Closes https://github.com/elixir-lang/elixir/issues/8564 This is my first attempt to contribute code to elixir, so please forgive me if this PR is completely naive. Besides adding the `is_list/1` guards, I have updated the error message to specify that record specifications must use an atom `literal` as the name, to make it clear that the type `atom` is also not valid. Signed-off-by: José Valim <jose.valim@plataformatec.com.br>