Bug Report

Inferring a type variable from a function passed to a union of generic callback protocols depends on which calls were checked earlier in the same process. The first call infers correctly; a later call matching the other protocol infers Never. With --num-workers this makes results nondeterministic, because it depends on which modules share a worker.

To Reproduce

Repro minimized using LLMs based on real world code. (playground)

from typing import Protocol, TypeVar

T = TypeVar("T")
PT = TypeVar("PT", contravariant=True)


class OneArg(Protocol[PT]):
    def __call__(self, x: PT, /) -> None: ...


class TwoArgs(Protocol[PT]):
    def __call__(self, x: PT, y: int, /) -> None: ...


def first_arg(fn: OneArg[T] | TwoArgs[T]) -> T:
    raise NotImplementedError


def one(x: int) -> None: ...
def two(x: str, y: int) -> None: ...


reveal_type(first_arg(one))  # int
reveal_type(first_arg(two))  # expected str, got Never

Expected Behavior

main.py:23: note: Revealed type is "int"
main.py:24: note: Revealed type is "str"

Actual Behavior

main.py:23: note: Revealed type is "int"
main.py:24: note: Revealed type is "Never"

• Swapping the last two lines gives str then Never; either call alone is correct.
• With Callable[[T], None] | Callable[[T, int], None] instead of the protocols, both infer correctly.
• Parallel mode: put the definitions in one module and the two calls (as x = first_arg(...)) in six modules each. Cold runs of mypy --num-workers=8 . then report between zero and four Need type annotation for "x" errors; 4 of 20 runs were clean on 2.3.0.

Your Environment

• Mypy version used: 2.3.0, 2.4.0, master (2.5.0+dev.6a6979c49a96d677396fe6df7301c903c7bcc01e)
• Mypy command-line flags: none (parallel variant: --num-workers=8)
• Mypy configuration options: none
• Python version used: 3.13