`functools.reduce` over sets becomes unacceptable when used in a larger expression

Reproducer:

from typing import Collection
from functools import reduce


def _pull_out_loop_nest(
            loop_nests: list[frozenset[str]],
            inames_to_pull_out: frozenset[str]
        ) -> None:
    emptyset: frozenset[str] = frozenset()

    # OK
    reduce(frozenset.union, loop_nests[:-1], emptyset)

    # Error
    inames_to_pull_out - reduce(frozenset.union, loop_nests[:-1], emptyset)

Here is what mypy says as of 1.11.1:

mypybug.py:15: error: Argument 1 to "reduce" has incompatible type "Callable[[frozenset[_T_co], VarArg(Iterable[_S])], frozenset[_T_co | _S]]"; expected "Callable[[AbstractSet[str], frozenset[str]], AbstractSet[str]]"  [arg-type]

This seems spurious: It's not clear to me why this should be acceptable as a stand-alone expression, but not within a larger expression. Pyright (1.1.376) also seems to like this code just fine.

See also #17693.

Reproduce the diagnostic from the code shown in mypybug.py, comparing the standalone reduce call with the same call used in the set subtraction expression. Trace how mypy infers and checks functools.reduce in that context; done means the valid expression is accepted without weakening errors for genuinely incompatible reduce calls.