     @dialyzer :no_opaque
+
  """

  @type value :: term

  # We don't use @opaque (or `:sets.set` which is opaque) because MapSets can be inlined,
  # either via module attributes or by the compiler.
-  @typep internal(value) :: %{optional(value) => term()}
+  # Defaulting to a broad `term()` type to prevent opaqueness violations.
+  @typep internal(_value) :: term()

  @type t(value) :: %__MODULE__{map: internal(value)}
  @type t :: t(term)
@@ -170,18 +180,14 @@ defmodule MapSet do
  @doc since: "1.14.0"
  @spec symmetric_difference(t(val1), t(val2)) :: t(val1 | val2) when val1: value, val2: value
  def symmetric_difference(%MapSet{map: set1} = map_set1, %MapSet{map: set2} = _map_set2) do
-    {small, large} = if :sets.size(set1) <= :sets.size(set2), do: {set1, set2}, else: {set2, set1}
-
-    disjointer_fun = fn elem, {small, acc} ->
-      if :sets.is_element(elem, small) do
-        {:sets.del_element(elem, small), acc}
+    map =
+      if :sets.is_disjoint(set1, set2) do
+        :sets.union(set1, set2)
      else
-        {small, [elem | acc]}
+        :sets.union(:sets.subtract(set1, set2), :sets.subtract(set2, set1))
      end
-    end

-    {new_small, list} = :sets.fold(disjointer_fun, {small, []}, large)
-    %{map_set1 | map: :sets.union(new_small, :sets.from_list(list, version: 2))}
+    %{map_set1 | map: map}
  end

  @doc """
@@ -204,7 +210,7 @@ defmodule MapSet do
  Checks if two sets are equal.

  The comparison between elements is done using `===/2`,
-  which a set with `1` is not equivalent to a set with
+  which means a set with `1` is not equivalent to a set with
  `1.0`.

  ## Examples
@@ -343,7 +349,7 @@ defmodule MapSet do
  > If you find yourself doing multiple calls to `MapSet.filter/2`
  > and `MapSet.reject/2` in a pipeline, it is likely more efficient
  > to use `Enum.map/2` and `Enum.filter/2` instead and convert to
-  > a map at the end using `MapSet.new/1`.
+  > a set at the end using `MapSet.new/1`.

  ## Examples

@@ -399,15 +405,15 @@ defmodule MapSet do
      iex> while_false
      MapSet.new([1, 3])

-      iex> {while_true, while_false} = MapSet.split_with(MapSet.new(), fn {_k, v} -> v > 50 end)
+      iex> {while_true, while_false} = MapSet.split_with(MapSet.new([10, 20, 60, 70]), fn v -> v > 50 end)
      iex> while_true
-      MapSet.new([])
+      MapSet.new([60, 70])
      iex> while_false
-      MapSet.new([])
+      MapSet.new([10, 20])

  """
  @doc since: "1.15.0"
-  @spec split_with(MapSet.t(), (term() -> as_boolean(term))) :: {MapSet.t(), MapSet.t()}
+  @spec split_with(t(a), (a -> as_boolean(term))) :: {t(a), t(a)} when a: value
  def split_with(%MapSet{map: map}, fun) when is_function(fun, 1) do
    {while_true, while_false} = Map.split_with(map, fn {key, _} -> fun.(key) end)
    {%MapSet{map: while_true}, %MapSet{map: while_false}}
@@ -190,7 +190,7 @@ defmodule Module do

    * a string (often a heredoc)
    * `false`, which will make the entity invisible to documentation-extraction
-      tools like [`ExDoc`](https://hexdocs.pm/ex_doc/)
+      tools like [`ExDoc`](https://ex-doc.hexdocs.pm/)
    * a keyword list, since Elixir 1.7.0

  For example:
@@ -216,7 +216,7 @@ defmodule Module do

  As can be seen in the example above, since Elixir 1.7.0 `@doc` and `@typedoc`
  also accept a keyword list that serves as a way to provide arbitrary metadata
-  about the entity. Tools like [`ExDoc`](https://hexdocs.pm/ex_doc/) and
+  about the entity. Tools like [`ExDoc`](https://ex-doc.hexdocs.pm/) and
  `IEx` may use this information to display annotations. A common use
  case is the `:since` key, which may be used to annotate in which version the
  function was introduced.
@@ -263,7 +263,7 @@ defmodule Module do

  Tools may use this information to ensure the module is recompiled
  in case any of the external resources change, see for example:
-  [`mix compile.elixir`](https://hexdocs.pm/mix/Mix.Tasks.Compile.Elixir.html).
+  [`mix compile.elixir`](https://mix.hexdocs.pm/Mix.Tasks.Compile.Elixir.html).

  The specified file path provided is interpreted as relative to
  the folder containing the project's `mix.exs`, which is the
@@ -321,7 +321,7 @@ defmodule Module do

  Accepts a string (often a heredoc) or `false` where `@moduledoc false`
  will make the module invisible to documentation extraction tools like
-  [`ExDoc`](https://hexdocs.pm/ex_doc/).
+  [`ExDoc`](https://ex-doc.hexdocs.pm/).

  Similarly to `@doc` also accepts a keyword list to provide metadata
  about the module. For more details, see the documentation of `@doc`
@@ -366,7 +366,8 @@ defmodule Module do
  Unlike other hooks, `@on_definition` will only invoke functions and
  never macros. This is to avoid `@on_definition` callbacks from
  redefining functions that have just been defined in favor of more
-  explicit approaches.
+  explicit approaches. They are also invoked in the reverse order of
+  registration.

  When just a module is provided, the function is assumed to be
  `__on_definition__/6`.
@@ -656,7 +657,8 @@ defmodule Module do
      @spec module_info(:attributes) :: keyword()
      @spec module_info(:compile) :: keyword()
      @spec module_info(:md5) :: binary()
-      @spec module_info(:nifs) :: module()
+      @spec module_info(:nifs) :: [function_info]
+        when function_info: {function_name :: atom(), arity :: non_neg_integer()}
      @spec module_info(:exports) :: [function_info]
        when function_info: {function_name :: atom(), arity :: non_neg_integer()}
      @spec module_info(:functions) :: [function_info]
@@ -679,7 +681,7 @@ defmodule Module do
  information. For documentation, see `c:Module.__info__/1`.
  '''

-  @type definition :: {atom, arity}
+  @type definition :: {function_name :: atom, arity}
  @type def_kind :: :def | :defp | :defmacro | :defmacrop

  @type create_opts :: [
@@ -752,7 +754,7 @@ defmodule Module do

  """
  @doc since: "1.12.0"
-  @spec reserved_attributes() :: map
+  @spec reserved_attributes() :: %{optional(atom()) => %{doc: binary()}}
  def reserved_attributes() do
    %{
      after_compile: %{
@@ -958,7 +960,7 @@ defmodule Module do

  It handles binaries and atoms.

-  > #### Untracked compile-time dependencies {. :warning}
+  > #### Untracked compile-time dependencies {: .warning}
  >
  > Use this function with care, as dynamically defining
  > module names at compilation time may lead to
@@ -984,7 +986,7 @@ defmodule Module do
  It handles binaries and atoms. If one of the aliases
  is nil, it is discarded.

-  > #### Untracked compile-time dependencies {. :warning}
+  > #### Untracked compile-time dependencies {: .warning}
  >
  > Use this function with care, as dynamically defining
  > module names at compilation time may lead to
@@ -1015,7 +1017,7 @@ defmodule Module do
  If the alias was not referenced yet, fails with `ArgumentError`.
  It handles binaries and atoms.

-  > #### Untracked compile-time dependencies {. :warning}
+  > #### Untracked compile-time dependencies {: .warning}
  >
  > Use this function with care, as dynamically defining
  > module names at compilation time may lead to
@@ -1039,7 +1041,7 @@ defmodule Module do
  If the alias was not referenced yet, fails with `ArgumentError`.
  It handles binaries and atoms.

-  > #### Untracked compile-time dependencies {. :warning}
+  > #### Untracked compile-time dependencies {: .warning}
  >
  > Use this function with care, as dynamically defining
  > module names at compilation time may lead to
@@ -1146,7 +1148,7 @@ defmodule Module do
  defp simplify_var(var, guess_priority) do
    case Atom.to_string(var) do
      "_" -> {:_, [], guess_priority}
-      "_" <> rest -> {String.to_atom(rest), [], guess_priority}
+      "_" <> rest -> {String.to_unsafe_atom(rest), [], guess_priority}
      _ -> {var, [], nil}
    end
  end
@@ -1157,7 +1159,7 @@ defmodule Module do
    rescue
      ArgumentError -> module
    else
-      module_name -> String.to_atom(Macro.underscore(List.last(module_name)))
+      module_name -> String.to_unsafe_atom(Macro.underscore(List.last(module_name)))
    end
  end

@@ -1190,7 +1192,7 @@ defmodule Module do
  defp expand_key(key, counters) do
    case counters do
      %{^key => count} when is_integer(count) and count >= 1 ->
-        {{:"#{key}#{count}", [], Elixir}, Map.put(counters, key, count - 1)}
+        {{String.to_unsafe_atom("#{key}#{count}"), [], Elixir}, Map.put(counters, key, count - 1)}

      _ ->
        {{key, [], Elixir}, counters}
@@ -1223,7 +1225,8 @@ defmodule Module do
  defp merge_signature({var, _, _} = older, {var, _, _}, _), do: older

  # Otherwise, returns a generic guess
-  defp merge_signature({_, meta, _}, _newer, i), do: {:"arg#{i}", meta, Elixir}
+  defp merge_signature({_, meta, _}, _newer, i),
+    do: {String.to_unsafe_atom("arg#{i}"), meta, Elixir}

  @doc """
  Checks if the module defines the given function or macro.
@@ -1249,17 +1252,18 @@ defmodule Module do

  """
  @spec defines?(module, definition) :: boolean
-  def defines?(module, {name, arity} = tuple)
-      when is_atom(module) and is_atom(name) and is_integer(arity) and arity >= 0 and arity <= 255 do
+  def defines?(module, {function_name, arity} = definition)
+      when is_atom(module) and is_atom(function_name) and is_integer(arity) and arity >= 0 and
+             arity <= 255 do
    {set, _bag} = data_tables_for!(module, __ENV__.function, @extra_error_msg_defines?)
-    :ets.member(set, {:def, tuple})
+    :ets.member(set, {:def, definition})
  end

  @doc """
  Checks if the module defines a function or macro of the
-  given `kind`.
+  given kind.

-  `kind` can be any of `:def`, `:defp`, `:defmacro`, or `:defmacrop`.
+  `def_kind` can be any of `:def`, `:defp`, `:defmacro`, or `:defmacrop`.

  This function can only be used on modules that have not yet been compiled.
  Use `Kernel.function_exported?/3` and `Kernel.macro_exported?/3` to check for
@@ -1275,12 +1279,13 @@ defmodule Module do

  """
  @spec defines?(module, definition, def_kind) :: boolean
-  def defines?(module, {name, arity} = tuple, def_kind)
-      when is_atom(module) and is_atom(name) and is_integer(arity) and arity >= 0 and arity <= 255 and
+  def defines?(module, {function_name, arity} = definition, def_kind)
+      when is_atom(module) and is_atom(function_name) and is_integer(arity) and arity >= 0 and
+             arity <= 255 and
             def_kind in [:def, :defp, :defmacro, :defmacrop] do
    {set, _bag} = data_tables_for!(module, __ENV__.function, @extra_error_msg_defines?)

-    case :ets.lookup(set, {:def, tuple}) do
+    case :ets.lookup(set, {:def, definition}) do
      [{_, ^def_kind, _, _, _, _}] -> true
      _ -> false
    end
@@ -1293,7 +1298,9 @@ defmodule Module do
  """
  @doc since: "1.7.0"
  @spec defines_type?(module, definition) :: boolean
-  def defines_type?(module, definition) when is_atom(module) do
+  def defines_type?(module, {function_name, arity} = definition)
+      when is_atom(module) and is_atom(function_name) and is_integer(arity) and arity >= 0 and
+             arity <= 255 do
    Kernel.Typespec.defines_type?(module, definition)
  end

@@ -1306,7 +1313,9 @@ defmodule Module do
  """
  @doc since: "1.7.0"
  @spec spec_to_callback(module, definition) :: boolean
-  def spec_to_callback(module, definition) do
+  def spec_to_callback(module, {function_name, arity} = definition)
+      when is_atom(module) and is_atom(function_name) and is_integer(arity) and arity >= 0 and
+             arity <= 255 do
    Kernel.Typespec.spec_to_callback(module, definition)
  end

@@ -1358,7 +1367,7 @@ defmodule Module do

  """
  @doc since: "1.13.0"
-  @spec overridables_in(module) :: [atom]
+  @spec overridables_in(module) :: [definition]
  def overridables_in(module) when is_atom(module) do
    assert_not_compiled!(__ENV__.function, module, :all)
    :elixir_overridable.overridables_for(module)
@@ -1407,10 +1416,10 @@ defmodule Module do

  """
  @spec definitions_in(module, def_kind) :: [definition]
-  def definitions_in(module, kind)
-      when is_atom(module) and kind in [:def, :defp, :defmacro, :defmacrop] do
+  def definitions_in(module, def_kind)
+      when is_atom(module) and def_kind in [:def, :defp, :defmacro, :defmacrop] do
    {set, _} = data_tables_for!(module, __ENV__.function, @extra_error_msg_definitions_in)
-    :ets.select(set, [{{{:def, :"$1"}, kind, :_, :_, :_, :_}, [], [:"$1"]}])
+    :ets.select(set, [{{{:def, :"$1"}, def_kind, :_, :_, :_, :_}, [], [:"$1"]}])
  end

  @doc """
@@ -1441,16 +1450,17 @@ defmodule Module do
           [{meta :: keyword, arguments :: [Macro.t()], guards :: [Macro.t()], Macro.t()}]}
          | nil
  @doc since: "1.12.0"
-  def get_definition(module, {name, arity}, options \\ [])
-      when is_atom(module) and is_atom(name) and is_integer(arity) and is_list(options) do
+  def get_definition(module, {function_name, arity} = _definition, options \\ [])
+      when is_atom(module) and is_atom(function_name) and is_integer(arity) and arity >= 0 and
+             arity <= 255 and is_list(options) do
    {set, bag} = data_tables_for!(module, __ENV__.function, "")

-    case :ets.lookup(set, {:def, {name, arity}}) do
+    case :ets.lookup(set, {:def, {function_name, arity}}) do
      [{_key, kind, meta, _, _, _}] ->
        clauses =
          if options[:skip_clauses],
            do: [],
-            else: bag_lookup_element(bag, {:clauses, {name, arity}}, 2)
+            else: bag_lookup_element(bag, {:clauses, {function_name, arity}}, 2)

