Make Union[list[T], int] work

This involves a rare change in typing.py.

Lib/test/test_genericalias.py

@@ -207,8 +207,7 @@ def test_union_generic(self):
         T = typing.TypeVar('T')
         a = typing.Union[list[T], tuple[T, ...]]
         self.assertEqual(a.__args__, (list[T], tuple[T, ...]))
-        # TODO: To make this work, would need to update typing.py to recognize list[T].
-        # self.assertEqual(a.__parameters__, (T,))
+        self.assertEqual(a.__parameters__, (T,))
 
 
 if __name__ == "__main__":

Lib/typing.py

@@ -26,7 +26,7 @@
 import re as stdlib_re  # Avoid confusion with the re we export.
 import sys
 import types
-from types import WrapperDescriptorType, MethodWrapperType, MethodDescriptorType
+from types import WrapperDescriptorType, MethodWrapperType, MethodDescriptorType, GenericAlias
 
 # Please keep __all__ alphabetized within each category.
 __all__ = [
@@ -180,7 +180,8 @@ def _collect_type_vars(types):
     for t in types:
         if isinstance(t, TypeVar) and t not in tvars:
             tvars.append(t)
-        if isinstance(t, _GenericAlias) and not t._special:
+        if ((isinstance(t, _GenericAlias) and not t._special)
+                or isinstance(t, GenericAlias)):
             tvars.extend([t for t in t.__parameters__ if t not in tvars])
     return tuple(tvars)