gh-149239: Deopt LOAD_ATTR_CLASS_WITH_METACLASS_CHECK on reassining __class__

Lib/test/test_capi/test_opt.py

@@ -1,4 +1,5 @@
 import contextlib
+import enum
 import itertools
 import sys
 import textwrap
@@ -3511,6 +3512,50 @@ def f(n):
         self.assertNotIn("_LOAD_ATTR_METHOD_NO_DICT", uops)
         self.assertIn("_LOAD_CONST_INLINE_BORROW", uops)
 
+    def test_load_attr_class_with_metaclass_check(self):
+        # LOAD_ATTR_CLASS_WITH_METACLASS_CHECK must check
+        # for `__class__` writes, see gh-149239
+        class ColorMeta(enum.EnumType):
+            pass
+
+        class Color(enum.IntEnum, metaclass=ColorMeta):
+            RED = 1
+
+        red = Color.RED
+
+        def f1(n):
+            for _ in range(n):
+                assert Color.RED == 1
+            return n
+
+        res, ex = self._run_with_optimizer(f1, TIER2_THRESHOLD)
+        self.assertIsNotNone(ex)
+        self.assertEqual(res, TIER2_THRESHOLD)
+        uops = get_opnames(ex)
+        self.assertIn("_CHECK_ATTR_CLASS", uops)
+        self.assertIn("_GUARD_TYPE_VERSION", uops)
+
+        # Reassign the `__class__` attr to deopt:
+        class Descriptor(enum.IntEnum):
+            RED = 1
+
+            def __get__(self, obj, owner):
+                return "descr"
+
+        red.__class__ = Descriptor
+
+        def f2(n):
+            for _ in range(n):
+                assert Color.RED == 'descr'
+            return n
+
+        res, ex = self._run_with_optimizer(f2, TIER2_THRESHOLD)
+        self.assertIsNotNone(ex)
+        self.assertEqual(res, TIER2_THRESHOLD)
+        uops = get_opnames(ex)
+        self.assertNotIn("_CHECK_ATTR_CLASS", uops)
+        self.assertNotIn("_GUARD_TYPE_VERSION", uops)
+
     def test_cached_load_special(self):
         class CM:
             def __enter__(self):

Misc/NEWS.d/next/Core_and_Builtins/2026-05-02-10-37-24.gh-issue-149239.lMoium.rst

@@ -0,0 +1,2 @@
+Deopt ``LOAD_ATTR_CLASS_WITH_METACLASS_CHECK`` opcode on ``__class__``
+reassigning.

Python/bytecodes.c

@@ -2976,6 +2976,11 @@ dummy_func(
         }
 
         op(_LOAD_ATTR_CLASS, (descr/4, owner -- attr)) {
+            PyTypeObject *descr_type = Py_TYPE(descr);
+            PyObject *owner_o = PyStackRef_AsPyObjectBorrow(owner);
+            EXIT_IF((descr_type->tp_flags & Py_TPFLAGS_IMMUTABLETYPE) == 0
+                && descr_type != (PyTypeObject *)owner_o);
+
             STAT_INC(LOAD_ATTR, hit);
             assert(descr != NULL);
             attr = PyStackRef_FromPyObjectNew(descr);