Inspired bymicrosoft/pyright#10673, I looked at this simple example: Code sample inpyright playground

fromcollections.abcimportSequencedeflistify[T](x:Sequence[T])->list[T]:returnlist(x)deffunc(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 imolist[str] | list[int].

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

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

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