Fix subscription of Unpack causing nested Unpacks to not be resolved correctly

CHANGELOG.md

@@ -16,6 +16,8 @@
   subscripted objects) had wrong parameters if they were directly
   subscripted with an `Unpack` object.
   Patch by [Daraan](https://github.com/Daraan).
+- Fix error in subscription of Unpack aliases causing nested Unpacks 
+  to not be resolved correctly. Patch by [Daraan](https://github.com/Daraan).
 
 # Release 4.12.2 (June 7, 2024)
 

src/test_typing_extensions.py

@@ -5779,6 +5779,42 @@ class D(Protocol[T1, T2, Unpack[Ts]]): pass
                     with self.assertRaises(TypeError):
                         klass[int]
 
+    def test_substitution(self):
+        Ts = TypeVarTuple("Ts")
+        unpacked_str = Unpack[Ts][str]
+        with self.subTest("Check substitution result"):
+            self.assertIs(unpacked_str, str)
+
+    @skipUnless(TYPING_3_11_0, "Needs Issue #103 for <3.11")
+    def test_nested_unpack(self):
+        T = TypeVar('T')
+        Ts = TypeVarTuple("Ts")
+        Variadic = Tuple[int, Unpack[Ts]]
+        # Tuple[int, int, Tuple[str, int]]
+        direct_subscription = Variadic[int, Tuple[str, int]]
+        # Tuple[int, int, Tuple[*Ts, int]]
+        TupleAliasTs = Variadic[int, Tuple[Unpack[Ts], int]]
+
+        # if this fails all below are likely to fail too
+        # Tuple[int, int, Tuple[str, int]]
+        recursive_unpack = TupleAliasTs[str]
+        self.assertEqual(direct_subscription, recursive_unpack)
+        self.assertEqual(get_args(recursive_unpack), (int, int, Tuple[str, int]))
+
+        TupleAliasTsT = Variadic[Tuple[Unpack[Ts], T]]
+        # Equivalent Forms
+        with self.subTest("Equivalence of variadic arguments"):
+            nested_tuple_bare = TupleAliasTsT[str, int, object]
+            self.assertEqual(nested_tuple_bare, TupleAliasTsT[Unpack[Tuple[str, int, object]]])
+            self.assertEqual(nested_tuple_bare, TupleAliasTsT[Unpack[Tuple[str, int]], object])
+            self.assertEqual(nested_tuple_bare, TupleAliasTsT[Unpack[Tuple[str]], Unpack[Tuple[int]], object])
+            self.assertEqual(get_args(nested_tuple_bare), (int, Tuple[str, int, object],))
+
+        with self.subTest("With Callable and Unpack"):
+            # Tuple[int, (str, int) -> object]
+            CallableAliasTsT = Variadic[Callable[[Unpack[Ts]], T]]
+            callable_fully_subscripted = CallableAliasTsT[Unpack[Tuple[str, int]], object]
+            self.assertEqual(get_args(callable_fully_subscripted), (int, Callable[[str, int], object],))
 
 class TypeVarTupleTests(BaseTestCase):
 

src/typing_extensions.py

@@ -2424,6 +2424,17 @@ def __typing_unpacked_tuple_args__(self):
                 return arg.__args__
             return None
 
+        @property
+        def __typing_is_unpacked_typevartuple__(self):
+            assert self.__origin__ is Unpack
+            assert len(self.__args__) == 1
+            return isinstance(self.__args__[0], TypeVarTuple)
+
+        def __getitem__(self, args):
+            if self.__typing_is_unpacked_typevartuple__:
+                return args
+            return super().__getitem__(args)
+
     @_UnpackSpecialForm
     def Unpack(self, parameters):
         item = typing._type_check(parameters, f'{self._name} accepts only a single type.')
@@ -2436,6 +2447,17 @@ def _is_unpack(obj):
     class _UnpackAlias(typing._GenericAlias, _root=True):
         __class__ = typing.TypeVar
 
+        @property
+        def __typing_is_unpacked_typevartuple__(self):
+            assert self.__origin__ is Unpack
+            assert len(self.__args__) == 1
+            return isinstance(self.__args__[0], TypeVarTuple)
+
+        def __getitem__(self, args):
+            if self.__typing_is_unpacked_typevartuple__:
+                return args
+            return super().__getitem__(args)
+
     class _UnpackForm(_ExtensionsSpecialForm, _root=True):
         def __getitem__(self, parameters):
             item = typing._type_check(parameters,