Fix error message when subclassing TypeVarTuple

Author: JanEricNitschke

CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
 
+- Fix error message when trying to subclass `typing_extensions.TypeVarTuple`.
 - Fix incorrect behaviour on Python 3.9 and Python 3.10 that meant that
   calling `isinstance` with `typing_extensions.Concatenate[...]` or
   `typing_extensions.Unpack[...]` as the first argument could have a different

src/test_typing_extensions.py

@@ -110,6 +110,10 @@
 KT = TypeVar("KT")
 VT = TypeVar("VT")
 
+CANNOT_SUBCLASS_TYPE = 'Cannot subclass special typing classes'
+NOT_A_BASE_TYPE = r"type '(?:typing|typing_extensions).%s' is not an acceptable base type"
+CANNOT_SUBCLASS_INSTANCE = 'Cannot subclass an instance of %s'
+
 # Flags used to mark tests that only apply after a specific
 # version of the typing module.
 TYPING_3_10_0 = sys.version_info[:3] >= (3, 10, 0)
@@ -6792,6 +6796,27 @@ def test_pickle(self):
                 self.assertEqual(z.__name__, typevartuple.__name__)
                 self.assertEqual(z.__default__, typevartuple.__default__)
 
+    def test_cannot_subclass(self):
+        with self.assertRaisesRegex(TypeError, NOT_A_BASE_TYPE % 'TypeVarTuple'):
+            class C(TypeVarTuple): pass
+        Ts = TypeVarTuple('Ts')
+        with self.assertRaisesRegex(TypeError,
+                CANNOT_SUBCLASS_INSTANCE % 'TypeVarTuple'):
+            class D(Ts): pass
+        with self.assertRaisesRegex(TypeError, CANNOT_SUBCLASS_TYPE):
+            class E(type(Unpack)): pass
+        with self.assertRaisesRegex(TypeError, CANNOT_SUBCLASS_TYPE):
+            class F(type(*Ts)): pass
+        with self.assertRaisesRegex(TypeError, CANNOT_SUBCLASS_TYPE):
+            class G(type(Unpack[Ts])): pass
+        with self.assertRaisesRegex(TypeError,
+                                    r'Cannot subclass typing\.Unpack'):
+            class H(Unpack): pass
+        with self.assertRaisesRegex(TypeError, r'Cannot subclass typing.Unpack\[Ts\]'):
+            class I(*Ts): pass
+        with self.assertRaisesRegex(TypeError, r'Cannot subclass typing.Unpack\[Ts\]'):
+            class J(Unpack[Ts]): pass
+
 
 class FinalDecoratorTests(BaseTestCase):
     def test_final_unmodified(self):

src/typing_extensions.py

@@ -2637,7 +2637,9 @@ def _typevartuple_prepare_subst(alias, args):
             return tvt
 
         def __init_subclass__(self, *args, **kwds):
-            raise TypeError("Cannot subclass special typing classes")
+            raise TypeError(
+                f"type '{__name__}.TypeVarTuple' is not an acceptable base type"
+            )
 
 else:  # <=3.10
     class TypeVarTuple(_DefaultMixin):