Avoid cascading unions in map difference

Author: josevalim

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

@@ -5632,11 +5632,13 @@ defmodule Module.Types.Descr do
        when is_tuple(bdd2) do
     case leaf_compare.(a1, bdd2) do
       :disjoint ->
-        bdd_union(
-          bdd_difference(u1, bdd2, leaf_compare),
-          bdd_difference({a1, :bdd_bot, :bdd_bot, d1}, bdd2)
-        )
-        |> bdd_union({a1, c1, :bdd_bot, :bdd_bot})
+        res =
+          bdd_union(
+            bdd_difference(u1, bdd2, leaf_compare),
+            bdd_difference({a1, :bdd_bot, :bdd_bot, d1}, bdd2)
+          )
+
+        if c1 == :bdd_bot, do: res, else: bdd_union(res, {a1, c1, :bdd_bot, :bdd_bot})
 
       :subtype ->
         bdd_union(bdd_difference(u1, bdd2, leaf_compare), bdd_difference(d1, bdd2, leaf_compare))

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

@@ -620,11 +620,21 @@ defmodule Module.Types.DescrTest do
       assert difference(closed_map(__struct__: atom_foo), closed_map(__struct__: atom_bar)) ==
                closed_map(__struct__: atom_foo)
 
-      refute difference(closed_map(__struct__: atom_foo), closed_map(__struct__: term())) ==
-               closed_map(__struct__: atom_foo)
+      assert difference(closed_map(__struct__: atom_foo), closed_map(__struct__: term())) ==
+               none()
 
-      refute difference(closed_map(__struct__: atom()), closed_map(__struct__: atom_bar)) ==
-               closed_map(__struct__: atom())
+      assert difference(closed_map(__struct__: atom()), closed_map(__struct__: atom_bar)) ==
+               closed_map(__struct__: difference(atom(), atom_bar))
+
+      # Explicitly assert we keep it as cascading differences
+      assert %{map: {{:closed, _}, :bdd_bot, :bdd_bot, _}} =
+               difference(
+                 difference(
+                   open_map(value: term()),
+                   closed_map(__struct__: atom_foo, value: term())
+                 ),
+                 closed_map(__struct__: atom_bar, name: term())
+               )
     end
 
     test "map with domain keys" do