Wrong type inference with unpacking and enumerate()

[JelleZijlstra]

Mypy infers that the type of [*enumerate(["x", "y"])] is List[str], when it should be List[Tuple[int, str]].

A repro is below, with a slightly simplified copy of the typeshed definition of enumerate. Note that type inference is right if I define enumerate() as a function instead of a class.

$ python -m mypy tmp/enumunpack.py 
tmp/enumunpack.py:14: note: Revealed type is 'builtins.list[builtins.str*]'
tmp/enumunpack.py:21: note: Revealed type is 'builtins.list[Tuple[builtins.int, builtins.str*]]'
$ cat tmp/enumunpack.py 
from typing import Iterable, Iterator, Tuple, TypeVar, Generic

_T = TypeVar("_T")


class enumerate(Iterable[Tuple[int, _T]], Generic[_T]):
    def __init__(self, iterable: Iterable[_T], start: int = ...) -> None:
        ...

    def __iter__(self) -> Iterator[Tuple[int, _T]]:
        ...


reveal_type([*enumerate(["x", "y"])])


def enumerate_fn(iterable: Iterable[_T]) -> Iterator[Tuple[int, _T]]:
    ...


reveal_type([*enumerate_fn(["x", "y"])])

I tried this on master and on 0.730.

[JukkaL]

This seems to happen only in certain contexts. I wonder if we are missing a map_instance_to_supertype call somewhere. In particular, for x, y in enumerate(...) seems to work.

[AlexWaygood]

Cannot reproduce on 0.931: https://mypy-play.net/?mypy=latest&python=3.10&gist=704eba04a9a7d8594856978d7452c98a

[JelleZijlstra]

Great!