        {:v1, kind, meta, clauses}

@@ -1467,10 +1477,11 @@ defmodule Module do
  """
  @doc since: "1.12.0"
  @spec delete_definition(module, definition) :: boolean()
-  def delete_definition(module, {name, arity})
-      when is_atom(module) and is_atom(name) and is_integer(arity) do
+  def delete_definition(module, {function_name, arity} = _definition)
+      when is_atom(module) and is_atom(function_name) and is_integer(arity) and arity >= 0 and
+             arity <= 255 do
    assert_not_compiled!(__ENV__.function, module, :writeable)
-    :elixir_def.take_definition(module, {name, arity}) != false
+    :elixir_def.take_definition(module, {function_name, arity}) != false
  end

  @doc """
@@ -1486,20 +1497,20 @@ defmodule Module do
  given.
  """
  @spec make_overridable(module, [definition]) :: :ok
-  def make_overridable(module, tuples) when is_atom(module) and is_list(tuples) do
+  def make_overridable(module, definitions) when is_atom(module) and is_list(definitions) do
    assert_not_compiled!(__ENV__.function, module, :writeable)

    func = fn
-      {function_name, arity} = tuple
+      {function_name, arity} = definition
      when is_atom(function_name) and is_integer(arity) and arity >= 0 and arity <= 255 ->
-        case :elixir_def.take_definition(module, tuple) do
+        case :elixir_def.take_definition(module, definition) do
          false ->
            raise ArgumentError,
                  "cannot make function #{function_name}/#{arity} " <>
                    "overridable because it was not defined"

          clause ->
-            :elixir_overridable.record_overridable(module, tuple, clause)
+            :elixir_overridable.record_overridable(module, definition, clause)
        end

      other ->
@@ -1508,7 +1519,7 @@ defmodule Module do
                "{function_name :: atom, arity :: 0..255} tuple, got: #{inspect(other)}"
    end

-    :lists.foreach(func, tuples)
+    :lists.foreach(func, definitions)
  end

  @spec make_overridable(module, module) :: :ok
@@ -1566,9 +1577,10 @@ defmodule Module do
  exists or one is pending.
  """
  @spec overridable?(module, definition) :: boolean
-  def overridable?(module, {function_name, arity} = tuple)
-      when is_atom(function_name) and is_integer(arity) and arity >= 0 and arity <= 255 do
-    :elixir_overridable.overridable_for(module, tuple) != :not_overridable
+  def overridable?(module, {function_name, arity} = definition)
+      when is_atom(module) and is_atom(function_name) and is_integer(arity) and arity >= 0 and
+             arity <= 255 do
+    :elixir_overridable.overridable_for(module, definition) != :not_overridable
  end

  @doc """
@@ -228,11 +228,7 @@ defmodule Module.Behaviour do
  defp behaviour_defined?(callbacks, behaviour) do
    callbacks
    |> Map.values()
-    |> List.flatten()
-    |> Enum.any?(fn
-      {_kind, ^behaviour, _optional?} -> true
-      {_kind, _behaviour, _optional?} -> false
-    end)
+    |> Enum.any?(fn list -> :lists.keymember(behaviour, 2, list) end)
  end

  defp warn_missing_impls(%{callbacks: callbacks} = context, _impl_contexts, _defs)
@@ -20,6 +20,7 @@ defmodule Module.ParallelChecker do
          {:max_concurrency, pos_integer()}
          | {:long_verification_threshold, pos_integer()}
          | {:each_long_verification, (module() -> term()) | (module(), pid() -> term())}
+          | {:profile, :time}
          | {atom(), term()}
        ]

@@ -67,17 +68,37 @@ defmodule Module.ParallelChecker do
  def spawn({pid, {checker, table}}, module, module_map, signatures, beam_location, log?) do
    # Protocols may have been consolidated. So if we know their beam location,
    # we discard their module map on purpose and start from file.
-    info =
-      if beam_location != [] and Keyword.has_key?(module_map.attributes, :__protocol__) do
-        List.to_string(beam_location)
-      else
-        cache_from_module_map(table, module_map, signatures)
-      end
-
-    inner_spawn(pid, checker, table, module, info, log?)
+    if beam_location != [] and Keyword.has_key?(module_map.attributes, :__protocol__) do
+      spawn_and_register_cache(pid, checker, table, module, List.to_string(beam_location), log?)
+    else
+      {mode, module_tuple} = cache_from_module_map(table, module_map, signatures)
+      ref = make_ref()
+      spawned = spawn_checker(ref, pid, checker, table, module, module_tuple, log?)
+      register_cache_and_checker(checker, mode, module, spawned, ref)
+      :ok
+    end
  end

-  defp inner_spawn(pid, checker, table, module, info, log?) do
+  defp spawn_checker(ref, pid, checker, table, module, module_tuple, log?) do
+    spawn(fn ->
+      mon_ref = Process.monitor(pid)
+
+      receive do
+        {^ref, :check, profile} ->
+          # Set the compiler info so we can collect warnings
+          Process.link(pid)
+          :erlang.put(:elixir_compiler_info, {pid, self()})
+          {warnings, errors} = check_module(module_tuple, {checker, table}, log?, profile)
+          send(pid, {__MODULE__, module, warnings, errors})
+          send(checker, {__MODULE__, :checked, ref})
+
+        {:DOWN, ^mon_ref, _, _, _} ->
+          :ok
+      end
+    end)
+  end
+
+  defp spawn_and_register_cache(pid, checker, table, module, info, log?) do
    ref = make_ref()

    spawned =
@@ -88,48 +109,28 @@ defmodule Module.ParallelChecker do
          {^ref, :cache} ->
            Process.link(pid)

-            {mode, module_tuple} =
-              cond do
-                is_binary(info) ->
-                  location =
-                    case :code.which(module) do
-                      [_ | _] = path -> path
-                      _ -> info
-                    end
-
-                  with {:ok, binary} <- File.read(location),
-                       {:ok,
-                        {_, [{:debug_info, {:debug_info_v1, backend, data}}, {~c"ExCk", checker}]}} <-
-                         :beam_lib.chunks(binary, [:debug_info, ~c"ExCk"]),
-                       {:ok, module_map} <- backend.debug_info(:elixir_v1, module, data, []),
-                       {@elixir_checker_version, contents} <- :erlang.binary_to_term(checker) do
-                    {cache_chunk(table, module, contents), module_map_to_module_tuple(module_map)}
-                  else
-                    _ -> {:not_found, nil}
-                  end
-
-                is_tuple(info) ->
-                  info
+            location =
+              case :code.which(module) do
+                [_ | _] = path -> path
+                _ -> info
              end

-            # We only make the module available now, so they are not visible during inference
-            :ets.insert(table, {module, mode})
-            send(checker, {ref, :cached})
-
-            receive do
-              {^ref, :check} ->
-                # Set the compiler info so we can collect warnings
-                :erlang.put(:elixir_compiler_info, {pid, self()})
-
-                {warnings, errors} =
-                  if module_tuple do
-                    check_module(module_tuple, {checker, table}, log?)
-                  else
-                    {[], []}
-                  end
-
-                send(pid, {__MODULE__, module, warnings, errors})
-                send(checker, {__MODULE__, :done, module})
+            with {:ok, binary} <- File.read(location),
+                 {:ok, {_, [{:debug_info, {:debug_info_v1, backend, data}}, {~c"ExCk", exck}]}} <-
+                   :beam_lib.chunks(binary, [:debug_info, ~c"ExCk"]),
+                 {:ok, module_map} <- backend.debug_info(:elixir_v1, module, data, []),
+                 {@elixir_checker_version, contents} <- :erlang.binary_to_term(exck) do
+              mode = cache_chunk(table, module, contents)
+              module_tuple = module_map_to_module_tuple(module_map)
+              spawned = spawn_checker(ref, pid, checker, table, module, module_tuple, log?)
+              :ets.insert(table, {module, mode})
+              send(checker, {__MODULE__, :cached, module, spawned, ref})
+            else
+              _ ->
+                # Nothing to check, so we notify everyone we are done
+                :ets.insert(table, {module, :uncached})
+                send(checker, {__MODULE__, :cached, module, nil, ref})
+                send(pid, {__MODULE__, module, [], []})
            end

          {:DOWN, ^mon_ref, _, _, _} ->
@@ -137,7 +138,7 @@ defmodule Module.ParallelChecker do
        end
      end)

-    register(checker, module, spawned, ref)
+    register_cache(checker, module, spawned, ref)
    :ok
  end

@@ -191,7 +192,7 @@ defmodule Module.ParallelChecker do
    log? = not match?({_, false}, value)

    for {module, file} <- runtime_files do
-      inner_spawn(self(), checker, table, module, file, log?)
+      spawn_and_register_cache(self(), checker, table, module, file, log?)
    end

    count = :gen_server.call(checker, :start, :infinity)
@@ -262,7 +263,7 @@ defmodule Module.ParallelChecker do

  ## Module checking

-  defp check_module(module_tuple, cache, log?) do
+  defp check_module(module_tuple, cache, log?, profile) do
    {module, file, line, definitions, no_warn_undefined, behaviours, impls, attrs, after_verify} =
      module_tuple

@@ -276,9 +277,13 @@ defmodule Module.ParallelChecker do
        definitions
      )

+    type_warnings =
+      profile(module, profile, fn ->
+        Module.Types.warnings(module, file, attrs, definitions, no_warn_undefined, cache)
+      end)
+
    {warnings, errors} =
-      module
-      |> Module.Types.warnings(file, attrs, definitions, no_warn_undefined, cache)
+      type_warnings
      |> Kernel.++(behaviour_warnings)
      |> group_diagnostics()
      |> emit_diagnostics(file, log?)
@@ -528,8 +533,12 @@ defmodule Module.ParallelChecker do
    :gen_server.call(server, {:unlock, module, mode}, :infinity)
  end

-  defp register(server, module, pid, ref) do
-    :gen_server.cast(server, {:register, module, pid, ref})
+  defp register_cache_and_checker(server, mode, module, pid, ref) do
+    :gen_server.cast(server, {:register_cache_and_checker, mode, module, pid, ref})
+  end
+
+  defp register_cache(server, module, pid, ref) do
+    :gen_server.cast(server, {:register_cache, module, pid, ref})
  end

  ## Server callbacks
@@ -572,11 +581,13 @@ defmodule Module.ParallelChecker do

    state = %{
      waiting: %{},
-      modules: [],
+      caches: [],
+      checkers: [],
      spawned: %{},
      schedulers: schedulers,
      threshold: threshold,
      callback: callback,
+      profile: Keyword.get(options, :profile),
      protocols: [],
      table: table
    }
@@ -584,20 +595,9 @@ defmodule Module.ParallelChecker do
    :gen_server.enter_loop(__MODULE__, [], state)
  end

-  def handle_call(:start, _from, %{modules: modules, protocols: protocols, table: table} = state) do
+  def handle_call(:start, _from, %{caches: caches, protocols: protocols, table: table} = state) do
    :ets.insert(table, Enum.map(protocols, &{&1, :uncached}))
-
-    for {_module, pid, ref} <- modules do
-      send(pid, {ref, :cache})
-    end
-
-    for {_module, _pid, ref} <- modules do
-      receive do
-        {^ref, :cached} -> :ok
-      end
-    end
-
-    {:reply, length(modules), run_checkers(%{state | protocols: []})}
+    {:reply, length(caches), run_caches(%{state | protocols: []})}
  end

  def handle_call({:lock, module}, from, %{waiting: waiting} = state) do
@@ -627,10 +627,21 @@ defmodule Module.ParallelChecker do
    {:noreply, state}
  end

-  def handle_info({__MODULE__, :done, module}, state) do
-    # Unfortunately we cannot assume uniqueness because the same module
-    # may be defined by mistake several times
-    {timer, spawned} = Map.pop(state.spawned, module)
+  def handle_info({__MODULE__, :cached, module, pid, ref}, state) do
+    {_nil, spawned} = Map.pop(state.spawned, ref)
+
+    state =
+      if pid do
+        %{state | spawned: spawned, checkers: [{module, pid, ref} | state.checkers]}
+      else
+        %{state | spawned: spawned}
+      end
+
+    {:noreply, run_caches(state)}
+  end
+
+  def handle_info({__MODULE__, :checked, ref}, state) do
+    {timer, spawned} = Map.pop(state.spawned, ref)
    timer && Process.cancel_timer(timer)
    {:noreply, run_checkers(%{state | spawned: spawned})}
  end
@@ -639,11 +650,41 @@ defmodule Module.ParallelChecker do
    {:stop, :normal, state}
  end

-  def handle_cast({:register, module, pid, ref}, %{modules: modules} = state) do
-    {:noreply, %{state | modules: [{module, pid, ref} | modules]}}
+  def handle_cast({:register_cache, module, pid, ref}, %{caches: caches} = state) do
+    {:noreply, %{state | caches: [{module, pid, ref} | caches]}}
  end

-  defp run_checkers(%{modules: []} = state) do
+  def handle_cast(
+        {:register_cache_and_checker, mode, module, pid, ref},
+        %{caches: caches, checkers: checkers} = state
+      ) do
+    {:noreply,
+     %{state | caches: [{module, mode} | caches], checkers: [{module, pid, ref} | checkers]}}
+  end
+
+  defp run_caches(%{caches: [], spawned: spawned} = state) do
+    if spawned == %{}, do: run_checkers(state), else: state
+  end
+
+  defp run_caches(%{spawned: spawned, schedulers: schedulers} = state)
+       when map_size(spawned) >= schedulers do
+    state
+  end
+
+  defp run_caches(%{caches: [cache | caches]} = state) do
+    case cache do
+      {_module, pid, ref} ->
+        send(pid, {ref, :cache})
+        spawned = Map.put(state.spawned, ref, nil)
+        run_caches(%{state | caches: caches, spawned: spawned})
+
+      {module, mode} ->
+        :ets.insert(state.table, {module, mode})
+        run_caches(%{state | caches: caches})
+    end
+  end
+
+  defp run_checkers(%{checkers: []} = state) do
    state
  end

