erroneous complaint that overloaded function implementation does not accept all possible arguments of signature #18596

If I change the implemention to this, mypy is happy:

    @classmethod
    def set(cls, *args: datetime | int, **kw: int | datetime) -> None:
        pass

But that's not correct: it's never valid to pass a datetime as a keyword parameter, note the "positional argument only" marker in signature 2.

The following also make mypy happy, but doesn't match the behaviour I need:

from datetime import datetime
from typing import overload

class Sample:

    @overload
    @classmethod
    def set(cls, *, year: int) -> None:
        ...

    @overload
    @classmethod
    def set(cls, instance: datetime, / ) -> None:
        ...

    @classmethod
    def set(cls, *args: datetime, **kw: int) -> None:
        pass

Smaller reproducer:

from typing import overload

@overload
def set(year: int) -> None:
    ...

@overload
def set() -> None:
    ...

def set(*args: object | int, **kw: int) -> None:  # E: Overloaded function implementation does not accept all possible arguments of signature 1
    pass

If you remove the object | then it passes type checking.

The error message is also frustratingly poor: why not specify which types are missing from the implementation? :-/

The very wish-washy answer is that it's because "is this implementation alright" uses a generalized subtype check, so there's not the available information. I think mypy could definitely provide more provenance information though.

Summary question (and hopefully eventually a thread!) on related problems:
python/typing#2040