gh-139905: Provide suggestion in error message if Generic.__init_subclass__ was not called

Lib/test/test_typing.py

@@ -4722,6 +4722,21 @@ class D(Generic[T]): pass
         with self.assertRaises(TypeError):
             D[()]
 
+    def test_generic_init_subclass_not_called_error(self):
+        class Base:
+            def __init_subclass__(cls) -> None:
+                # Oops, I forgot super().__init_subclass__()!
+                pass
+
+        class Sub(Base, Generic[T]):
+            pass
+
+        with self.assertRaisesRegex(TypeError,
+                                  r"type object '.*Sub' has no attribute "
+                                    r"'__parameters__'; Maybe you forgot to call "
+                                    r"super\(\).__init_subclass__\(\)\?"):
+            Sub[int]
+
     def test_generic_subclass_checks(self):
         for typ in [list[int], List[int],
                     tuple[int, str], Tuple[int, str],

Lib/typing.py

@@ -1158,14 +1158,20 @@ def _generic_class_getitem(cls, args):
                 f"Parameters to {cls.__name__}[...] must all be unique")
     else:
         # Subscripting a regular Generic subclass.
-        for param in cls.__parameters__:
+        try:
+            parameters = cls.__parameters__
+        except AttributeError:
+            raise TypeError(f"type object '{cls.__qualname__}' has no attribute "
+                            f"'__parameters__'; Maybe you forgot to call "
+                            f"super().__init_subclass__()?") from None
+        for param in parameters:
             prepare = getattr(param, '__typing_prepare_subst__', None)
             if prepare is not None:
                 args = prepare(cls, args)
         _check_generic_specialization(cls, args)
 
         new_args = []
-        for param, new_arg in zip(cls.__parameters__, args):
+        for param, new_arg in zip(parameters, args):
             if isinstance(param, TypeVarTuple):
                 new_args.extend(new_arg)
             else:

Misc/NEWS.d/next/Library/2025-10-11-10-02-56.gh-issue-139905.UyJIR_.rst

@@ -0,0 +1,2 @@
+Add suggestion to inscrutable error about ``cls.__parameters__`` not
+existing, because ``super().__init_subclass__()`` was not called.