Type error randomly raised when using a decorator and a type: ignore comment

[Viicos]

To Reproduce

1. Create a new environment with pydantic==2.9.2 installed.
2. Create a file t.py:

from typing import Any

from typing_extensions import Self

from pydantic import BaseModel, model_validator
from pydantic.functional_validators import ModelWrapValidatorHandler


class WrapModelValidator(BaseModel):
    @model_validator(mode='wrap')  # type: ignore[arg-type]  # pyright: ignore[reportArgumentType]
    def no_classmethod(cls, value: Any, handler: ModelWrapValidatorHandler[Self]) -> Self: ...

3. Run mypy --cache-dir=/dev/null --python-version 3.10 --disable-error-code empty-body --warn-unused-ignores t.py

Mypy will randomly raise:

t.py:10: error: Unused "type: ignore" comment  [unused-ignore]

And sometimes suceed.

Sorry I couldn't provide more details, the issue is really weird and it seems like mypy doesn't produce fully reproducible results in this case.

Your Environment

• Mypy version used: 1.13.0
• Mypy command-line flags: --cache-dir=/dev/null --python-version 3.10 --disable-error-code empty-body --warn-unused-ignores
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

[brianschubert]

Thanks! I can reproduce this locally. This is certainly a weird one!

So far I've narrowed this down to some non-deterministic behavior of is_subtype (specifically during this invocation).

What's quite interesting is that the nondeterministic behavior isn't just between mypy runs, but also between invocations of is_subtype with the same arguments during the same run (here's a video demo in my debugger).