fix TypeAliasType union with typing.TypeAliasType (python#575)

Author: jorenham

CHANGELOG.md

@@ -1,3 +1,9 @@
+# Unreleased
+
+- Fix `TypeError` when taking the union of `typing_extensions.TypeAliasType` and a
+  `typing.TypeAliasType` on Python 3.12 and 3.13.
+  Patch by [Joren Hammudoglu](https://github.com/jorenham).
+
 # Release 4.13.1 (April 3, 2025)
 
 Bugfixes:

src/test_typing_extensions.py

@@ -7819,6 +7819,10 @@ def test_or(self):
             self.assertEqual(Alias | None, Union[Alias, None])
             self.assertEqual(Alias | (int | str), Union[Alias, int | str])
             self.assertEqual(Alias | list[float], Union[Alias, list[float]])
+
+            if sys.version_info >= (3, 12):
+                Alias2 = typing.TypeAliasType("Alias2", str)
+                self.assertEqual(Alias | Alias2, Union[Alias, Alias2])
         else:
             with self.assertRaises(TypeError):
                 Alias | int

src/typing_extensions.py

@@ -3827,14 +3827,27 @@ def __ror__(self, other):
     TypeAliasType = typing.TypeAliasType
 # 3.8-3.13
 else:
-    def _is_unionable(obj):
-        """Corresponds to is_unionable() in unionobject.c in CPython."""
-        return obj is None or isinstance(obj, (
-            type,
-            _types.GenericAlias,
-            _types.UnionType,
-            TypeAliasType,
-        ))
+    if sys.version_info >= (3, 12):
+        # 3.12-3.14
+        def _is_unionable(obj):
+            """Corresponds to is_unionable() in unionobject.c in CPython."""
+            return obj is None or isinstance(obj, (
+                type,
+                _types.GenericAlias,
+                _types.UnionType,
+                typing.TypeAliasType,
+                TypeAliasType,
+            ))
+    else:
+        # 3.8-3.11
+        def _is_unionable(obj):
+            """Corresponds to is_unionable() in unionobject.c in CPython."""
+            return obj is None or isinstance(obj, (
+                type,
+                _types.GenericAlias,
+                _types.UnionType,
+                TypeAliasType,
+            ))
 
     if sys.version_info < (3, 10):
         # Copied and pasted from https://github.com/python/cpython/blob/986a4e1b6fcae7fe7a1d0a26aea446107dd58dd2/Objects/genericaliasobject.c#L568-L582,