Confusing error message for a missing classmethod decorator

Please consider

import typing as tp
import enum

@enum.unique
class ColorIndex(enum.IntEnum):
    BLACK = 0
    GREY = 1
    WHITE = 2

    def _missing_(cls, value: object) -> tp.Any:
        return ColorIndex.BLACK

then mypy shows very confusing error (both signatures, for the subclass and the superclass, are the same)

$ mypy t.py
t.py:10: error: Signature of "_missing_" incompatible with supertype "Enum"
t.py:10: note:      Superclass:
t.py:10: note:          def _missing_(cls, value: object) -> Any
t.py:10: note:      Subclass:
t.py:10: note:          def _missing_(cls, value: object) -> Any
Found 1 error in 1 file (checked 1 source file)

The real issue is that _missing_ should be decorated with classmethod.

Python 3.10.1, mypy 0.930.

Start by reproducing the supplied Python example with mypy and trace the override diagnostic that reports the identical superclass and subclass signatures. Look for the existing regression tests around method override or decorator handling. Done means the diagnostic explains that missing needs a classmethod decorator and the example is covered by a test.