From 357f1d981f027289943d954a90e5b3b136993939 Mon Sep 17 00:00:00 2001
From: Xuanteng Huang <xuanteng.huang@outlook.com>
Date: Sat, 11 Oct 2025 17:39:55 +0800
Subject: [PATCH 1/2] better error msg for generic alias subclass

---
 Lib/test/test_genericalias.py | 14 ++++++++++++++
 Lib/typing.py                 |  3 +++
 2 files changed, 17 insertions(+)

diff --git a/Lib/test/test_genericalias.py b/Lib/test/test_genericalias.py
index 4e08adaca05cdd..50731fac212940 100644
--- a/Lib/test/test_genericalias.py
+++ b/Lib/test/test_genericalias.py
@@ -203,6 +203,20 @@ class C(list[int]):
         self.assertEqual(C.__bases__, (list,))
         self.assertEqual(C.__class__, type)
 
+        class Foo:
+            def __init_subclass__(cls) -> None:
+                pass
+
+        class Bar:
+            def __init_subclass__(cls) -> None:
+                super().__init_subclass__()
+
+        class FooSub[T](Foo): pass
+        class BarSub[T](Bar): pass
+        with self.assertRaisesRegex(RuntimeError, '__parameters__'):
+            _ = FooSub[int]
+        _ = BarSub[int]
+
     def test_class_methods(self):
         t = dict[int, None]
         self.assertEqual(dict.fromkeys(range(2)), {0: None, 1: None})  # This works
diff --git a/Lib/typing.py b/Lib/typing.py
index 4311a77b8db749..48009bf0380f00 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1158,6 +1158,9 @@ def _generic_class_getitem(cls, args):
                 f"Parameters to {cls.__name__}[...] must all be unique")
     else:
         # Subscripting a regular Generic subclass.
+        if not hasattr(cls, "__parameters__"):
+            raise RuntimeError(f"{cls.__name__} has no __parameters__, "
+                               f"you may forget to call super().__init_subclass__()")
         for param in cls.__parameters__:
             prepare = getattr(param, '__typing_prepare_subst__', None)
             if prepare is not None:

From facf010e410bd8aea3b6dc5620742ec30cecada7 Mon Sep 17 00:00:00 2001
From: Xuanteng Huang <xuanteng.huang@outlook.com>
Date: Sat, 11 Oct 2025 17:53:34 +0800
Subject: [PATCH 2/2] add news entry

---
 .../2025-10-11-17-53-24.gh-issue-139905.qCm4jW.rst              | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2025-10-11-17-53-24.gh-issue-139905.qCm4jW.rst

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-10-11-17-53-24.gh-issue-139905.qCm4jW.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-10-11-17-53-24.gh-issue-139905.qCm4jW.rst
new file mode 100644
index 00000000000000..611b7610d48f6d
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-10-11-17-53-24.gh-issue-139905.qCm4jW.rst
@@ -0,0 +1,2 @@
+Print better error message when subclassing `GenericAlias` without calling
+``super().__init_subclass__``.