Fix map union optimization for open maps (#15322)

Author: gldubc

lib/elixir/lib/module/types/descr.ex

@@ -3000,8 +3000,12 @@ defmodule Module.Types.Descr do
     end
   end
 
-  defp map_union_strategy([], [], _tag1, _tag2, status) do
-    status
+  defp map_union_strategy([], [], tag1, tag2, status) do
+    case status do
+      :left_subtype_of_right when tag1 == :open and tag2 == :closed -> :none
+      :right_subtype_of_left when tag1 == :closed and tag2 == :open -> :none
+      _ -> status
+    end
   end
 
   defp map_union_strategy(l1, l2, tag1, tag2, status) do

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

@@ -115,6 +115,14 @@ defmodule Module.Types.DescrTest do
       a_integer_open = open_map(a: integer())
       assert equal?(union(closed_map(a: integer()), a_integer_open), a_integer_open)
 
+      closed = closed_map(a: integer(), b: atom())
+      open = open_map(a: integer(), b: boolean())
+
+      assert subtype?(closed, union(closed, open))
+      assert subtype?(open, union(closed, open))
+      assert subtype?(closed, union(open, closed))
+      assert subtype?(open, union(open, closed))
+
       # Domain key types
       atom_to_atom = open_map([{domain_key(:atom), atom()}])
       atom_to_integer = open_map([{domain_key(:atom), integer()}])