Cannot subclass from Protocol and Enum at the same time.

Bug report

Bug description:

x-ref: https://github.com/python/cpython/issues/119946#issuecomment-4418882668

When trying to create an Enum of classes that implement a certain Protocol, an unexpected 'Protocols cannot be instantiated' error is raised.

from enum import Enum, EnumType
from typing import Protocol

class Example(Protocol):
    def method(self) -> None: ...

class Impl(Example):
    def method(self) -> None: ...

Impl()  # OK

class CompatEnumType(type(Protocol), EnumType): ...

class ProtoEnum(Example, Enum, metaclass=CompatEnumType):
    impl = Impl()  # ERROR 'Protocols cannot be instantiated'

I have debugged this a bit and the issue is that _is_protocol is only set during Protocol.__init_subclass__, but Enum instantiates its members before this method is called.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-149830

Start with Protocol.init_subclass and Enum member creation, using the reproducer in the issue to observe their ordering. Review linked PR gh-149830 for the current work. Done means the Protocol-and-Enum combination no longer raises the unexpected instantiation error and the relevant tests pass.