@@ -652,10 +693,25 @@ defmodule Module.ParallelChecker do
    state
  end

-  defp run_checkers(%{modules: [{module, pid, ref} | modules]} = state) do
-    send(pid, {ref, :check})
-    timer = Process.send_after(self(), {__MODULE__, :timeout, module, pid}, state.threshold)
-    spawned = Map.put(state.spawned, module, timer)
-    run_checkers(%{state | modules: modules, spawned: spawned})
+  defp run_checkers(%{checkers: [{module, pid, ref} | checkers]} = state) do
+    send(pid, {ref, :check, state.profile})
+    timer = :erlang.send_after(state.threshold, self(), {__MODULE__, :timeout, module, pid})
+    spawned = Map.put(state.spawned, ref, timer)
+    run_checkers(%{state | checkers: checkers, spawned: spawned})
+  end
+
+  defp profile(module, :time, fun) do
+    {time, result} = :timer.tc(fun)
+
+    IO.puts(
+      :stderr,
+      "[profile] Type checked #{inspect(module)} in #{div(time, 1000)}ms"
+    )
+
+    result
+  end
+
+  defp profile(_module, _profile, fun) do
+    fun.()
  end
 end
@@ -4,7 +4,7 @@

 defmodule Module.Types do
  @moduledoc false
-  alias Module.Types.{Descr, Expr, Pattern, Helpers}
+  alias Module.Types.{Apply, Descr, Expr, Helpers, Pattern}

  # The mode controls what happens on function application when
  # there are gradual arguments. Non-gradual arguments always
@@ -86,7 +86,7 @@ defmodule Module.Types do
          {_kind, inferred, context} = local_handler(meta, fun_arity, stack, context, finder)

          if infer_signatures? and kind == :def and fun_arity not in @no_infer do
-            {[{fun_arity, inferred} | types], private, context}
+            {[{fun_arity, group_clauses_by_return(inferred)} | types], private, context}
          else
            {types, private, context}
          end
@@ -136,8 +136,8 @@ defmodule Module.Types do
  defp impl_for(attrs) do
    case List.keyfind(attrs, :__impl__, 0) do
      {:__impl__, [protocol: protocol, for: for]} ->
-        if Code.ensure_loaded?(protocol) and function_exported?(protocol, :behaviour_info, 1) do
-          {for, protocol.behaviour_info(:callbacks)}
+        if Code.ensure_loaded?(protocol) and function_exported?(protocol, :__protocol__, 1) do
+          {for, protocol.__protocol__(:functions)}
        else
          nil
        end
@@ -232,7 +232,7 @@ defmodule Module.Types do

    context =
      Enum.reduce(defs, context(), fn {fun_arity, _kind, meta, _clauses} = def, context ->
-        # Optimized version of finder, since we already the definition
+        # Optimized version of finder, since we already have the definition
        finder = fn _ -> default_domain(:dynamic, def, fun_arity, impl) end
        {_kind, _inferred, context} = local_handler(meta, fun_arity, stack, context, finder)
        context
@@ -251,15 +251,21 @@ defmodule Module.Types do
      context ->
        {_kind, info, mapping} = Map.fetch!(context.local_sigs, fun_arity)

-        clauses_indexes =
-          for type_index <- pending,
-              not skip_unused_clause?(info, type_index),
-              {clause_index, ^type_index} <- mapping,
-              do: clause_index
+        {used_indexes, unused_indexes} =
+          Enum.reduce(mapping, {[], []}, fn {clause_index, type_index},
+                                            {used_indexes, unused_indexes} ->
+            if type_index in pending and not skip_unused_clause?(info, type_index) do
+              {used_indexes, [clause_index | unused_indexes]}
+            else
+              {[clause_index | used_indexes], unused_indexes}
+            end
+          end)

-        Enum.reduce(clauses_indexes, context, fn clause_index, context ->
+        unused_indexes = Enum.uniq(unused_indexes) -- used_indexes
+
+        Enum.reduce(unused_indexes, context, fn clause_index, context ->
          {meta, _args, _guards, _body} = Enum.fetch!(clauses, clause_index)
-          stack = %{stack | function: fun_arity}
+          stack = %{stack | function: fun_arity} |> with_file_meta(meta)
          Helpers.warn(__MODULE__, {:unused_clause, kind, fun_arity}, meta, stack, context)
        end)
    end
@@ -289,11 +295,11 @@ defmodule Module.Types do

      local_sigs ->
        case finder.(fun_arity) do
-          {mode, {fun_arity, kind, meta, clauses}, expected} ->
+          {mode, {fun_arity, kind, _meta, clauses}, expected} ->
            context = put_in(context.local_sigs, Map.put(local_sigs, fun_arity, kind))

            {inferred, mapping, context} =
-              local_handler(mode, fun_arity, kind, meta, clauses, expected, stack, context)
+              local_handler(mode, fun_arity, kind, clauses, expected, stack, context)

            context =
              update_in(context.local_sigs, &Map.put(&1, fun_arity, {kind, inferred, mapping}))
@@ -306,9 +312,10 @@ defmodule Module.Types do
    end
  end

-  defp local_handler(:traverse, {_, arity}, _kind, _meta, clauses, _expected, stack, context) do
+  defp local_handler(:traverse, {_, arity}, _kind, clauses, _expected, stack, context) do
    context =
-      Enum.reduce(clauses, context, fn {_meta, _args, _guards, body}, context ->
+      Enum.reduce(clauses, context, fn {meta, _args, _guards, body}, context ->
+        stack = with_file_meta(stack, meta)
        Module.Types.Traverse.of_expr(body, stack, context)
      end)

@@ -316,59 +323,234 @@ defmodule Module.Types do
    {inferred, [{0, 0}], context}
  end

-  defp local_handler(mode, fun_arity, kind, meta, clauses, expected, stack, context) do
+  defp local_handler(mode, fun_arity, kind, clauses, expected, stack, context) do
    {fun, _arity} = fun_arity
-    stack = stack |> fresh_stack(mode, fun_arity) |> with_file_meta(meta)
+    stack = fresh_stack(stack, mode, fun_arity)
    base_info = {:def, kind, fun, expected}

-    {_, _, _, mapping, clauses_types, clauses_context} =
-      Enum.reduce(clauses, {0, 0, Pattern.init_previous(), [], [], context}, fn
-        {meta, args, guards, body}, {index, total, previous, mapping, inferred, acc_context} ->
+    case clauses do
+      [{meta, args, [], {:super, _, [_ | _]} = body}] ->
+        default_local_handler(meta, args, body, base_info, kind, fun, expected, stack, context)
+
+      _ ->
+        infer_local_handler(clauses, base_info, kind, fun, expected, stack, context)
+    end
+  end
+
+  defp default_local_handler(meta, args, body, base_info, kind, fun, expected, stack, context) do
+    stack = with_file_meta(stack, meta)
+    guards = []
+    previous = Pattern.init_previous()
+    fresh_context = fresh_context(context)
+    info = {base_info, args, guards}
+
+    try do
+      {trees, _, _, _, head_context} =
+        Pattern.of_head(args, guards, expected, previous, info, meta, stack, fresh_context)
+
+      # Compute the intersected arrows from the function call
+      {:super, meta, call_args} = body
+      {_kind, call_fun} = Keyword.fetch!(meta, :super)
+      term = Descr.term()
+      of_fun = &Expr.of_expr/5
+
+      {arrows, body_context} =
+        Apply.local_arrows(call_fun, call_args, term, body, stack, head_context, of_fun)
+
+      # For each arrow, compute the default arrow
+      {_, _, mapping, inferred} =
+        Enum.reduce(arrows, {0, 0, [], []}, fn
+          {clause_domain, return_type}, {index, total, mapping, inferred} ->
+            of_fun = &Expr.of_expr(&1, &2, body, stack, &3)
+
+            {_clause_args, clause_context} =
+              Helpers.zip_map_reduce(call_args, clause_domain, head_context, of_fun)
+
+            clause_types = Pattern.of_domain(trees, stack, clause_context)
+
+            {type_index, inferred} =
+              add_inferred(inferred, clause_types, return_type, total - 1, [])
+
+            total = if type_index == -1, do: total + 1, else: total
+            {index + 1, total, [{0, index} | mapping], inferred}
+        end)
+
+      domain =
+        case inferred do
+          [_] ->
+            nil
+
+          _ ->
+            inferred
+            |> Enum.map(fn {args, _} -> args end)
+            |> Enum.zip_with(fn types -> Enum.reduce(types, &Descr.opt_union/2) end)
+        end
+
+      {{:infer, domain, Enum.reverse(inferred)}, mapping, restore_context(body_context, context)}
+    rescue
+      e ->
+        internal_error!(e, __STACKTRACE__, kind, meta, fun, args, guards, body, stack)
+    end
+  end
+
+  defp infer_local_handler(clauses, base_info, kind, fun, expected, stack, context) do
+    {_, clauses_types, clauses_context} =
+      Enum.reduce(clauses, {Pattern.init_previous(), [], context}, fn
+        {meta, args, guards, body}, {previous, inferred, acc_context} ->
+          stack = with_file_meta(stack, meta)
          fresh_context = fresh_context(acc_context)
          info = {base_info, args, guards}

          try do
-            {trees, previous, context} =
+            {trees, precise?, head_no_previous_args_types, previous, head_context} =
              Pattern.of_head(args, guards, expected, previous, info, meta, stack, fresh_context)

            {return_type, context} =
-              Expr.of_expr(body, Descr.term(), body, stack, context)
+              Expr.of_expr(body, Descr.term(), body, stack, head_context)

            args_types = Pattern.of_domain(trees, stack, context)

-            {type_index, inferred} =
-              add_inferred(inferred, args_types, return_type, total - 1, [])
+            head_args_types =
+              case inferred do
+                [] -> nil
+                _ -> Pattern.of_domain(trees, stack, head_context)
+              end

-            if type_index == -1 do
-              {index + 1, total + 1, previous, [{index, total} | mapping], inferred, context}
-            else
-              {index + 1, total, previous, [{index, type_index} | mapping], inferred, context}
-            end
+            args_triplet = {args_types, head_args_types, head_no_previous_args_types}
+            inferred = [{args_triplet, return_type, precise?} | inferred]
+            {previous, inferred, context}
          rescue
            e ->
              internal_error!(e, __STACKTRACE__, kind, meta, fun, args, guards, body, stack)
          end
      end)

+    {clauses_types, mapping, domain} =
+      clauses_types
+      |> Enum.reverse()
+      |> group_clauses()
+
    domain =
      case clauses_types do
-        [_] ->
-          nil
-
-        _ ->
-          clauses_types
-          |> Enum.map(fn {args, _} -> args end)
-          |> Enum.zip_with(fn types -> Enum.reduce(types, &Descr.union/2) end)
+        [_] -> nil
+        _ -> domain
      end

-    inferred = {:infer, domain, Enum.reverse(clauses_types)}
+    inferred = {:infer, domain, clauses_types}
    {inferred, mapping, restore_context(clauses_context, context)}
  end

+  defp group_clauses(clauses) do
+    {_, all_clauses, filtered_clauses, non_empty?} =
+      Enum.reduce(clauses, {0, [], [], false}, fn
+        {_args_triplet, return, precise?} = clause,
+        {index, all_clauses, filtered_clauses, non_empty?} ->
+          empty? = Descr.empty?(return)
+          indexed_clause = {clause, index}
+
+          filtered_clauses =
+            if precise? and empty? do
+              filtered_clauses
+            else
+              [indexed_clause | filtered_clauses]
+            end
+
+          {index + 1, [indexed_clause | all_clauses], filtered_clauses, non_empty? or not empty?}
+      end)
+
+    clauses =
+      if non_empty? do
+        Enum.reverse(filtered_clauses)
+      else
+        Enum.reverse(all_clauses)
+      end
+
+    [
+      {{{args, _head_args, _head_no_previous_args}, _return, _precise?}, _index}
+      | clauses_tail
+    ] = clauses
+
+    domain =
+      Enum.reduce(clauses_tail, args, fn
+        {{{args, head_args, head_no_previous_args}, _return, _precise?}, _index}, domain ->
+          compute_domain(args, head_args, head_no_previous_args, domain)
+      end)
+
+    {_, mapping, inferred} =
+      Enum.reduce(clauses, {0, [], []}, fn
+        {{{args, _head_args, _head_no_previous_args}, return, _precise?}, index},
+        {total, mapping, inferred} ->
+          {type_index, inferred} = add_inferred(inferred, args, return, total - 1, [])
+
+          if type_index == -1 do
+            {total + 1, [{index, total} | mapping], inferred}
+          else
+            {total, [{index, type_index} | mapping], inferred}
+          end
+      end)
+
+    {Enum.reverse(inferred), mapping, domain}
+  end
+
+  defp compute_domain(
+         [arg | args_types],
+         [head_arg | head_args_types],
+         [no_prev_arg | no_prev_args_types],
+         [d | domain]
+       ) do
+    [
+      # This is an optimization that broadens the domain, but it is acceptable
+      # because the domain is used for reverse arrows and not type checking.
+      #
+      # The overall idea is that, if we have a function with three clauses,
+      # the domain is computed by unioning their inferred types. However, their
+      # inferred types often have the different of the previous clauses:
+      #
+      #     opt_union(r3 ^ (c3 - c2 - c1), r2 ^ (c2 - c1), r1 ^ c1)
+      #
+      # Where `rN` represents the refinement in every function body.
+      #
+      # What this function does is, if the type of a given arg in a clause
+      # before and after the body is the same (meaning r3 is term), then
+      # we replace all of `(c3 - c2 - c1)` by just `c3`, which removes
+      # many of the differences in the node. However, keep in mind that,
+      # because `r2` may have refine `c2` in the previous clause, the domain
+      # may end-up being broader. Take this example:
+      #
+      #     % %{..., foo: integer()} -> binary()
+      #     def example(%{foo: var}), do: Integer.to_string(var)
+      #
+      #     % %{...} and not %{..., foo: term()} -> :error
+      #     def example(%{}), do: :error
+      #
+      # The actual domain is:
+      #
+      #     %{..., foo: not_set()} or %{..., foo: integer()}
+      #     #=> %{..., foo: if_set(integer())}
+      #
+      # But we will infer:
+      #
+      #     %{...} or %{..., foo: integer()}
+      #     #=> %{...}
+      #
+      # We lose precision but this is exactly what we want: to have simpler types.
+      # Furthermore, the signature used in type checking is not refined in any way,
+      # so type checking is still sound.
+      if arg == head_arg do
+        Descr.opt_union(Descr.upper_bound(no_prev_arg), d)
+      else
+        Descr.opt_union(arg, d)
+      end
+      | compute_domain(args_types, head_args_types, no_prev_args_types, domain)
+    ]
+  end
+
+  defp compute_domain([], [], [], []), do: []
+
  # We check for term equality of types as an optimization
  # to reduce the amount of check we do at runtime.
  defp add_inferred([{args, existing_return} | tail], args, return, index, acc),
-    do: {index, Enum.reverse(acc, [{args, Descr.union(existing_return, return)} | tail])}
+    do: {index, Enum.reverse(acc, [{args, Descr.opt_union(existing_return, return)} | tail])}

  defp add_inferred([head | tail], args, return, index, acc),
    do: add_inferred(tail, args, return, index - 1, [head | acc])
@@ -376,6 +558,58 @@ defmodule Module.Types do
  defp add_inferred([], args, return, -1, acc),
    do: {-1, [{args, return} | Enum.reverse(acc)]}

