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functools.reduce over sets: empty set not accepted without annotation #17693
Description
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.
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