`ABCMeta` and its subclasses are treated differently

[sobolevn]

It is a common practice to create subclasses of ABCMeta for your own abstract metaclasses when you also have other metaclass(es). It helps solving metaclass conflict error.

For example:

from djang.db.models import ModelBase
from abc import ABCMeta

class AbstractModel(ABCMeta, ModelBase): ...

Or any other existing metaclass.

But, mypy treats ABCMeta and AbstractModel differently:

from abc import ABCMeta, abstractmethod
from typing import Protocol

class A:  # OK, has @abstractmethod
    @abstractmethod
    def f(self) -> None:
        pass

# Expected:
class B(A):  # E: Class a.B has abstract attributes "f"  # N: If it is meant to be abstract, add 'abc.ABCMeta' as an explicit metaclass
    pass

class C(A, metaclass=ABCMeta):  # OK, has ABCMeta as a metaclass
    pass

class CustomABC(ABCMeta):
    pass

# Unexpected:
class D(A, metaclass=CustomABC):  # E: Class a.D has abstract attributes "f"  # N: If it is meant to be abstract, add 'abc.ABCMeta' as an explicit metaclass
    pass

I think that needs to be fixed.