+  # Compact clauses that have the same return and differ in exactly one
+  # argument by unioning that argument. For example:
+  #
+  #   (integer(), atom() -> boolean()) and (float(), atom() -> boolean())
+  #
+  # becomes:
+  #
+  #   (number(), atom() -> boolean())
+  #
+  # Arity-zero clauses have no argument position to widen.
+  defp group_clauses_by_return({:infer, domain, [{[_ | _], _} | _] = clauses}) do
+    clauses =
+      Enum.reduce(clauses, [], fn {args, return}, acc ->
+        group_clause_by_return(acc, args, return)
+      end)
+
+    {:infer, domain, clauses}
+  end
+
+  defp group_clauses_by_return(info), do: info
+
+  defp group_clause_by_return([{existing_args, return} | tail], args, return) do
+    case union_args(existing_args, args, [], false) do
+      nil ->
+        [{existing_args, return} | group_clause_by_return(tail, args, return)]
+
+      new_args ->
+        [{new_args, return} | tail]
+    end
+  end
+
+  defp group_clause_by_return([head | tail], args, return) do
+    [head | group_clause_by_return(tail, args, return)]
+  end
+
+  defp group_clause_by_return([], args, return), do: [{args, return}]
+
+  defp union_args([arg | existing], [arg | args], acc, changed?) do
+    union_args(existing, args, [arg | acc], changed?)
+  end
+
+  # Allow exactly one differing argument. That one position is widened
+  # with union/2. A second difference means the clauses must stay separate.
+  defp union_args([existing_arg | existing], [arg | args], acc, false) do
+    union_args(existing, args, [Descr.opt_union(existing_arg, arg) | acc], true)
+  end
+
+  defp union_args([_ | _], [_ | _], _acc, true), do: nil
+
+  # In theory fully equal args are merged on add_inferred
+  defp union_args([], [], acc, _changed?), do: Enum.reverse(acc)
+
  defp with_file_meta(stack, meta) do
    case Keyword.fetch(meta, :file) do
      {:ok, {meta_file, _}} -> %{stack | file: meta_file}
@@ -437,7 +671,9 @@ defmodule Module.Types do
      # The mode to be used, see the @modes attribute
      mode: mode,
      # The function for handling local calls
-      local_handler: handler
+      local_handler: handler,
+      # Reverse arrow handling (nil | :cache | :except_none | :include_none)
+      reverse_arrow: nil
    }
  end

@@ -459,20 +695,26 @@ defmodule Module.Types do
      # Local signatures used by local handler
      local_sigs: %{},
      # Track which clauses have been used across private local calls
-      local_used: %{}
+      local_used: %{},
+      # Cached reverse arrows
+      reverse_arrows: %{}
    }
  end

  defp fresh_stack(stack, mode, function) when mode in @modes do
-    %{stack | mode: mode, function: function}
+    %{stack | mode: mode, function: function, reverse_arrow: nil}
  end

  defp fresh_context(context) do
-    %{context | vars: %{}, failed: false}
+    %{context | vars: %{}, failed: false, reverse_arrows: %{}}
  end

-  defp restore_context(later_context, %{vars: vars, failed: failed}) do
-    %{later_context | vars: vars, failed: failed}
+  defp restore_context(later_context, %{
+         vars: vars,
+         failed: failed,
+         reverse_arrows: reverse_arrows
+       }) do
+    %{later_context | vars: vars, failed: failed, reverse_arrows: reverse_arrows}
  end

  ## Diagnostics
@@ -480,7 +722,7 @@ defmodule Module.Types do
  def format_diagnostic({:unused_clause, kind, {fun, arity}}) do
    %{
      message:
-        "this clause of #{kind} #{fun}/#{arity} is never used (or it will always fail when invoked)"
+        "this clause of #{kind} #{fun}/#{arity} is never used (or it will always fail/warn when invoked)"
    }
  end

@@ -57,7 +57,7 @@ defmodule Module.Types.Helpers do
  ## Warnings

  @doc """
-  Converts an itneger into ordinal.
+  Converts an integer into ordinal.
  """
  def integer_to_ordinal(i) do
    case rem(i, 10) do
@@ -329,7 +329,7 @@ defmodule Module.Types.Helpers do
            end
        end

-      {{:., _, [:lists, :member]}, meta, [expr, [_ | _] = args]} = call ->
+      {{:., _, [:lists, :member]}, meta, [expr, args]} = call when is_list(args) ->
        if Enum.any?(args, &match?({:|, _, [_, _]}, &1)) do
          call
        else
@@ -363,7 +363,7 @@ defmodule Module.Types.Helpers do

      {:case, meta, [expr, [do: clauses]]} ->
        case meta[:type_check] do
-          {:case, _} ->
+          {:case, op} ->
            case clauses do
              [
                {:->, _,
@@ -382,7 +382,8 @@ defmodule Module.Types.Helpers do
                   true
                 ]},
                {:->, _, [[{:_, _, Kernel}], false]}
-              ] ->
+              ]
+              when op == :! ->
                {:!, meta, [expr]}

              [
@@ -402,7 +403,8 @@ defmodule Module.Types.Helpers do
                   right_side
                 ]},
                {:->, _, [[{var, _, Kernel}], {var, _, Kernel}]}
-              ] ->
+              ]
+              when op == :|| ->
                {:||, meta, [expr, right_side]}

              [
@@ -422,7 +424,8 @@ defmodule Module.Types.Helpers do
                   {var, _, Kernel}
                 ]},
                {:->, _, [[{:_, _, Kernel}], right_side]}
-              ] ->
+              ]
+              when op == :&& ->
                {:&&, meta, [expr, right_side]}

              [
@@ -442,15 +445,33 @@ defmodule Module.Types.Helpers do
                   else_block
                 ]},
                {:->, _, [[{:_, _, Kernel}], do_block]}
-              ] ->
+              ]
+              when op == :if ->
                {:if, meta, [expr, [do: do_block, else: else_block]]}

              [
                {:->, _, [[false], else_block]},
                {:->, _, [[true], do_block]}
-              ] ->
+              ]
+              when op == :if ->
                {:if, meta, [expr, [do: do_block, else: else_block]]}

+              [
+                {:->, _, [[false], false]},
+                {:->, _, [[true], right]}
+                | _
+              ]
+              when op == :and ->
+                {:and, meta, [expr, right]}
+
+              [
+                {:->, _, [[false], right]},
+                {:->, _, [[true], true]}
+                | _
+              ]
+              when op == :or ->
+                {:or, meta, [expr, right]}
+
              _ ->
                {:case, meta, [expr, [do: {:..., [], []}]]}
            end
@@ -11,7 +11,7 @@ defmodule Module.Types.Of do
  @prefix quote(do: ...)
  @suffix quote(do: ...)

-  @integer_or_float union(integer(), float())
+  @integer_or_float opt_union(integer(), float())
  @integer integer()
  @float float()
  @binary binary()
@@ -31,7 +31,7 @@ defmodule Module.Types.Of do
  @doc """
  Marks a variable with error.

-  This purposedly deletes all traces of the variable,
+  This purposely deletes all traces of the variable,
  as it is often invoked when the cause for error is elsewhere.
  """
  def error_var({_, meta, _}, context) do
@@ -48,7 +48,7 @@ defmodule Module.Types.Of do
  @doc """
  Declares a variable.
  """
-  def declare_var(var, context) do
+  def declare_var(var, type \\ term(), context) do
    {var_name, meta, var_context} = var
    version = Keyword.fetch!(meta, :version)

@@ -58,7 +58,7 @@ defmodule Module.Types.Of do

      vars ->
        data = %{
-          type: term(),
+          type: type,
          name: var_name,
          context: var_context,
          off_traces: [],
@@ -106,15 +106,15 @@ defmodule Module.Types.Of do
          context
      end

-    case context do
+    case stack do
      _ when type in @skip_refinement_for or is_map_key(data, :errored) ->
        {old_type, context}

-      %{pattern_info: %{guard_context: guard_context}} ->
-        new_type = intersection(old_type, type)
+      %{reverse_arrow: reverse_arrow} when reverse_arrow in [:except_none, :include_none] ->
+        new_type = opt_intersection(old_type, type)

        case empty?(new_type) do
-          true when guard_context == :orelse ->
+          true when reverse_arrow == :include_none ->
            data = %{
              data
              | type: none(),
@@ -171,7 +171,7 @@ defmodule Module.Types.Of do
        {:ok, error_type(), context}

      %{^version => %{type: old_type, off_traces: off_traces} = data} = vars ->
-        new_type = intersection(type, old_type)
+        new_type = opt_intersection(type, old_type)

        data = %{
          data
@@ -245,7 +245,7 @@ defmodule Module.Types.Of do
        type =
          Enum.reduce(vars_conds, type, fn {vars, _cond}, acc ->
            %{^version => %{type: type}} = vars
-            union(acc, type)
+            opt_union(acc, type)
          end)

        {_, context} = refine_body_var(version, type, expr, stack, context)
@@ -264,8 +264,8 @@ defmodule Module.Types.Of do
    {Float, float()},
    {Function, fun()},
    {Integer, integer()},
-    {List, union(empty_list(), non_empty_list(term(), term()))},
-    {Map, open_map(__struct__: if_set(negation(atom())))},
+    {List, opt_union(empty_list(), non_empty_list(term(), term()))},
+    {Map, open_map(__struct__: {Module.Types.Descr.opt_negation(atom()), true})},
    {Port, port()},
    {PID, pid()},
    {Reference, reference()},
@@ -294,7 +294,7 @@ defmodule Module.Types.Of do
    if info = mode == :closed && Code.ensure_loaded?(struct) && struct.__info__(:struct) do
      struct_type(struct, info)
    else
-      open_map(__struct__: atom([struct]))
+      open_map(__struct__: {atom([struct]), false})
    end
  end

@@ -365,7 +365,7 @@ defmodule Module.Types.Of do
                # Because a multiple key may override single keys, we can only
                # collect single keys while there are no multiples.
                [key] when multiple == [] ->
-                  {dynamic?, domain, [{key, value_type} | single], multiple}
+                  {dynamic?, domain, [{key, {value_type, false}} | single], multiple}

                _ ->
                  {dynamic?, domain, single, [{pos, value_type} | multiple]}
@@ -381,37 +381,47 @@ defmodule Module.Types.Of do
          closed_map(non_multiple)

        [{keys, type} | tail] ->
-          for key <- keys, t <- cartesian_map(tail) do
-            closed_map(non_multiple ++ [{key, type} | t])
+          products = cartesian_map(tail)
+
+          for key <- keys, product <- products do
+            closed_map(non_multiple ++ [{key, {type, false}} | product])
          end
-          |> Enum.reduce(&union/2)
+          |> Enum.reduce(&opt_union/2)
      end

    {if(dynamic?, do: dynamic(map), else: map), context}
  end

  defp union_negated([], new_type, single, multiple) do
-    single = Enum.map(single, fn {key, old_type} -> {key, union(old_type, new_type)} end)
-    multiple = Enum.map(multiple, fn {keys, old_type} -> {keys, union(old_type, new_type)} end)
+    single =
+      Enum.map(single, fn
+        {key, {old_type, optional?}} ->
+          {key, {opt_union(old_type, new_type), optional?}}
+      end)
+
+    multiple =
+      Enum.map(multiple, fn {keys, old_type} -> {keys, opt_union(old_type, new_type)} end)
+
    {single, multiple}
  end

  defp union_negated(negated, new_type, single, multiple) do
    {single, matched} =
-      Enum.map_reduce(single, [], fn {key, old_type}, matched ->
-        if key in negated do
-          {{key, old_type}, [key | matched]}
-        else
-          {{key, union(old_type, new_type)}, matched}
-        end
+      Enum.map_reduce(single, [], fn
+        {key, {old_type, optional?}}, matched ->
+          if key in negated do
+            {{key, {old_type, optional?}}, [key | matched]}
+          else
+            {{key, {opt_union(old_type, new_type), optional?}}, matched}
+          end
      end)

    multiple =
      Enum.map(multiple, fn {keys, old_type} ->
-        {keys, union(old_type, new_type)}
+        {keys, opt_union(old_type, new_type)}
      end)

-    {Enum.map(negated -- matched, fn key -> {key, not_set()} end) ++ single, multiple}
+    {Enum.map(negated -- matched, fn key -> {key, {none(), true}} end) ++ single, multiple}
  end

  defp pairs(pairs, expected, stack, context, of_fun) do
@@ -420,7 +430,7 @@ defmodule Module.Types.Of do

      expected_value_type =
        with {[key], [], []} <- pos_neg_domain,
-             {_, expected_value_type} <- map_fetch_key(expected, key) do
+             {_optional?, expected_value_type} <- map_fetch_key(expected, key) do
          expected_value_type
        else
          _ -> term()
@@ -455,7 +465,8 @@ defmodule Module.Types.Of do
        [[]]

      [{keys, type} | tail] ->
-        for key <- keys, t <- cartesian_map(tail), do: [{key, type} | t]
+        products = cartesian_map(tail)
+        for key <- keys, product <- products, do: [{key, {type, false}} | product]
    end
  end

@@ -467,35 +478,35 @@ defmodule Module.Types.Of do
  # TODO: Type check the fields match the struct
  def struct_instance(struct, args, expected, meta, stack, context, of_fun)
      when is_atom(struct) do
-    {info, context} = struct_info(struct, :expr, meta, stack, context)
+    {info, context} = struct_info(struct, :expr, meta, stack, context, true)

    if is_nil(info) do
-      raise "expected #{inspect(struct)} to return struct metadata, but got none"
+      {dynamic(), context}
+    else
+      # The compiler has already checked the keys are atoms and which ones are required.
+      {args_types, context} =
+        Enum.map_reduce(args, context, fn {key, value}, context when is_atom(key) ->
+          value_type =
+            case map_fetch_key(expected, key) do
+              {_optional?, expected_value_type} -> expected_value_type
+              _ -> term()
+            end
+
+          {type, context} = of_fun.(value, value_type, stack, context)
+          {{key, {type, false}}, context}
+        end)
+
+      {closed_map([__struct__: {atom([struct]), false}] ++ args_types), context}
    end
-
-    # The compiler has already checked the keys are atoms and which ones are required.
-    {args_types, context} =
-      Enum.map_reduce(args, context, fn {key, value}, context when is_atom(key) ->
-        value_type =
-          case map_fetch_key(expected, key) do
-            {_, expected_value_type} -> expected_value_type
-            _ -> term()
-          end
-
-        {type, context} = of_fun.(value, value_type, stack, context)
-        {{key, type}, context}
-      end)
-
-    {closed_map([{:__struct__, atom([struct])} | args_types]), context}
  end

