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).

[sterliakov]

Okay, this is really indeterminate. mypy.solve.solve_one is order-sensitive and iterates over sets. The following patch (not a fix, but only a proof) makes mypy output on this snippet completely reproducible.

diff --git a/mypy/solve.py b/mypy/solve.py
index 57988790a..a6d9cfcd7 100644
--- a/mypy/solve.py
+++ b/mypy/solve.py
@@ -214,7 +214,7 @@ def solve_iteratively(
             break
         # Solve each solvable type variable separately.
         s_batch.remove(solvable_tv)
-        result = solve_one(lowers[solvable_tv], uppers[solvable_tv])
+        result = solve_one(sorted(lowers[solvable_tv], key=str), sorted(uppers[solvable_tv], key=str))
         solutions[solvable_tv] = result
         if result is None:
             # TODO: support backtracking lower/upper bound choices and order within SCCs.

The problematic case is solving the following set of constraints:

[1677 :> type[Self`0], 1677 :> Self`0, 1677 <: Self`0]

for Self`1667 where Self`0 has default=Any, upper_bound=p.WrapModelValidator. If the iteration starts with Self`0, we solve 1677 to Any, otherwise we conclude that deps can't be satisfied (None).

This problem stems from non-commutative join and meet implementation.

[hauntsaninja]

Ah I ran into that in #16979 (comment) as well

[hauntsaninja]

Spent some time today looking at a few of these nondeterminism issues.

#19149 is a fix for this (it makes mypy consistently emit the error)