Type inference by container membership

[kojiromike]

Feature

If a is contained by B[T], then a must be of type T. I think mypy should make this inference, but it does not do so today.

from typing import Container


def check(o: object, c: Container[str]) -> str | None:
    if o in c:
        return o  # Incompatible return value type (got "object", expected "Optional[str]")
    return None

In the above case, I think it's clear that if o in c is True, then o must have the generic of c, in this case str.

Pitch

Any opportunity to correctly infer a type improves mypy and typechecking in general. This is just such an opportunity.

[AlexWaygood]

Duplicate of #10977