gh-148450: abc.register needs to update type_version when tp_flags is changed.

Lib/test/test_type_cache.py

@@ -1,4 +1,5 @@
 """ Tests for the internal type cache in CPython. """
+import collections.abc
 import dis
 import unittest
 import warnings
@@ -114,6 +115,25 @@ class HolderSub(Holder):
             Holder.set_value()
             HolderSub.value
 
+    def test_abc_register_invalidates_subclass_versions(self):
+        class Parent:
+            pass
+
+        class Child(Parent):
+            pass
+
+        type_assign_version(Parent)
+        type_assign_version(Child)
+        parent_version = type_get_version(Parent)
+        child_version = type_get_version(Child)
+        if parent_version == 0 or child_version == 0:
+            self.skipTest("Could not assign valid type versions")
+
+        collections.abc.Mapping.register(Parent)
+
+        self.assertEqual(type_get_version(Parent), 0)
+        self.assertEqual(type_get_version(Child), 0)
+
 @support.cpython_only
 class TypeCacheWithSpecializationTests(unittest.TestCase):
     def tearDown(self):

Misc/NEWS.d/next/Core_and_Builtins/2026-04-15-15-48-04.gh-issue-148450.2MEVqH.rst

@@ -0,0 +1 @@
+Fix ``abc.register()`` so it invalidates type version tags for registered classes.

Objects/typeobject.c

@@ -6419,9 +6419,25 @@ set_flags_recursive(PyTypeObject *self, unsigned long mask, unsigned long flags)
 void
 _PyType_SetFlagsRecursive(PyTypeObject *self, unsigned long mask, unsigned long flags)
 {
+    BEGIN_TYPE_LOCK();
+    /* Ideally, changing flags and invalidating the old version tag would
+       happen in one step. But type_modified_unlocked() is re-entrant and
+       cannot run with the world stopped, so we must invalidate first.
+       Immutable/static-builtin types are skipped because
+       set_flags_recursive() does not modify them. */
+    if (!PyType_HasFeature(self, Py_TPFLAGS_IMMUTABLETYPE) &&
+        (self->tp_flags & mask) != flags)
+    {
+        type_modified_unlocked(self);
+    }
+    /* Keep TYPE_LOCK held while waiting for stop-the-world so no thread
+       can reassign a version tag before the flag update. */
+    type_lock_prevent_release();
     types_stop_world();
     set_flags_recursive(self, mask, flags);
     types_start_world();
+    type_lock_allow_release();
+    END_TYPE_LOCK();
 }
 
 /* This is similar to PyObject_GenericGetAttr(),