Handle None case correctly, also check list

Author: Daraan

src/test_typing_extensions.py

@@ -7332,6 +7332,10 @@ def test_alias_types_and_substitutions(self):
                 subscripted = alias[int, float]
                 self.assertEqual(get_args(subscripted), (int, float))
                 self.assertEqual(subscripted.__parameters__, ())
+            with self.subTest(alias=alias, args=[int, float]):
+                subscripted = alias[[int, float]]
+                self.assertEqual(get_args(subscripted), ([int, float],))
+                self.assertEqual(subscripted.__parameters__, ())
             for expected_args, expected_parameters in test_argument_cases.items():
                 with self.subTest(alias=alias, args=expected_args):
                     self.assertEqual(get_args(alias[expected_args]), (expected_args,))

src/typing_extensions.py

@@ -3667,6 +3667,8 @@ def _check_single_param(self, param, recursion=0):
                 # Allow [], [int], [int, str], [int, ...], [int, T]
                 if param is ...:
                     return ...
+                if param is None:
+                    return None
                 # Note in <= 3.9 _ConcatenateGenericAlias inherits from list
                 if isinstance(param, list) and recursion == 0:
                     return [self._check_single_param(arg, recursion+1)