New LOAD_ATTR_CLASS_WITH_METACLASS_CHECK does not guard against __class__ reassignment

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Bug description:

This was found by Codex. Consider the following:

import enum
import _testinternalcapi

class ColorMeta(enum.EnumType):
    pass

class Color(enum.IntEnum, metaclass=ColorMeta):
    RED = 1

red = Color.RED

class Descriptor(enum.IntEnum):
    RED = 1

    def __get__(self, obj, owner):
        return "descr"

def get_enum_member():
    return Color.RED

for _ in range(_testinternalcapi.SPECIALIZATION_THRESHOLD):
    assert get_enum_member() == 1

red.__class__ = Descriptor

for _ in range(_testinternalcapi.SPECIALIZATION_THRESHOLD):
    assert get_enum_member() == "descr"

This passes without specializing interpreter, but fails with. @kumaraditya303

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-149239: Deopt LOAD_ATTR_CLASS_WITH_METACLASS_CHECK on reassining __class__ #149269

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A possible fix (also suggested by Codex) is to add to _LOAD_ATTR_CLASS:

PyTypeObject *descr_type = Py_TYPE(descr);
EXIT_IF((descr_type->tp_flags & Py_TPFLAGS_IMMUTABLETYPE) == 0 && descr_type != (PyTypeObject *)owner);

which seems correct, as it re-checks the specialization invariant.