  @doc """
  Returns `__info__(:struct)` information about a struct.
  """
-  def struct_info(struct, kind, meta, stack, context) do
+  def struct_info(struct, kind, meta, stack, context, must_exist? \\ false) do
    case stack.no_warn_undefined do
      %Macro.Env{} = env ->
-        case :elixir_map.maybe_load_struct_info(meta, struct, env) do
+        case :elixir_map.maybe_load_struct_info(meta, struct, :soft, env) do
          {:ok, info} -> {info, context}
          {:error, _desc} -> {nil, context}
        end
@@ -511,7 +522,7 @@ defmodule Module.Types.Of do

          {info, context}
        else
-          error = {:unknown_struct, kind, struct}
+          error = {:unknown_struct, kind, struct, must_exist?}
          {nil, error(error, meta, stack, context)}
        end
    end
@@ -524,9 +535,16 @@ defmodule Module.Types.Of do
  # we introduce typed structs. They are only used by exceptions.
  def struct_type(struct, info, args_types \\ []) do
    term = dynamic()
-    pairs = for %{field: field} <- info, do: {field, term}
-    pairs = [{:__struct__, atom([struct])} | pairs]
-    pairs = if args_types == [], do: pairs, else: pairs ++ args_types
+    pairs = for %{field: field} <- info, do: {field, {term, false}}
+    pairs = [{:__struct__, {atom([struct]), false}} | pairs]
+
+    pairs =
+      if args_types == [] do
+        pairs
+      else
+        pairs ++ args_types
+      end
+
    closed_map(pairs)
  end

@@ -848,19 +866,26 @@ defmodule Module.Types.Of do
    }
  end

-  def format_diagnostic({:unknown_struct, kind, module}) do
-    message =
-      if Code.ensure_loaded?(module) do
-        "struct #{inspect(module)} is undefined (there is such module but it does not define a struct)"
-      else
-        "struct #{inspect(module)} is undefined " <>
-          "(module #{inspect(module)} is not available or is yet to be defined)"
+  def format_diagnostic({:unknown_struct, kind, module, must_exist?}) do
+    detail =
+      case {Code.ensure_loaded?(module), must_exist?} do
+        {true, false} ->
+          "there is such module but it does not define a struct"
+
+        {false, false} ->
+          "module #{inspect(module)} is not available or is yet to be defined"
+
+        {true, true} ->
+          "the module may have been redefined as it no longer defines a struct"
+
+        {false, true} ->
+          "the module was also only available but may have been removed during compilation"
      end

    %{
-      message: message,
+      message: "struct #{inspect(module)} is undefined (#{detail})",
      group: true,
-      severity: if(kind == :pattern, do: :error, else: :warning)
+      severity: if(kind == :pattern or must_exist?, do: :error, else: :warning)
    }
  end

@@ -12,60 +12,49 @@ defmodule Module.Types.Pattern do
  Returns the initial value for previous clause information.
  """
  def init_previous do
-    []
+    {[], none()}
  end

-  defp empty_previous?([]), do: true
-  defp empty_previous?([_ | _]), do: false
+  defp empty_previous?({[], _}), do: true
+  defp empty_previous?({[_ | _], _}), do: false

-  defp previous_subtype?(_, []), do: false
+  # Previous is always an upper bound (static), so we can use subtype?
+  defp previous_subtype?(_, {[], _}), do: false
  defp previous_subtype?([], _), do: true
-
-  defp previous_subtype?([type], previous),
-    do: subtype?(type, Enum.reduce(previous, none(), &union(&2, hd(&1))))
-
-  defp previous_subtype?(args, previous) do
-    subtype?(
-      args_to_domain(args),
-      Enum.reduce(previous, none(), &union(&2, args_to_domain(&1)))
-    )
-  end
+  defp previous_subtype?(args, {_, descr}), do: subtype?(args_to_previous(args), descr)

  defp concat_previous([], previous),
    do: previous

-  defp concat_previous(types, previous),
-    do: [types | previous]
+  defp concat_previous(types, {list, descr}),
+    do: {[types | list], opt_union(descr, args_to_previous(types))}

-  defp of_pattern_previous(types, [], _trees, _pattern_info, _tag, _stack, _context) do
-    {:ok, types}
+  defp args_to_previous([type]), do: upper_bound(type)
+  defp args_to_previous(types), do: args_to_static_domain(types)
+
+  defp of_pattern_previous(types, {[], _}, _stack) do
+    {types, false}
  end

-  defp of_pattern_previous(types, previous, trees, pattern_info, tag, stack, context) do
-    types =
+  defp of_pattern_previous(types, {_, descr}, stack) do
+    refined_types =
      case types do
-        [type] ->
-          [Enum.reduce(previous, type, &difference(&2, hd(&1)))]
-
-        [_ | _] ->
-          previous
-          |> Enum.reduce(args_to_domain(types), &difference(&2, args_to_domain(&1)))
-          |> domain_to_flat_args(types)
+        [type] -> [opt_difference(type, descr)]
+        [_ | _] -> args_to_domain(types) |> opt_difference(descr) |> domain_to_flat_args(types)
      end

-    if index = Enum.find_index(types, &empty?/1) do
-      {_, _, pattern} = Enum.fetch!(trees, index)
-      context = badpattern_error(pattern, index, tag, stack, context)
-      {:error, error_vars(pattern_info, context)}
-    else
-      {:ok, types}
-    end
+    # check_previous? is an optimization. If types have not changed,
+    # it means args_types and previous are disjoint, and any further
+    # refinement will keep them disjoint, so no need to check for previous.
+    {refined_types, stack.mode != :infer and types != refined_types}
  end

-  defp previous_to_string(previous) do
-    Enum.map_join(previous, "\n    ", fn types ->
+  defp previous_to_string({list, _}) do
+    list
+    |> Enum.reverse()
+    |> Enum.map_join("\n    ", fn types ->
      types
-      |> Enum.map_join(", ", &to_quoted_string/1)
+      |> Enum.map_join(", ", &(&1 |> upper_bound() |> to_quoted_string()))
      |> indent(4)
    end)
  end
@@ -194,7 +183,7 @@ defmodule Module.Types.Pattern do
  def of_head(patterns, guards, expected, previous, tag, meta, stack, original) do
    stack = %{stack | meta: meta}

-    {trees, precise?, args_types, context} =
+    {trees, precise?, check_previous?, args_types, context} =
      of_precise_head(patterns, guards, expected, previous, tag, stack, original)

    if context.failed and stack.mode != :infer and not empty_previous?(previous) and
@@ -202,28 +191,29 @@ defmodule Module.Types.Pattern do
      # If it failed, let's try to break it down to a better error message.
      # First we check if it fails without previous, if it doesn't, check if it is redundant.
      case of_precise_head(patterns, guards, expected, init_previous(), tag, stack, original) do
-        {other_trees, _, _, %{failed: true} = other_context} ->
-          {other_trees, previous, other_context}
+        {other_trees, _, _, _, %{failed: true} = other_context} ->
+          {other_trees, false, args_types, previous, other_context}

-        {other_trees, _, args_types, other_context} ->
+        {other_trees, _, _, args_types, other_context} ->
          if previous_subtype?(args_types, previous) do
            warning = {:redundant, tag, expected, args_types, previous, other_context}
-            {other_trees, previous, warn(__MODULE__, warning, meta, stack, other_context)}
+            context = warn(__MODULE__, warning, meta, stack, other_context)
+            {other_trees, false, args_types, previous, context}
          else
-            {trees, previous, context}
+            {trees, false, args_types, previous, context}
          end
      end
    else
      cond do
-        stack.mode != :infer and previous_subtype?(args_types, previous) ->
+        check_previous? and previous_subtype?(args_types, previous) ->
          warning = {:redundant, tag, expected, args_types, previous, context}
-          {trees, previous, warn(__MODULE__, warning, meta, stack, context)}
+          {trees, false, args_types, previous, warn(__MODULE__, warning, meta, stack, context)}

        precise? ->
-          {trees, concat_previous(args_types, previous), context}
+          {trees, true, args_types, concat_previous(args_types, previous), context}

        true ->
-          {trees, previous, context}
+          {trees, false, args_types, previous, context}
      end
    end
  end
@@ -231,7 +221,7 @@ defmodule Module.Types.Pattern do
  defp of_precise_head([], guards, _expected, _previous, _tag, stack, context) do
    %{vars: vars} = context
    {guard_precise?, changed, context} = of_guards(guards, vars, stack, context)
-    {[], guard_precise?, [], of_changed(Map.keys(changed), stack, context)}
+    {[], guard_precise?, false, [], of_changed(Map.keys(changed), stack, context)}
  end

  defp of_precise_head(patterns, guards, expected, previous, tag, stack, context) do
@@ -248,48 +238,39 @@ defmodule Module.Types.Pattern do
           of_pattern_intersect(trees, 0, [], pattern_info, tag, stack, context),
         # We compute the args types before we do the intersection with previous clauses
         args_types =
-           (with [_ | _] <- previous,
-                 {:ok, _types, context} <-
+           (with false <- empty_previous?(previous),
+                 {:ok, context} <-
                   of_pattern_refine(types, changed, pattern_info, tag, stack, context) do
              trees_to_args_types(trees, stack, context)
            else
              _ -> nil
            end),
-         {:ok, types} <-
-           of_pattern_previous(types, previous, trees, pattern_info, tag, stack, context),
-         {:ok, _types, context} <-
-           of_pattern_refine(types, changed, pattern_info, tag, stack, context) do
-      {trees, pattern_precise? and guard_precise?,
+         {types, check_previous?} = of_pattern_previous(types, previous, stack),
+         {:ok, context} <- of_pattern_refine(types, changed, pattern_info, tag, stack, context) do
+      {trees, pattern_precise? and guard_precise?, check_previous?,
       args_types || trees_to_args_types(trees, stack, context), context}
    else
      {:error, context} ->
-        {trees, false, trees_to_args_types(trees, stack, context), context}
+        {trees, false, false, trees_to_args_types(trees, stack, context), context}
    end
  end

  defp trees_to_args_types(trees, stack, context) do
    Enum.map(trees, fn {tree, _, _} ->
-      tree
-      |> of_pattern_tree(stack, context)
-      |> upper_bound()
+      of_pattern_tree(tree, stack, context)
    end)
  end

  @doc """
  Computes the domain from the pattern tree and expected types.

-  Note we use `upper_bound` because the user of dynamic in the signature
+  Note we use `upper_bound` because the use of dynamic in the signature
  won't make a difference.
  """
-  def of_domain([{tree, expected, _pattern} | trees], stack, context) do
-    [
-      intersection(of_pattern_tree(tree, stack, context), expected) |> upper_bound()
-      | of_domain(trees, stack, context)
-    ]
-  end
-
-  def of_domain([], _stack, _context) do
-    []
+  def of_domain(trees, stack, context) do
+    Enum.map(trees, fn {tree, _, _} ->
+      tree |> of_pattern_tree(stack, context) |> upper_bound()
+    end)
  end

  defp of_pattern_args_zip(
@@ -315,7 +296,8 @@ defmodule Module.Types.Pattern do
  @doc """
  Handles the match operator.
  """
-  def of_match(pattern, expected_fun, expr, stack, context) do
+  def of_match(pattern, expected_fun, expr, meta, stack, context) do
+    stack = %{stack | meta: meta}
    context = init_pattern_info(context, [])

    {tree, _precise?, context} =
@@ -329,9 +311,9 @@ defmodule Module.Types.Pattern do

    with {:ok, types} <-
           of_pattern_intersect(args, 0, [], pattern_info, tag, stack, context),
-         {:ok, [type], context} <-
+         {:ok, context} <-
           of_pattern_refine(types, %{}, pattern_info, tag, stack, context) do
-      {type, context}
+      {of_pattern_tree(tree, stack, context), context}
    else
      {:error, context} -> {expected, context}
    end
@@ -340,30 +322,34 @@ defmodule Module.Types.Pattern do
  @doc """
  Handles matches in generators.
  """
-  def of_generator(pattern, guards, expected, tag, expr, stack, %{vars: vars} = context) do
+  def of_generator(pattern, guards, expected, op, expr, meta, stack, %{vars: vars} = context)
+      when is_atom(op) do
+    stack = %{stack | meta: meta}
    context = init_pattern_info(context, [])

-    {tree, _precise?, context} =
+    {tree, pattern_precise?, context} =
      of_pattern(pattern, [%{root: {:arg, 0}, expr: expr}], stack, context)

    {pattern_info, context} = pop_pattern_info(context)
+    {guard_precise?, changed, context} = of_guards(guards, vars, stack, context)
+
    args = [{tree, expected, pattern}]
-    {_precise?, changed, context} = of_guards(guards, vars, stack, context)
+    tag = {op, expr, expected}

    with {:ok, types} <-
           of_pattern_intersect(args, 0, [], pattern_info, tag, stack, context),
-         {:ok, _types, context} <-
+         {:ok, context} <-
           of_pattern_refine(types, changed, pattern_info, tag, stack, context) do
-      context
+      {args, pattern_precise? and guard_precise?, context}
    else
-      {:error, context} -> context
+      {:error, context} -> {args, false, context}
    end
  end

  defp of_pattern_intersect([head | tail], index, acc, pattern_info, tag, stack, context) do
    {tree, expected, pattern} = head
    actual = of_pattern_tree(tree, stack, context)
-    type = intersection(actual, expected)
+    type = opt_intersection(actual, expected)

    if empty?(type) do
      context = badpattern_error(pattern, index, tag, stack, context)
@@ -415,7 +401,7 @@ defmodule Module.Types.Pattern do
    context -> {:error, error_vars(pattern_info, context)}
  else
    {changed, context} ->
-      {:ok, types, of_changed(Map.keys(changed), stack, context)}
+      {:ok, of_changed(Map.keys(changed), stack, context)}
  end

  defp error_vars(pattern_info, context) do
@@ -425,6 +411,13 @@ defmodule Module.Types.Pattern do
    end)
  end

+  defp match_var do
+    version = make_ref()
+    {version, match_var(version)}
+  end
+
+  defp match_var(version), do: {:match, [version: version], __MODULE__}
+
  defp match_error?({:match, _, __MODULE__}, _type), do: true
  defp match_error?(_var, type), do: empty?(type)

