Fixes type inference for generic calls in if expr

[sobolevn]

Closes #11049

[sobolevn]

mypy_primer output seems related, but it is not directly caused by this change: this also happens on latest master.

[sobolevn]

Ok, I will try to figure out what is going on. Looks like other commits into master do not produce this problem. 🤔

[sobolevn]

Yes, this problem is reproduced on master with:

from typing import TypeVar, Union, List

T1 = TypeVar("T1")

def promote_list(item: Union[T1, List[T1]]) -> List[T1]:
    ...

exprs: List[int]
reveal_type(promote_list(exprs))

I've opened a new issue: #11149

So, this can be considered as "ready to be reviewed".

[JukkaL]

Thanks for the PR! This seems like a promising approach. I want to check this with our big internal repository before merging, however. Even minor changes to type inference often cause unexpected false positives or other regressions.

[JelleZijlstra]

FYI this has a merge conflict.

[sobolevn]

@JelleZijlstra thanks! Resolved.