gh-149239: Deopt LOAD_ATTR_CLASS_WITH_METACLASS_CHECK on reassining __class__

Lib/test/test_opcache.py

@@ -1,3 +1,4 @@
+import enum
 import copy
 import pickle
 import dis
@@ -2114,6 +2115,45 @@ def load_enum_member():
         self.assert_specialized(load_enum_member,
                                 "LOAD_ATTR_CLASS_WITH_METACLASS_CHECK")
 
+    @cpython_only
+    @requires_specialization
+    def test_load_attr_class_with_metaclass_check_149239(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():
+            for _ in range(_testinternalcapi.SPECIALIZATION_THRESHOLD):
+                assert Color.RED == 1
+
+        f1()
+        self.assert_specialized(f1,
+                                "LOAD_ATTR_CLASS_WITH_METACLASS_CHECK")
+
+        # Reassign the `__class__` attr to deopt:
+        class Descriptor(enum.IntEnum):
+            RED = 1
+
+            def __get__(self, obj, owner):
+                return "descr"
+
+        red.__class__ = Descriptor
+
+        def f2():
+            for _ in range(_testinternalcapi.SPECIALIZATION_THRESHOLD):
+                assert Color.RED == 'descr'
+
+        f2()
+        self.assert_no_opcode(f2,
+                              "LOAD_ATTR_CLASS_WITH_METACLASS_CHECK")
+
+
 
 if __name__ == "__main__":
     unittest.main()

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);