@@ -505,7 +498,7 @@ defmodule Module.Types.Pattern do

  defp of_pattern_var([{:subpattern, key} | rest], type, context) do
    %{^key => subpattern} = context.subpatterns
-    of_pattern_var(rest, intersection(type, subpattern), context)
+    of_pattern_var(rest, opt_intersection(type, subpattern), context)
  end

  defp of_pattern_var([:tail | rest], type, context) do
@@ -525,7 +518,7 @@ defmodule Module.Types.Pattern do
    # This logic mirrors the code in `Apply.compare`
    cond do
      # If it is a singleton, we can always be precise
-      singleton?(type) -> if polarity, do: type, else: negation(type)
+      singleton?(type) -> if polarity, do: type, else: Module.Types.Descr.opt_negation(type)
      # We are checking for `not x == 1` or similar, we can't say anything about x
      polarity == false -> term()
      # We are checking for `x == 1`, make sure x is integer or float
@@ -541,14 +534,18 @@ defmodule Module.Types.Pattern do

  defp of_pattern_tree({:open_map, static, dynamic}, stack, context) do
    dynamic =
-      Enum.map(dynamic, fn {key, value} -> {key, of_pattern_tree(value, stack, context)} end)
+      Enum.map(dynamic, fn {key, value} ->
+        {key, {of_pattern_tree(value, stack, context), false}}
+      end)

    open_map(static ++ dynamic)
  end

  defp of_pattern_tree({:closed_map, static, dynamic}, stack, context) do
    dynamic =
-      Enum.map(dynamic, fn {key, value} -> {key, of_pattern_tree(value, stack, context)} end)
+      Enum.map(dynamic, fn {key, value} ->
+        {key, {of_pattern_tree(value, stack, context), false}}
+      end)

    closed_map(static ++ dynamic)
  end
@@ -557,7 +554,7 @@ defmodule Module.Types.Pattern do
    tail
    |> Enum.reduce(
      of_pattern_tree(head, stack, context),
-      &union(of_pattern_tree(&1, stack, context), &2)
+      &opt_union(of_pattern_tree(&1, stack, context), &2)
    )
    |> non_empty_list(of_pattern_tree(suffix, stack, context))
  end
@@ -565,7 +562,7 @@ defmodule Module.Types.Pattern do
  defp of_pattern_tree({:intersection, entries}, stack, context) do
    entries
    |> Enum.map(&of_pattern_tree(&1, stack, context))
-    |> Enum.reduce(&intersection/2)
+    |> Enum.reduce(&opt_intersection/2)
  end

  defp of_pattern_tree({:var, version}, _stack, context) do
@@ -668,8 +665,8 @@ defmodule Module.Types.Pattern do
      |> Enum.split_while(&(not is_versioned_var(&1)))
      |> case do
        {matches, []} ->
-          version = make_ref()
-          {true, matches, version, {:match, [version: version], __MODULE__}}
+          {version, var} = match_var()
+          {true, matches, version, var}

        {pre, [{_, meta, _} = var | post]} ->
          version = Keyword.fetch!(meta, :version)
@@ -697,7 +694,7 @@ defmodule Module.Types.Pattern do
      if static == [] do
        {:intersection, dynamic}
      else
-        {:intersection, [Enum.reduce(static, &intersection/2) | dynamic]}
+        {:intersection, [Enum.reduce(static, &opt_intersection/2) | dynamic]}
      end

    context = of_var(var, version, [%{root: return, expr: match}], context)
@@ -728,20 +725,20 @@ defmodule Module.Types.Pattern do

      pairs = Map.new(pairs)
      term = term()
-      static = [__struct__: atom([struct])]
+      static = [__struct__: {atom([struct]), false}]
      dynamic = []

      {static, dynamic} =
        Enum.reduce(info, {static, dynamic}, fn %{field: field}, {static, dynamic} ->
          case pairs do
            %{^field => value_type} when is_descr(value_type) ->
-              {[{field, value_type} | static], dynamic}
+              {[{field, {value_type, false}} | static], dynamic}

            %{^field => value_type} ->
              {static, [{field, value_type} | dynamic]}

            _ ->
-              {[{field, term} | static], dynamic}
+              {[{field, {term, false}} | static], dynamic}
          end
        end)

@@ -779,7 +776,9 @@ defmodule Module.Types.Pattern do
    {refined, context} = of_match_var(var, atom(), expr, stack, context)

    if compatible?(refined, atom()) do
-      of_open_map(args, singleton?(refined), [__struct__: refined], [], path, stack, context)
+      fields = [__struct__: {refined, false}]
+      precise? = match?({:_, _, _}, var) or singleton?(refined)
+      of_open_map(args, precise?, fields, [], path, stack, context)
    else
      error = {:badstruct, refined, expr, context}
      {error_type(), false, error(__MODULE__, error, meta, stack, context)}
@@ -793,8 +792,9 @@ defmodule Module.Types.Pattern do

  # <<...>>>
  defp of_pattern({:<<>>, _meta, args} = node, _path, stack, context) do
+    precise? = of_precise_bitstring?(node, context.vars)
    {type, context} = Of.bitstring(args, :match, stack, context)
-    {type, of_precise_bitstring?(node), context}
+    {type, precise?, context}
  end

  # left ++ right
@@ -812,17 +812,16 @@ defmodule Module.Types.Pattern do
    version = Keyword.fetch!(meta, :version)

    case Enum.reverse(reverse_path) do
-      [%{root: :key} | _] -> {Of.var(var, context), false, context}
-      path -> {{:var, version}, false, of_shared_var(var, version, true, path, context)}
+      [%{root: :key} | _] ->
+        {Of.var(var, context), false, context}
+
+      path ->
+        precise? = singleton?(context.vars[version].type)
+        {{:var, version}, precise?, of_shared_var(var, version, true, path, context)}
    end
  end

-  # _
-  defp of_pattern({:_, _meta, _var_context}, _path, _stack, context) do
-    {term(), true, context}
-  end
-
-  # var
+  # var (includes underscores)
  defp of_pattern({name, meta, ctx} = var, path, _stack, context)
       when is_atom(name) and is_atom(ctx) do
    version = Keyword.fetch!(meta, :version)
@@ -872,7 +871,7 @@ defmodule Module.Types.Pattern do
          precise? = precise? and value_precise?

          if is_descr(value_type) do
-            {precise?, [{key, value_type} | static], dynamic, context}
+            {precise?, [{key, {value_type, false}} | static], dynamic, context}
          else
            {precise?, static, [{key, value_type} | dynamic], context}
          end
@@ -962,13 +961,13 @@ defmodule Module.Types.Pattern do
    type =
      case {static, dynamic} do
        {static, []} when is_descr(suffix_type) ->
-          non_empty_list(Enum.reduce(static, &union/2), suffix_type)
+          non_empty_list(Enum.reduce(static, &opt_union/2), suffix_type)

        {[], dynamic} ->
          {:non_empty_list, dynamic, suffix_type}

        {static, dynamic} ->
-          {:non_empty_list, [Enum.reduce(static, &union/2) | dynamic], suffix_type}
+          {:non_empty_list, [Enum.reduce(static, &opt_union/2) | dynamic], suffix_type}
      end

    {type, false, context}
@@ -999,12 +998,19 @@ defmodule Module.Types.Pattern do
    {type, precise?, put_in(context.subpatterns[key], of_pattern_tree(type, stack, context))}
  end

-  defp of_precise_bitstring?({:<<>>, _meta, [{:"::", _, [expr, {type, _, _}]}]})
+  defp of_precise_bitstring?({:<<>>, _meta, [{:"::", _, [expr, {type, _, _}]}]}, vars)
       when type in [:binary, :bitstring, :bytes, :bits] do
-    is_var(expr) or of_precise_bitstring?(expr)
+    new_var?(expr, vars) or of_precise_bitstring?(expr, vars)
  end

-  defp of_precise_bitstring?(_), do: false
+  defp of_precise_bitstring?(_expr, _vars), do: false
+
+  defp new_var?({_, meta, _} = var, vars) when is_var(var) do
+    version = Keyword.fetch!(meta, :version)
+    not is_map_key(vars, version)
+  end
+
+  defp new_var?(_expr, _vars), do: false

  ## Guards
  #
@@ -1030,11 +1036,13 @@ defmodule Module.Types.Pattern do
  end

  defp of_guards(guards, vars, stack, context) do
+    stack = %{stack | reverse_arrow: :except_none}
+
    context =
      init_pattern_info(context, %{
-        guard_context: :andalso,
        parent_version: nil,
        vars: vars,
+        subpatterns_vars: %{},
        changed: %{}
      })

@@ -1060,7 +1068,12 @@ defmodule Module.Types.Pattern do
      end)

    expr = Enum.reduce(guards, {:_, [], []}, &{:when, [], [&2, &1]})
-    context = %{context | vars: vars, conditional_vars: conditional_vars}
+
+    context = %{
+      context
+      | vars: Map.merge(vars, context.pattern_info.subpatterns_vars),
+        conditional_vars: conditional_vars
+    }

    {precise? and Of.all_same_conditional_vars?(vars_conds),
     Of.reduce_conditional_vars(vars_conds, expr, stack, context)}
@@ -1071,10 +1084,6 @@ defmodule Module.Types.Pattern do
     %{context | pattern_info: %{pattern_info | parent_version: parent_version}}}
  end

-  defp enable_conditional_mode(%{pattern_info: pattern_info} = context) do
-    %{context | pattern_info: %{pattern_info | guard_context: :orelse}, conditional_vars: %{}}
-  end
-
  defp maybe_badguard(type, guard, stack, context) do
    case booleaness(type) do
      :maybe_both ->
@@ -1126,7 +1135,7 @@ defmodule Module.Types.Pattern do
      Enum.map_reduce(prefix, context, &of_guard(&1, term(), expr, stack, &2))

    {suffix, context} = of_guard(suffix, term(), expr, stack, context)
-    {non_empty_list(Enum.reduce(prefix, &union/2), suffix), context}
+    {non_empty_list(Enum.reduce(prefix, &opt_union/2), suffix), context}
  end

  # {left, right}
@@ -1166,7 +1175,7 @@ defmodule Module.Types.Pattern do
  # var.field
  def of_guard({{:., _, [callee, key]}, _, []} = map_fetch, expected, expr, stack, context)
      when not is_atom(callee) do
-    {type, context} = of_guard(callee, open_map([{key, expected}]), expr, stack, context)
+    {type, context} = of_guard(callee, open_map([{key, {expected, false}}]), expr, stack, context)
    Of.map_fetch(map_fetch, type, key, stack, context)
  end

@@ -1258,14 +1267,26 @@ defmodule Module.Types.Pattern do
    end
  end

+  @dynamic_fail dynamic(atom([:fail]))
+
+  # `x or :fail` is a pattern used in Elixir to make guards fail.
+  # For example, `not is_struct(x, Foo)` checks if `Foo` is an atom
+  # or fails, but in the negation case `:fail` becomes a possible
+  # return type leading to a violation, we don't want. So we mark
+  # `:fail` as dynamic as its whole purpose is to cause failures.
+  defp of_remote(:orelse, [left, :fail], _call, expected, stack, context) do
+    {type, context} = of_guard(left, expected, left, stack, context)
+    {opt_union(type, @dynamic_fail), context}
+  end
+
  defp of_remote(fun, _args, call, expected, stack, context)
       when fun in [:and, :or, :andalso, :orelse] do
-    {both_domain, abort_domain, always_rhs?} =
+    {boolean, abort_domain, always_rhs?} =
      case fun do
-        :andalso -> {@atom_true, @atom_false, false}
-        :orelse -> {@atom_false, @atom_true, false}
-        :and -> {@atom_true, @atom_false, true}
-        :or -> {@atom_false, @atom_true, true}
+        :andalso -> {true, @atom_false, false}
+        :orelse -> {false, @atom_true, false}
+        :and -> {true, @atom_false, true}
+        :or -> {false, @atom_true, true}
      end

    # If we have multiple operations in a row,
@@ -1280,46 +1301,108 @@ defmodule Module.Types.Pattern do
    # For example, if the expected type is true for andalso, then it can
    # only be true if both clauses are executed, so we know the first
    # argument has to be true and the second has to be expected.
-    if subtype?(expected, both_domain) do
-      of_logical_all([left | right], true, both_domain, abort_domain, stack, context)
-    else
-      cond_context = enable_conditional_mode(context)
+    cond do
+      is_nil(context.pattern_info) or booleaness(expected) == {boolean, :always} ->
+        of_logical_all([left | right], true, expected, abort_domain, stack, context)

-      # Compute the sure types, which are stored directly in the context
-      {_type, context} = of_guard(left, boolean(), left, stack, context)
+      right == [] ->
+        of_guard(left, expected, left, stack, context)

-      # andalso/orelse may not execute the rhs, so we cannot get sure types from it
-      context =
-        case always_rhs? do
-          true ->
-            Enum.reduce(right, context, fn expr, context ->
-              {_, context} = of_guard(expr, boolean(), expr, stack, context)
+      true ->
+        cond_context = %{context | conditional_vars: %{}}
+        stack = %{stack | reverse_arrow: :include_none}
+
+        # Compute the sure types, which are stored directly in the context
+        {_type, context} = of_guard(left, boolean(), left, stack, context)
+
+        # andalso/orelse may not execute the rhs, so we cannot get sure types from it
+        context =
+          case always_rhs? do
+            true ->
+              Enum.reduce(right, context, fn expr, context ->
+                {_, context} = of_guard(expr, boolean(), expr, stack, context)
+                context
+              end)
+
+            false ->
              context
-            end)
+          end

-          false ->
-            context
-        end
+        {type, vars_conds} =
+          of_logical_cond([left | right], none(), [], expected, stack, cond_context)

-      {type, vars_conds} =
-        of_logical_cond([left | right], none(), [], expected, stack, cond_context)
+        # We will be precise if all branches changed the same variable
+        context =
+          update_in(context.pattern_info.vars, fn
+            false -> false
+            vars -> Of.all_same_conditional_vars?(vars_conds) and vars
+          end)

-      # We will be precise if all branches changed the same variable
-      context =
-        update_in(context.pattern_info.vars, fn
-          false -> false
-          vars -> Of.all_same_conditional_vars?(vars_conds) and vars
-        end)
-
-      {type, Of.reduce_conditional_vars(vars_conds, call, stack, context)}
+        {type, Of.reduce_conditional_vars(vars_conds, call, stack, context)}
    end
  end

