pyrefly raises on `iter` on `Mapping`

[MarcoGorelli]

from typing import Mapping, TypeAlias

B: TypeAlias = Mapping[str, int]

def func(a: Mapping[str, str] | B) -> None:
    _ = next(iter(a.items()))

ERROR No matching overload found for function `iter` [no-matching-overload]
 --> /home/marcogorelli/scratch/t.py:6:18
  |
6 |     _ = next(iter(a.items()))
  |                  ^^^^^^^^^^^
  |
  Possible overloads:
  (object: SupportsIter[_SupportsNextT_co], /) -> _SupportsNextT_co [closest match]
  (object: _GetItemIterable[_T], /) -> Iterator[_T]
  (object: () -> _T | None, sentinel: None, /) -> Iterator[_T]
  (object: () -> _T, sentinel: object, /) -> Iterator[_T]
 INFO 1 error

pyrefly 0.31.1

[grievejia]

The type alias is not required for the example to work. Simpler repro:

def func2(a: list[int] | list[str]) -> None:
    _ = next(iter(a))