Fix overriding with Elixir. prefixed defmodule (#13011)

Closes #12456.

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

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