functools.partialmethod not supported properly? #8619

• Are you reporting a bug, or opening a feature request?

Bug

• Please insert below the code you are checking with mypy.

import operator
from functools import partialmethod


class IntCmp:
    def __init__(self, value):
        self._value = value

    def _cmp(self, opr, other: int) -> bool:
        if isinstance(other, int):
            return opr(self._value, other)
        return NotImplemented

    __eq__ = partialmethod(_cmp, operator.eq)  # Incompatible types in assignment (expression has type "partialmethod[bool]", base class "object" defined the type as "Callable[[object, object], bool]")
    __ne__ = partialmethod(_cmp, operator.ne)  # Incompatible types in assignment (expression has type "partialmethod[bool]", base class "object" defined the type as "Callable[[object, object], bool]")
    # rest of the comparison operators (lt, le, gt, ge) type checks correctly


c = IntCmp(1)
assert c == 1  # Cannot determine type of '__eq__'

• What is the actual behavior/output?

MyPy reports an error on the comparison, saying Cannot determine type of '__eq__', as well as incompatible types on assignment as shown.
The code correctly passes at runtime.

• What is the behavior/output you expect?

__eq__ and __ne__ should correctly type-check the partialmethod, and comparison should give no error. This particular partialmethod usage is shown in Python docs: https://docs.python.org/3/library/functools.html#functools.partialmethod

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

Using latest git master, Python 3.8.1

• What are the mypy flags you are using? (For example --strict-optional)

--follow-imports=silent
--show-column-numbers
--ignore-missing-imports
--check-untyped-defs

EDIT: I am aware of #2967 and #1484 being a thing, however this covers partialmethod usage which wasn't mentioned as a topic in the two other issues.