`functools.reduce` over sets: empty set not accepted without annotation

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:
    # OK
    emptyset: frozenset[str] = frozenset()
    assert inames_to_pull_out <= reduce(frozenset.union, loop_nests, emptyset)

    # Error
    assert inames_to_pull_out <= reduce(frozenset.union, loop_nests, frozenset())

Here is what mypy says as of 1.11.1:

mypybug.py:14: error: Argument 1 to "reduce" has incompatible type "Callable[[frozenset[_T_co], VarArg(Iterable[_S])], frozenset[_T_co | _S]]"; expected "Callable[[frozenset[Never], frozenset[str]], frozenset[Never]]"  [arg-type]

This seems spurious: It seems that something picks up the type of the empty set somewhat too eagerly. Pyright (1.1.376) also seems to like this code just fine.

Start with the reproducer in mypybug.py and run it against mypy 1.11.1, comparing the annotated and unannotated empty frozenset cases. Trace the reduce type inference involved and add a regression test showing that the unannotated form no longer produces the reported diagnostic.