Distribute return type over union

[randolf-scholz]

Inspired by microsoft/pyright#10673, I looked at this simple example: Code sample in pyright playground

from collections.abc import Sequence

def listify[T](x: Sequence[T]) -> list[T]:
    return list(x)

def func(x: Sequence[str] | Sequence[int]):
    y = listify(x)
    reveal_type(y)  # list[str | int]
    print(y)

It feels like the revealed type is not actually what we want here. The desirable inference is imo list[str] | list[int].

Is there simple way to annotate listify to make the return type distribute over unions, or is this a general type-checker limitation?

What seems to happen here is that we assign Sequence[str] | Sequence[int] <: Sequence[str | int] = Sequence[T], so we pick T = int | str. However, in principle wouldn't it be more precise if the type checker, when seeing a UnionType as an argument, passes every member separately and yields the UnionType of the individual return types?

I guess with multiple arguments this runs into combinatoric explosion...