Subclass of both `ABC` and `ABCMeta` breaks on `__subclasscheck__`

[erezinman]

BPO	43594
Nosy	@gvanrossum, @terryjreedy, @erezinman, @kamilturek

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-03-22.13:42:00.949>
labels = ['3.8', 'library', '3.9', '3.10']
title = 'Subclass of  both `ABC`  and `ABCMeta` breaks on `__subclasscheck__`'
updated_at = <Date 2021-03-26.23:17:05.469>
user = 'https://github.com/erezinman'

bugs.python.org fields:

activity = <Date 2021-03-26.23:17:05.469>
actor = 'gvanrossum'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-03-22.13:42:00.949>
creator = 'erezinman'
dependencies = []
files = []
hgrepos = []
issue_num = 43594
keywords = []
message_count = 3.0
messages = ['389314', '389562', '389573']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'terry.reedy', 'erezinman', 'kamilturek']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43594'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[erezinman]

Consider the following example:

from abc import ABCMeta, ABC

class MetaclassMixin(ABC):
    pass 

class Meta(MetaclassMixin, ABCMeta):
    pass

class A(metaclass=Meta):
    pass

Then the call isinstance(A, Meta) returns True but isinstance(1, Meta) raises

>>> TypeError: __subclasscheck__() missing 1 required positional argument: 'subclass'

Checked on 3.6.9, 3.8.0 & 3.8.8

[terryjreedy]

Verified on Win10 for 3.8-3.10. 3.6 only gets security fixes.

Guido, is the above a bug? (There is no listed abc or 'metaclass' expert and I don't know.)

[gvanrossum]

I think it's strange code -- the mixin wants to be a metaclass but also an ABC (which is not a meta class). Unless there's a serious use case I'm inclinded to reply "don't care".