Raise TypeError when TypeAliasType is subscripted without having type_params (#473)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Author: Daraan

CHANGELOG.md

@@ -7,6 +7,8 @@
   by PEP 649. Patches by Jelle Zijlstra and Alex Waygood.
 - Copy the coroutine status of functions and methods wrapped
   with `@typing_extensions.deprecated`. Patch by Sebastian Rittau.
+- Fix bug where `TypeAliasType` instances could be subscripted even
+  where they were not generic. Patch by [Daraan](https://github.com/Daraan).
 
 # Release 4.12.2 (June 7, 2024)
 

src/test_typing_extensions.py

@@ -7247,6 +7247,20 @@ def test_getitem(self):
         self.assertEqual(get_args(fully_subscripted), (Iterable[float],))
         self.assertIs(get_origin(fully_subscripted), ListOrSetT)
 
+    def test_subscription_without_type_params(self):
+        Simple = TypeAliasType("Simple", int)
+        with self.assertRaises(TypeError, msg="Only generic type aliases are subscriptable"):
+            Simple[int]
+
+        # A TypeVar in the value does not allow subscription
+        T = TypeVar('T')
+        MissingTypeParamsErr = TypeAliasType("MissingTypeParamsErr", List[T])
+        self.assertEqual(MissingTypeParamsErr.__type_params__, ())
+        self.assertEqual(MissingTypeParamsErr.__parameters__, ())
+        with self.assertRaises(TypeError, msg="Only generic type aliases are subscriptable"):
+            MissingTypeParamsErr[int]
+
+
     def test_pickle(self):
         global Alias
         Alias = TypeAliasType("Alias", int)

src/typing_extensions.py

@@ -3525,6 +3525,8 @@ def __repr__(self) -> str:
             return self.__name__
 
         def __getitem__(self, parameters):
+            if not self.__type_params__:
+                raise TypeError("Only generic type aliases are subscriptable")
             if not isinstance(parameters, tuple):
                 parameters = (parameters,)
             parameters = [