From ec4dafae0c71f9d3e5e2f721b99b672bee3a370e Mon Sep 17 00:00:00 2001
From: Jean Klingler <sabiwara@gmail.com>
Date: Tue, 17 Oct 2023 17:53:10 +0900
Subject: [PATCH] Fix overriding with Elixir. prefixed defmodule (#13011)

Closes #12456.
---
 lib/elixir/lib/kernel.ex                     | 15 +++++-----
 lib/elixir/test/elixir/kernel/alias_test.exs | 31 ++++++++++++++++++++
 2 files changed, 38 insertions(+), 8 deletions(-)

diff --git a/lib/elixir/lib/kernel.ex b/lib/elixir/lib/kernel.ex
index 2c9ab81c7c..f55b3d827d 100644
--- a/lib/elixir/lib/kernel.ex
+++ b/lib/elixir/lib/kernel.ex
@@ -4889,14 +4889,13 @@ defmodule Kernel do
     expanded = expand_module_alias(alias, env)
 
     {expanded, with_alias} =
-      case is_atom(expanded) do
-        true ->
+      case alias_defmodule(alias, expanded, env) do
+        {full, old, new} ->
           # Expand the module considering the current environment/nesting
-          {full, old, new} = alias_defmodule(alias, expanded, env)
           meta = [defined: full, context: env.module] ++ alias_meta(alias)
           {full, {:alias, meta, [old, [as: new, warn: false]]}}
 
-        false ->
+        nil ->
           {expanded, nil}
       end
 
@@ -4955,13 +4954,13 @@ defmodule Kernel do
 
   defp expand_module_alias(other, env), do: Macro.expand(other, env)
 
+  defp alias_defmodule(_raw, module, _env) when not is_atom(module), do: nil
+
   # defmodule Elixir.Alias
-  defp alias_defmodule({:__aliases__, _, [:"Elixir", _ | _]}, module, _env),
-    do: {module, module, nil}
+  defp alias_defmodule({:__aliases__, _, [:"Elixir", _ | _]}, _module, _env), do: nil
 
   # defmodule Alias in root
-  defp alias_defmodule({:__aliases__, _, _}, module, %{module: nil}),
-    do: {module, module, nil}
+  defp alias_defmodule({:__aliases__, _, _}, _module, %{module: nil}), do: nil
 
   # defmodule Alias nested
   defp alias_defmodule({:__aliases__, _, [h | t]}, _module, env) when is_atom(h) do
diff --git a/lib/elixir/test/elixir/kernel/alias_test.exs b/lib/elixir/test/elixir/kernel/alias_test.exs
index e91b31d157..a1e3d0f5cb 100644
--- a/lib/elixir/test/elixir/kernel/alias_test.exs
+++ b/lib/elixir/test/elixir/kernel/alias_test.exs
@@ -147,3 +147,34 @@ defmodule Macro.AliasTest.User do
     assert is_map(struct(Macro.AliasTest.User.Second, []).baz)
   end
 end
+
+defmodule Kernel.AliasNestingEnvTest do
+  use ExUnit.Case, async: true
+
+  alias Another.AliasEnv, warn: false
+
+  def aliases_before, do: __ENV__.aliases
+
+  defmodule Elixir.AliasEnv do
+    def aliases_nested, do: __ENV__.aliases
+  end
+
+  def aliases_after, do: __ENV__.aliases
+
+  test "keeps env after overriding nested Elixir module of the same name" do
+    assert aliases_before() == [
+             {Elixir.Nested, Kernel.AliasTest.Nested},
+             {Elixir.AliasEnv, Another.AliasEnv}
+           ]
+
+    assert Elixir.AliasEnv.aliases_nested() == [
+             {Elixir.Nested, Kernel.AliasTest.Nested},
+             {Elixir.AliasEnv, Another.AliasEnv}
+           ]
+
+    assert aliases_after() == [
+             {Elixir.Nested, Kernel.AliasTest.Nested},
+             {Elixir.AliasEnv, Another.AliasEnv}
+           ]
+  end
+end