Correct subscription handling when type_params are empty

Author: Daraan

src/test_typing_extensions.py

@@ -7457,6 +7457,24 @@ def test_invalid_cases_before_3_11(self):
         with self.assertRaises(TypeError):
             ListOrSetT[(Generic[T], )]
 
+    def test_subscription_without_type_params(self):
+        Simple = TypeAliasType("Simple", int)
+        with self.assertRaises(TypeError, msg="Only generic type aliases are subscriptable"):
+            Simple[int]
+        with self.assertRaises(TypeError, msg="Only generic type aliases are subscriptable"):
+            Simple[[]]
+        with self.assertRaises(TypeError, msg="Only generic type aliases are subscriptable"):
+            Simple[()]
+
+        # no TypeVar in type_params, however in value still allows subscription
+        T = TypeVar("T")
+        MissingTypeParams = TypeAliasType("MissingTypeParams", List[T], type_params=())
+        self.assertEqual(MissingTypeParams.__type_params__, ())
+        self.assertEqual(MissingTypeParams.__parameters__, ())
+        # These should not raise:
+        MissingTypeParams[int]
+        MissingTypeParams[[]]
+        MissingTypeParams[()]
 
     def test_pickle(self):
         global Alias

src/typing_extensions.py

@@ -3526,9 +3526,22 @@ def _raise_attribute_error(self, name: str) -> Never:
         def __repr__(self) -> str:
             return self.__name__
 
+        def _is_subscriptable(self):
+            if len(self.__parameters__) > 0:
+                return True
+            if _should_collect_from_parameters(self.__value__):
+                if hasattr(typing, '_collect_type_vars'):
+                    more_parameters = _collect_type_vars((self.__value__,),
+                                                         (TypeVar, ParamSpec))
+                else:
+                    more_parameters = _collect_parameters((self.__value__,))
+                if more_parameters:
+                    return True
+            return False
+
         if sys.version_info >= (3, 11):
             def __getitem__(self, parameters):
-                if len(self.__type_params__) == 0:
+                if len(self.__parameters__) == 0 and not self._is_subscriptable():
                     raise TypeError("Only generic type aliases are subscriptable")
                 if not isinstance(parameters, tuple):
                     parameters = (parameters,)
@@ -3562,7 +3575,7 @@ def _check_parameter(self, item, recursion=0):
                     )
 
             def __getitem__(self, parameters):
-                if len(self.__type_params__) == 0:
+                if len(self.__parameters__) == 0 and not self._is_subscriptable():
                     raise TypeError("Only generic type aliases are subscriptable")
                 if not isinstance(parameters, tuple):
                     parameters = (parameters,)