Enums: false-positive "No base classes allowed after ReprEnum" error

[AlexWaygood]

Bug Report

Minimal repro on 3.11 (playground link):

from enum import ReprEnum, Flag
class MyFlag(ReprEnum, Flag): ...

Minimal repro on <=3.10 (playground link):

from enum import Enum, Flag
class ReprEnum(Enum): ...
class MyFlag(ReprEnum, Flag): ...

mypy output for both (with small line-number differences):

main.py:4: error: No base classes are allowed after "__main__.ReprEnum"  [misc]

Expected Behavior

No error should be reported. This use of multiple inheritance works fine at runtime on 3.10 and 3.11. In fact, in 3.11, the source code for the enum module in the stdlib uses exactly this inheritance structure:

class ReprEnum(Enum):
    """
    Only changes the repr(), leaving str() and format() to the mixed-in type.
    """

# Much further down...

class IntFlag(int, ReprEnum, Flag, boundary=EJECT):
    """
    Support for integer-based Flags
    """

Your Environment

I can reproduce this bug on mypy 0.940+ (0.940 was the first released version with @sobolevn's #12026, which introduced this error message).

[AlexWaygood]

An even simpler repro that doesn't involve enum.Flag:

from enum import Enum

class Foo(Enum): ...
class Bar(Enum): ...
class Baz(Foo, Bar): ...

This works fine at runtime, but mypy says:

main.py:5: error: No base classes are allowed after "__main__.Foo"