`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.

mypybug.py の reproducer から始め、mypy 1.11.1 に対して実行し、注釈付きと注釈なしの空の frozenset のケースを比較します。関係する reduce の型推論を追跡し、注釈なしの形式で報告された診断が生成されなくなったことを示す回帰テストを追加します。