+  # We cannot track precision for lists, so we assign match variables
+  # to hd/tl and refine them accordingly.
+  defp of_remote(
+         fun,
+         [arg],
+         call,
+         expected,
+         stack,
+         context
+       )
+       when fun in [:hd, :tl] do
+    arg_key =
+      Macro.prewalk(arg, fn
+        {left, meta, right} -> {left, Keyword.take(meta, [:version]), right}
+        node -> node
+      end)
+
+    subpattern_key = {fun, arg_key}
+
+    {found?, var_version, context} =
+      case context.subpatterns do
+        %{^subpattern_key => var_version} ->
+          case context.vars do
+            %{^var_version => _} -> {true, var_version, context}
+            %{} -> {false, var_version, context}
+          end
+
+        %{} ->
+          var_version = make_ref()
+          {false, var_version, put_in(context.subpatterns[subpattern_key], var_version)}
+      end
+
+    var = match_var(var_version)
+
+    context =
+      if found? do
+        context
+      else
+        {type, context} =
+          Apply.remote(:erlang, fun, [arg], expected, call, stack, context, &of_guard/5)
+
+        context = Of.declare_var(var, type, context)
+
+        update_in(context.pattern_info, fn %{subpatterns_vars: subpatterns_vars} = pattern_info ->
+          %{
+            pattern_info
+            | vars: false,
+              subpatterns_vars:
+                Map.put(subpatterns_vars, var_version, Map.fetch!(context.vars, var_version))
+          }
+        end)
+      end
+
+    of_guard(var, expected, call, stack, context)
+  end
+
  defp of_remote(fun, args, call, expected, stack, context) do
    Apply.remote(:erlang, fun, args, expected, call, stack, context, &of_guard/5)
  end

-  defp unpack_op({{:., _, [:erlang, fun]}, _, [left, right]}, fun, acc) do
+  defp unpack_op({{:., _, [_mod, fun]}, _, [left, right]}, fun, acc) do
    unpack_op(left, fun, unpack_op(right, fun, acc))
  end

@@ -1365,7 +1448,7 @@ defmodule Module.Types.Pattern do

    case disjoint? do
      true -> {type, context}
-      false -> {union(to_abort, type), context}
+      false -> {opt_union(to_abort, type), context}
    end
  end

@@ -1380,7 +1463,7 @@ defmodule Module.Types.Pattern do
      of_guard(head, expected, head, stack, context)

    acc_vars = [{vars, cond_vars} | acc_vars]
-    of_logical_cond(tail, union(acc_type, type), acc_vars, expected, stack, context)
+    of_logical_cond(tail, opt_union(acc_type, type), acc_vars, expected, stack, context)
  end

  defp of_logical_cond([], acc_type, acc_vars, _expected, _stack, _context) do
@@ -1479,30 +1562,54 @@ defmodule Module.Types.Pattern do
    message =
      with {_op, meta, expr, type} <- info,
           true <- previous_subtype?(expected, previous) do
-        if match?({:case, :||}, meta[:type_check]) do
-          """
-          the right-hand side of || will always execute:
+        case meta[:type_check] do
+          {:case, :||} ->
+            """
+            the right-hand side of || will always execute:

-              #{expr_to_string(expr) |> indent(4)}
+                #{expr_to_string(expr) |> indent(4)}

-          because the left-hand side always evaluates to:
+            because the left-hand side always evaluates to:

-              #{to_quoted_string(type) |> indent(4)}
-          """
-        else
-          """
-          the following clause cannot match because the previous clauses already matched all possible values:
+                #{to_quoted_string(type) |> indent(4)}
+            """

-              #{args_to_string(args) |> indent(4)} ->
+          {:case, :!} ->
+            """
+            the following conditional expression:

-          it attempts to match on the result of:
+                #{expr_to_string({:!, [], [expr]}) |> indent(4)}

-              #{expr_to_string(expr) |> indent(4)}
+            will always evaluate to true because its inner expression has type:

-          which has the already matched type:
+                #{to_quoted_string(type) |> indent(4)}
+            """

-              #{to_quoted_string(type) |> indent(4)}
-          """
+          {:case, :"!!"} ->
+            """
+            the following conditional expression:
+
+                #{expr_to_string({:!, [], [{:!, [], [expr]}]}) |> indent(4)}
+
+            will always evaluate to false because its inner expression has type:
+
+                #{to_quoted_string(type) |> indent(4)}
+            """
+
+          _ ->
+            """
+            the following clause cannot match because the previous clauses already matched all possible values:
+
+                #{args_to_string(args) |> indent(4)} ->
+
+            it attempts to match on the result of:
+
+                #{expr_to_string(expr) |> indent(4)}
+
+            which has the already matched type:
+
+                #{to_quoted_string(type) |> indent(4)}
+            """
        end
      else
        _ ->
@@ -1544,8 +1651,9 @@ defmodule Module.Types.Pattern do
  #
  # $ typep head_pattern =
  #     {{:def, kind, fun, types}, args, guards} or
-  #     {{:case | :try_else, meta, expr, type}, [arg]} or
-  #     {:for_reduce | :receive | :try_catch | :with_else | :fn, [arg]}
+  #     {{:case, meta, expr, type}, [arg]} or
+  #     {{:try_else | :with_else, type}, [arg]} or
+  #     {:for_reduce | :receive | :try_catch | :fn, [arg]}
  #
  # $ typep match_pattern =
  #     {:with or :for or :match, pattern, type}
@@ -1591,62 +1699,85 @@ defmodule Module.Types.Pattern do
    end
  end

-  defp badpattern({{op, meta, expr, type}, args}, _index) when op in [:case, :try_else] do
-    with {:case, op} <- meta[:type_check] do
-      message =
-        cond do
-          op == :|| ->
-            additional =
-              with {:case, meta, [_, _]} <- expr,
-                   {:case, :||} <- meta[:type_check] do
-                "(shown as ... below) "
-              else
-                _ -> ""
-              end
+  defp badpattern({{:case, meta, expr, type}, args}, _index) do
+    case meta[:type_check] do
+      {:case, op} ->
+        message =
+          cond do
+            op == :|| ->
+              additional =
+                with {:case, meta, [_, _]} <- expr,
+                     {:case, :||} <- meta[:type_check] do
+                  "(shown as ... below) "
+                else
+                  _ -> ""
+                end

-            """
-            the right-hand side of || #{additional}will never be executed:
+              """
+              the right-hand side of || #{additional}will never be executed:

-                #{expr_to_string({:||, [], [expr, {:..., [], []}]}) |> indent(4)}
+                  #{expr_to_string({:||, [], [expr, {:..., [], []}]}) |> indent(4)}

-            because the left-hand side always evaluates to:
+              because the left-hand side always evaluates to:

-                #{to_quoted_string(type) |> indent(4)}
-            """
+                  #{to_quoted_string(type) |> indent(4)}
+              """

-          op in [:and, :or] ->
-            {first_message, second_message} =
-              case booleaness(type) do
-                {true, _} -> {" will always succeed", "because it evaluates to"}
-                {false, _} -> {" will never succeed", "because it evaluates to"}
-                :none -> {" will always fail", "because it evaluates to"}
-                _ -> {"", "will always evaluate to"}
-              end
+            op in [:and, :or] ->
+              {first_message, second_message} =
+                case booleaness(type) do
+                  {true, _} -> {" will always succeed", "because it evaluates to"}
+                  {false, _} -> {" will never succeed", "because it evaluates to"}
+                  :none -> {" will always fail", "because it evaluates to"}
+                  _ -> {"", "will always evaluate to"}
+                end

-            """
-            the following conditional expression#{first_message}:
+              """
+              the following conditional expression#{first_message}:

-                #{expr_to_string(expr) |> indent(4)}
+                  #{expr_to_string(expr) |> indent(4)}

-            #{second_message}:
+              #{second_message}:

-                #{to_quoted_string(type) |> indent(4)}
-            """
+                  #{to_quoted_string(type) |> indent(4)}
+              """

-          true ->
-            """
-            the following conditional expression:
+            op == :! ->
+              """
+              the following conditional expression:

-                #{expr_to_string(expr) |> indent(4)}
+                  #{expr_to_string({:!, [], [expr]}) |> indent(4)}

-            will always evaluate to:
+              will always evaluate to false because its inner expression has type:

-                #{to_quoted_string(type) |> indent(4)}
-            """
-        end
+                  #{to_quoted_string(type) |> indent(4)}
+              """
+
+            op == :"!!" ->
+              """
+              the following conditional expression:
+
+                  #{expr_to_string({:!, [], [{:!, [], [expr]}]}) |> indent(4)}
+
+              will always evaluate to true because its inner expression has type:
+
+                  #{to_quoted_string(type) |> indent(4)}
+              """
+
+            true ->
+              """
+              the following conditional expression:
+
+                  #{expr_to_string(expr) |> indent(4)}
+
+              will always evaluate to:
+
+                  #{to_quoted_string(type) |> indent(4)}
+              """
+          end
+
+        {expr, message}

-      {expr, message}
-    else
      _ ->
        {args,
         """
@@ -1665,8 +1796,22 @@ defmodule Module.Types.Pattern do
    end
  end

-  defp badpattern({op, args}, index)
-       when op in [:for_reduce, :receive, :try_catch, :with_else, :fn] do
+  defp badpattern({{op, type}, args}, index) when op in [:try_else, :with_else] do
+    arg = Enum.fetch!(args, index)
+
+    {arg,
+     """
+     the following clause will never match:
+
+         #{args_to_string(args) |> indent(4)} ->
+
+     it is expected to match on type:
+
+         #{to_quoted_string(type) |> indent(4)}
+     """}
+  end
+
+  defp badpattern({op, args}, index) when op in [:for_reduce, :receive, :try_catch, :fn] do
    arg = Enum.fetch!(args, index)

    {arg,
@@ -39,7 +39,7 @@ defmodule Node do
  Currently supported options are:

  * `:name_domain` - determines the host name part of the node name. If `:longnames`,
-    fully qualified domain names will be   used which also is the default.
+    fully qualified domain names will be used, which is also the default.
    If `:shortnames`, only the short name of the host will be used.

  * `:net_ticktime` - The tick time to use in seconds. Defaults to the value of the
@@ -47,7 +47,7 @@ defmodule Node do
    See [the `kernel` documentation](https://www.erlang.org/doc/apps/kernel/kernel_app.html)
    for more information.

-  * `net_tickintensity` - The tick intensity to use. Defaults to the value of the
+  * `:net_tickintensity` - The tick intensity to use. Defaults to the value of the
    `net_tickintensity` configuration under Erlang's `kernel` application.
    See [the `kernel` documentation](https://www.erlang.org/doc/apps/kernel/kernel_app.html)
    for more information.
@@ -338,8 +338,6 @@ defmodule Node do

  The default node is `Node.self/0`, the local node. If `node` is the local node,
  the function also sets the cookie of all other unknown nodes to `cookie`.
-
-  This function will raise `FunctionClauseError` if the given `node` is not alive.
  """
  @spec set_cookie(t, atom) :: true
  def set_cookie(node \\ Node.self(), cookie) when is_atom(cookie) do
@@ -43,7 +43,7 @@ defmodule OptionParser do

  defmodule ParseError do
    @moduledoc """
-    An exception raised when parsing option fails.
+    An exception raised when parsing an option fails.

    For example, see `OptionParser.parse!/2`.
    """
@@ -387,7 +387,7 @@ defmodule OptionParser do
        do_parse(rest, config, new_opts, args, invalid, all?)

      {:invalid, option, value, rest} ->
-        # the option exist but it has wrong value
+        # the option exists but it has wrong value
        do_parse(rest, config, opts, args, [{option, value} | invalid], all?)

      {:undefined, option, _value, rest} ->
@@ -463,7 +463,7 @@ defmodule OptionParser do
  defp next_with_config(["--" <> option | rest], config) do
    {option, value} = split_option(option)

-    if String.contains?(option, ["_"]) do
+    if String.contains?(option, "_") do
      {:undefined, "--" <> option, value, rest}
    else
      tagged = tag_option(option, config)
@@ -852,7 +852,7 @@ defmodule OptionParser do
    |> to_existing_key(allow_nonexistent_atoms?)
  end

-  defp to_existing_key(option, true), do: String.to_atom(option)
+  defp to_existing_key(option, true), do: String.to_unsafe_atom(option)

  defp to_existing_key(option, false) do
    try do
@@ -952,8 +952,14 @@ defmodule OptionParser do

        case types |> List.delete(:keep) |> List.first(:string) do
          :boolean ->
-            base = "#{to_switch(name)}, #{to_switch(name, "--no-")}"
-            add_aliases(base, name, reverse_aliases)
+            case to_switch(name) do
+              "--no-" <> _ = switch ->
+                add_aliases(switch, name, reverse_aliases)
+
+              switch ->
+                base = "#{switch}, #{to_switch(name, "--no-")}"
+                add_aliases(base, name, reverse_aliases)
+            end

          type ->
            base = "#{to_switch(name)} #{String.upcase(Atom.to_string(type))}"
@@ -170,6 +170,8 @@ defmodule PartitionSupervisor do
          | {:max_seconds, non_neg_integer()}
          | {:with_arguments, (args :: [term()], partition() -> updated_args :: [term()])}

+  defguardp is_name(name) when is_atom(name) or elem(name, 0) == :via
+
  @doc false
  def child_spec(opts) when is_list(opts) do
    id =
@@ -366,7 +368,7 @@ defmodule PartitionSupervisor do
  """
  @doc since: "1.18.0"
  @spec resize!(name(), non_neg_integer()) :: non_neg_integer()
-  def resize!(name, partitions) when is_integer(partitions) do
+  def resize!(name, partitions) when is_name(name) and is_integer(partitions) do
    supervisor =
      GenServer.whereis(name) || exit({:noproc, {__MODULE__, :resize!, [name, partitions]}})

@@ -422,7 +424,7 @@ defmodule PartitionSupervisor do
  """
  @doc since: "1.14.0"
  @spec partitions(name()) :: non_neg_integer()
-  def partitions(name) do
+  def partitions(name) when is_name(name) do
    name |> table() |> partitions(name)
  end

@@ -470,7 +472,7 @@ defmodule PartitionSupervisor do
          # Inlining [module()] | :dynamic here because :supervisor.modules() is not exported
          {integer(), pid | :restarting, :worker | :supervisor, [module()] | :dynamic}
        ]
-  def which_children(name) when is_atom(name) or elem(name, 0) == :via do
+  def which_children(name) when is_name(name) do
    Supervisor.which_children(name)
  end

@@ -498,7 +500,7 @@ defmodule PartitionSupervisor do
          supervisors: non_neg_integer,
          workers: non_neg_integer
        }
-  def count_children(supervisor) when is_atom(supervisor) do
+  def count_children(supervisor) when is_name(supervisor) do
    Supervisor.count_children(supervisor)
  end

@@ -514,7 +516,7 @@ defmodule PartitionSupervisor do
  """
  @doc since: "1.14.0"
  @spec stop(name(), reason :: term, timeout) :: :ok
-  def stop(supervisor, reason \\ :normal, timeout \\ :infinity) when is_atom(supervisor) do
+  def stop(supervisor, reason \\ :normal, timeout \\ :infinity) when is_name(supervisor) do
    Supervisor.stop(supervisor, reason, timeout)
  end

@@ -485,7 +485,7 @@ defmodule Path do
  def relative_to_cwd(path, opts \\ []) when is_list(opts) do
    case :file.get_cwd() do
      {:ok, base} -> relative_to(path, IO.chardata_to_string(base), opts)
-      _ -> path
+      _ -> IO.chardata_to_string(path)
    end
  end

@@ -699,6 +699,32 @@ defmodule Path do
    end
  end

+  @doc """
+  Safely joins two paths.
+
+  Returns `{:ok, path}` if `right` is safe to append to `left`, or `:error`
+  otherwise. See `safe_relative/2` for the exact safety rules applied to `right`.
+
+  ## Examples
+
+      iex> Path.safe_join("foo", "bar")
+      {:ok, "foo/bar"}
+
+      iex> Path.safe_join("foo", "../bar")
+      :error
+
+      iex> Path.safe_join("foo", "/bar")
+      :error
+
+  """
+  @doc since: "1.21.0"
+  @spec safe_join(t, t) :: {:ok, t} | :error
+  def safe_join(left, right) do
+    with {:ok, right} <- safe_relative(right, left) do
+      {:ok, join(left, right)}
+    end
+  end
+
  @doc ~S"""
  Splits the path into a list at the path separator.

