Improve error message for invalid default arguments

lib/elixir/lib/module/types/apply.ex

@@ -785,7 +785,7 @@ defmodule Module.Types.Apply do
   def format_diagnostic({:badlocal, expr, args_types, domain, clauses, context}) do
     traces = collect_traces(expr, context)
     converter = &Function.identity/1
-    {fun, _, _} = expr
+    {fun, meta, _} = expr
 
     explanation =
       empty_arg_reason(args_types) ||
@@ -794,16 +794,29 @@ defmodule Module.Types.Apply do
         #{clauses_args_to_quoted_string(clauses, converter)}
         """
 
-    %{
-      details: %{typing_traces: traces},
-      message:
-        IO.iodata_to_binary([
+    banner =
+      case fun == :super && meta[:default] && meta[:super] do
+        {_kind, fun} ->
+          """
+          incompatible types given as default arguments to #{fun}/#{length(args_types)}:
+          """
+
+        _ ->
           """
           incompatible types given to #{fun}/#{length(args_types)}:
 
               #{expr_to_string(expr) |> indent(4)}
 
           given types:
+          """
+      end
+
+    %{
+      details: %{typing_traces: traces},
+      message:
+        IO.iodata_to_binary([
+          banner,
+          """
 
               #{args_to_quoted_string(args_types, domain, converter) |> indent(4)}
 

lib/elixir/src/elixir_def.erl

@@ -314,7 +314,7 @@ unpack_defaults(Kind, Meta, Name, Args, S, E) ->
 unpack_expanded(Kind, Meta, Name, [{'\\\\', DefaultMeta, [Expr, _]} | T] = List, VersionOffset, Acc, Clauses) ->
   Base = match_defaults(Acc, length(Acc) + VersionOffset, []),
   {Args, Invoke} = extract_defaults(List, length(Base) + VersionOffset, [], []),
-  Clause = {Meta, Base ++ Args, [], {super, [{super, {Kind, Name}} | DefaultMeta], Base ++ Invoke}},
+  Clause = {Meta, Base ++ Args, [], {super, [{super, {Kind, Name}}, {default, true} | DefaultMeta], Base ++ Invoke}},
   unpack_expanded(Kind, Meta, Name, T, VersionOffset, [Expr | Acc], [Clause | Clauses]);
 unpack_expanded(Kind, Meta, Name, [H | T], VersionOffset, Acc, Clauses) ->
   unpack_expanded(Kind, Meta, Name, T, VersionOffset, [H | Acc], Clauses);

lib/elixir/test/elixir/module/types/integration_test.exs

@@ -244,6 +244,39 @@ defmodule Module.Types.IntegrationTest do
       assert_no_warnings(files)
     end
 
+    test "incompatible default argument" do
+      files = %{
+        "a.ex" => """
+        defmodule A do
+          def ok(x = :ok \\\\ nil) do
+            x
+          end
+        end
+        """
+      }
+
+      warnings = [
+        ~S"""
+            warning: incompatible types given as default arguments to ok/1:
+
+                -nil-
+
+            but expected one of:
+
+                dynamic(:ok)
+
+            typing violation found at:
+            │
+          2 │   def ok(x = :ok \\ nil) do
+            │                  ~
+            │
+            └─ a.ex:2:18: A.ok/0
+        """
+      ]
+
+      assert_warnings(files, warnings)
+    end
+
     test "returns diagnostics with source and file" do
       files = %{
         "a.ex" => """