enum: Mixin and int base class regression in 3.8.6

[puddly]

BPO	41889
Nosy	@warsaw, @ethanfurman, @WildCard65, @miss-islington, @puddly
PRs	bpo-41889: enum: fix multiple-inheritance regression #22487 [3.8] bpo-41889: [Enum] fix multiple-inheritance regression (GH-22487) #23672 [3.9] bpo-41889: [Enum] fix multiple-inheritance regression (GH-22487) #23673

^{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 = 'https://github.com/ethanfurman'
closed_at = <Date 2020-12-08.03:21:43.846>
created_at = <Date 2020-09-30.04:21:36.560>
labels = ['3.8', 'type-bug', 'library', '3.9', '3.10']
title = 'enum: Mixin and int base class regression in 3.8.6'
updated_at = <Date 2020-12-08.03:21:43.845>
user = 'https://github.com/puddly'

bugs.python.org fields:

activity = <Date 2020-12-08.03:21:43.845>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2020-12-08.03:21:43.846>
closer = 'ethan.furman'
components = ['Library (Lib)']
creation = <Date 2020-09-30.04:21:36.560>
creator = 'puddly'
dependencies = []
files = []
hgrepos = []
issue_num = 41889
keywords = ['patch']
message_count = 5.0
messages = ['377692', '377709', '382623', '382699', '382700']
nosy_count = 6.0
nosy_names = ['barry', 'eli.bendersky', 'ethan.furman', 'WildCard65', 'miss-islington', 'puddly']
pr_nums = ['22487', '23672', '23673']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41889'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[puddly]

The following code worked in 3.8.5 but does not in 3.8.6 due to the fix for bpo-39587:

import enum


class MyInt(int):
    def __new__(cls, value):
        return super().__new__(cls, value)

class HexMixin:
    def __repr__(self):
        return hex(self)

class MyIntEnum(HexMixin, MyInt, enum.Enum):
    pass


class Foo(MyIntEnum):
    TEST = 1

assert isinstance(Foo.TEST, MyInt)
assert repr(Foo.TEST) == "0x1"

In 3.8.6, the Foo enum itself fails to be created because HexMixin is now considered the member type instead of MyInt:

Traceback (most recent call last):
  File "enum_test.py", line 18, in <module>
    class Foo(MyIntEnum):
  File "/usr/local/Cellar/python@3.8/3.8.6/Frameworks/Python.framework/Versions/3.8/lib/python3.8/enum.py", line 215, in __new__
    enum_member = __new__(enum_class)
TypeError: object.__new__(Foo) is not safe, use int.__new__()

[WildCard65]

Actually, this is an issue with native types in general that define a 'tp_new' slot value ('!= NULL').

[ethanfurman]

New changeset c266736 by Ethan Furman in branch 'master':
bpo-41889: [Enum] fix multiple-inheritance regression (GH-22487)
c266736