@@ -61,7 +61,7 @@ defmodule Port do
    * `{pid, {:connect, new_pid}}` - sets the `new_pid` as the new owner of
      the port. Once a port is opened, the port is linked and connected to the
      caller process and communication to the port only happens through the
-      connected process. This message makes `new_pid` the new connected processes.
+      connected process. This message makes `new_pid` the new connected process.
      Unless the port is dead, the port will reply to the old owner with
      `{port, :connected}`. See `connect/2`.

@@ -88,10 +88,10 @@ defmodule Port do
  > On Unix systems, arguments are passed to a new operating system
  > process as an array of strings but on Windows it is up to the child
  > process to parse them and some Windows programs may apply their own
-  > rules, which are inconsistent with the standard C runtime `argv` parsing
+  > rules, which are inconsistent with the standard C runtime `argv` parsing.
  >
-  > This is particularly troublesome when invoking `.bat` or `.com` files
-  > as these run implicitly through `cmd.exe`, whose argument parsing is
+  > This is particularly troublesome when invoking `.bat`, `.cmd`, or `.com`
+  > files as these run implicitly through `cmd.exe`, whose argument parsing is
  > vulnerable to malicious input and can be used to run arbitrary shell
  > commands.
  >
@@ -99,7 +99,7 @@ defmodule Port do
  > files or `.com` applications, you must not pass untrusted input as
  > arguments to the program. You may avoid accidentally executing them
  > by explicitly passing the extension of the program you want to run,
-  > such as `.exe`, and double check the program is indeed not a batch
+  > such as `.exe`, and double-checking the program is indeed not a batch
  > file or `.com` application.
  >
  > This affects both `spawn` and `spawn_executable`.
@@ -156,7 +156,7 @@ defmodule Port do

  We do not always have control over how third-party software terminates.
  If necessary, one workaround is to wrap the child application in a script that
-  checks whether stdin has been closed.  Here is such a script that has been
+  checks whether stdin has been closed. Here is such a script that has been
  verified to work on bash shells:

      #!/usr/bin/env bash
@@ -79,10 +79,11 @@ defmodule Process do
  @typedoc """
  A process destination.

-  A remote or local PID, a local port, a locally registered name, or a tuple in
-  the form of `{registered_name, node}` for a registered name at another node.
+  A remote or local PID, a local port, a process alias, a locally registered name,
+  or a tuple in the form of `{registered_name, node}` for a registered name at
+  another node.
  """
-  @type dest :: pid | port | (registered_name :: atom) | {registered_name :: atom, node}
+  @type dest :: pid | port | alias | (registered_name :: atom) | {registered_name :: atom, node}

  @doc """
  Tells whether the given process is alive on the local node.
@@ -226,7 +227,7 @@ defmodule Process do
  >
  > The functions `Kernel.exit/1` and `Process.exit/2` are
  > named similarly but provide very different functionalities. The
-  > `Kernel:exit/1` function should be used when the intent is to stop the current
+  > `Kernel.exit/1` function should be used when the intent is to stop the current
  > process while `Process.exit/2` should be used when the intent is to send an
  > exit signal to another process. Note also that `Kernel.exit/1` can be caught
  > with `try/1` while `Process.exit/2` can only be handled by trapping exits and
@@ -339,7 +340,7 @@ defmodule Process do
  @doc """
  Sends a message to the given `dest`.

-  `dest` may be a remote or local PID, a local port, a locally
+  `dest` may be a remote or local PID, a local port, a process alias, a locally
  registered name, or a tuple in the form of `{registered_name, node}` for a
  registered name at another node.

@@ -387,8 +388,6 @@ defmodule Process do
  automatically canceled when `dest` is an atom (as the atom resolution is done
  on delivery).

-  Inlined by the compiler.
-
  ## Options

    * `:abs` - (boolean) when `false`, `time` is treated as relative to the
@@ -832,12 +831,16 @@ defmodule Process do

  Inlined by the compiler.
  """
+  @spec flag(:async_dist, boolean) :: boolean
  @spec flag(:error_handler, module) :: module
+  @spec flag(:fullsweep_after, non_neg_integer) :: non_neg_integer
  @spec flag(:max_heap_size, heap_size) :: heap_size
  # :off_heap | :on_heap twice because :erlang.message_queue_data() is not exported
  @spec flag(:message_queue_data, :off_heap | :on_heap) :: :off_heap | :on_heap
  @spec flag(:min_bin_vheap_size, non_neg_integer) :: non_neg_integer
  @spec flag(:min_heap_size, non_neg_integer) :: non_neg_integer
+  @spec flag(:monitor_nodes, term) :: term
+  @spec flag({:monitor_nodes, term}, term) :: term
  @spec flag(:priority, priority_level) :: priority_level
  @spec flag(:save_calls, 0..10_000) :: 0..10_000
  @spec flag(:sensitive, boolean) :: boolean
@@ -980,10 +983,34 @@ defmodule Process do
  defdelegate unalias(alias), to: :erlang

  @doc """
-  Add a descriptive term to the current process.
+  Returns the label set for the process `pid` as set with `set_label/1`
+  or `:proc_lib.set_label/1`.

-  The term does not need to be unique, and in Erlang/OTP 27+ will be shown in
-  Observer and in crash logs.
+  Defaults to the current process when `pid` is not passed.
+
+  ## Examples
+
+      Process.set_label({:any, "term"})
+      Process.get_label()
+      #=> {:any, "term"}
+
+  Returns `nil` when not set:
+
+      Process.get_label(pid)
+      #=> nil
+
+  """
+  @doc since: "1.20.0"
+  @spec get_label(pid()) :: term()
+  def get_label(pid \\ self()) do
+    nilify(:proc_lib.get_label(pid))
+  end
+
+  @doc """
+  Adds a descriptive term to the current process.
+
+  The term does not need to be unique, and will be shown in Observer and in
+  crash logs.
  This label may be useful for identifying a process as one of multiple in a
  given role, such as `:queue_worker` or `{:live_chat, user_id}`.

@@ -997,12 +1024,7 @@ defmodule Process do
  """
  @doc since: "1.17.0"
  @spec set_label(term()) :: :ok
-  def set_label(label) do
-    # TODO: switch to `:proc_lib.set_label/2` when we require Erlang/OTP 27+
-    Process.put(:"$process_label", label)
-    # mimic return value of `:proc_lib.set_label/2`
-    :ok
-  end
+  defdelegate set_label(label), to: :proc_lib

  @compile {:inline, nilify: 1}
  defp nilify(:undefined), do: nil
@@ -215,7 +215,7 @@ defmodule Protocol do
        ...
      end

-  If you are using `Mix.install/2`, you can do by passing the `consolidate_protocols`
+  If you are using `Mix.install/2`, you can do so by passing the `consolidate_protocols`
  option:

      Mix.install(
@@ -282,7 +282,9 @@ defmodule Protocol do
    type_args = :lists.map(fn _ -> quote(do: term) end, :lists.seq(2, arity))
    type_args = [quote(do: t) | type_args]

-    to_var = fn pos -> Macro.var(String.to_atom("arg" <> Integer.to_string(pos)), __MODULE__) end
+    to_var = fn pos ->
+      Macro.var(String.to_unsafe_atom("arg" <> Integer.to_string(pos)), __MODULE__)
+    end

    call_args = :lists.map(to_var, :lists.seq(2, arity))
    call_args = [quote(do: term) | call_args]
@@ -689,15 +691,15 @@ defmodule Protocol do
          structs_domain =
            case structs do
              [] -> Descr.none()
-              _ -> Descr.open_map(__struct__: Descr.atom(structs))
+              _ -> Descr.open_map(__struct__: {Descr.atom(structs), false})
            end

          domain =
            Enum.reduce(types_minus_any -- structs, structs_domain, fn impl, acc ->
-              Descr.union(Module.Types.Of.impl(impl, :open), acc)
+              Descr.opt_union(Module.Types.Of.impl(impl, :open), acc)
            end)

-          not_domain = Descr.negation(domain)
+          not_domain = Descr.opt_negation(domain)

          if Any in types do
            clauses =
@@ -786,7 +788,7 @@ defmodule Protocol do
  end

  defp fallback_clause_for(value, _protocol, meta) do
-    {meta, [quote(do: _)], [], value}
+    {meta, [{:_, [version: -1], __MODULE__}], [], value}
  end

  # Finally compile the module and emit its bytecode.
@@ -818,11 +820,9 @@ defmodule Protocol do
        @fallback_to_any false
        @undefined_impl_description ""

-        # Invoke the user given block
-        _ = unquote(block)
-
-        # Finalize expansion
+        res = unquote(block)
        unquote(after_defprotocol())
+        res
      end
    end
  end
@@ -932,7 +932,7 @@ defmodule Protocol do
      quote bind_quoted: [built_in: built_in()] do
        any_impl_for =
          if @fallback_to_any do
-            __MODULE__.Any
+            Protocol.__concat__(__MODULE__, "Any")
          else
            nil
          end
@@ -1225,10 +1225,12 @@ defmodule Protocol do
  end

  @doc false
-  def __concat__(left, right) do
-    String.to_atom(
-      ensure_prefix(Atom.to_string(left)) <> "." <> remove_prefix(Atom.to_string(right))
-    )
+  def __concat__(left, right) when is_atom(right) do
+    __concat__(left, remove_prefix(Atom.to_string(right)))
+  end
+
+  def __concat__(left, right) when is_binary(right) do
+    String.to_unsafe_atom(ensure_prefix(Atom.to_string(left)) <> "." <> right)
  end

  defp ensure_prefix("Elixir." <> _ = left), do: left
@@ -258,7 +258,7 @@ defmodule Range do
  def size(range)
  def size(first..last//step) when step > 0 and first > last, do: 0
  def size(first..last//step) when step < 0 and first < last, do: 0
-  def size(first..last//step), do: abs(div(last - first, step)) + 1
+  def size(first..last//step), do: div(last - first, step) + 1

  # TODO: Remove me on v2.0
  def size(%{__struct__: Range, first: first, last: last} = range) do
@@ -476,8 +476,8 @@ defmodule Range do
  """
  @doc since: "1.8.0"
  @spec disjoint?(t, t) :: boolean
-  def disjoint?(first1..last1//step1 = range1, first2..last2//step2 = range2) do
-    if size(range1) == 0 or size(range2) == 0 do
+  def disjoint?(first1..last1//step1, first2..last2//step2) do
+    if empty?(first1, last1, step1) or empty?(first2, last2, step2) do
      true
    else
      {first1, last1, step1} = normalize(first1, last1, step1)
@@ -487,7 +487,7 @@ defmodule Range do
        last2 < first1 or last1 < first2 ->
          true

-        abs(step1) == 1 and abs(step2) == 1 ->
+        step1 == 1 and step2 == 1 ->
          false

        true ->
@@ -498,9 +498,9 @@ defmodule Range do

          if rem(first2 - first1, gcd) == 0 do
            c = first1 - first2 + step2 - step1
-            t1 = -c / step2 * u
-            t2 = -c / step1 * v
-            t = max(floor(t1) + 1, floor(t2) + 1)
+            t1 = Integer.floor_div(-c * u, step2)
+            t2 = Integer.floor_div(-c * v, step1)
+            t = max(t1 + 1, t2 + 1)
            x = div(c * u + t * step2, gcd) - 1
            y = div(c * v + t * step1, gcd) - 1

@@ -513,9 +513,20 @@ defmodule Range do
    end
  end

-  @compile inline: [normalize: 3]
+  @compile inline: [empty?: 3, normalize: 3]
+  defp empty?(first, last, step) do
+    case step > 0 do
+      true -> first > last
+      false -> first < last
+    end
+  end
+
  defp normalize(first, last, step) when first > last,
-    do: {first - abs(div(first - last, step) * step), first, -step}
+    do: {first - div(first - last, step) * step, first, -step}
+
+  # A single-element range holds the same element regardless of the step, so
+  # make the step positive to keep the progression in disjoint?/2 increasing.
+  defp normalize(first, last, step) when first == last, do: {first, last, abs(step)}

  defp normalize(first, last, step), do: {first, last, step}

@@ -340,7 +340,7 @@ defmodule Record do
    fields
  end

-  # Normalizes of record fields to have default values.
+  # Normalizes record fields to have default values.
  defp fields(kind, fields) do
    normalizer_fun = fn
      {key, value} when is_atom(key) ->
@@ -40,7 +40,7 @@ defmodule Record.Extractor do
  defp from_lib_file(file) do
    [app | path] = :filename.split(String.to_charlist(file))

-    case :code.lib_dir(List.to_atom(app)) do
+    case :code.lib_dir(List.to_unsafe_atom(app)) do
      {:error, _} ->
        raise ArgumentError, "lib file #{file} could not be found"

--- a/Show More
+++ b/Show More