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

Bug Report

Erroneous complaint that overloaded function implementation does not accept all possible arguments of signature.

To Reproduce

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 | int, **kw: int) -> None:
        pass

Expected Behavior

The above should pass type checking.

Pyright's happy enough:

$ pyright typing_bug.py 
0 errors, 0 warnings, 0 informations

Actual Behavior

$ mypy typing_bug.py 
typing_bug.py:16: error: Overloaded function implementation does not accept all possible arguments of signature 1  [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Mypy version used: 1.14.1 (compiled: yes)
Mypy command-line flags: None, see above
Mypy configuration options from mypy.ini (and other config files): None.
Python version used: Python 3.12.1

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

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

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

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cjw296 commented on Feb 4, 2025

A5rocks commented on Feb 4, 2025

cjw296 commented on Feb 4, 2025

A5rocks commented on Feb 7, 2025

cjw296 commented on Jul 8, 2025

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