Metaclass confusion with properties

I'm using python 3.8 and mypy 0.740. The script below prints "21" and "42" when run, but fails to type check with mypy:

class meta (type):
    pass

class value (metaclass=meta):
    pass


class submeta (meta):
    @property
    def attr(cls) -> int:
        return 42

class subvalue (metaclass=submeta):
    @property
    def attr(self) -> int:
        return 21

o = subvalue()
a = o.attr
print(a)
a = subvalue.attr
print(a)

The mypy diagnostics say:

meta-test.py:24: error: Incompatible types in assignment (expression has type "Callable[[subvalue], int]", variable has type "int")
Found 1 error in 1 file (checked 1 source file)

The error is incorrect, "subvalue.attr" has type "int", as it resolves to the property defined on the metaclass, not to the property defined on the class.

This is fairly esoteric as written, but is the structure used by PyObjC to expose Objective-C classes. PyObjC uses metaclasses to expose class methods because ObjC classes can have instance- and class-methods with the same name.

issue で説明されている meta-test.py の例を使い、Python 3.8 と mypy 0.740 で診断を再現します。まず、mypy が subvalue とその submeta メタクラスに対してプロパティをどのように解決するかを追跡します。完了の条件は、報告されている incompatible-assignment エラーなしに subvalue.attr が int と推論され、インスタンスアクセスは引き続き正しく動作することです。