To motivate this PR, consider the following code (which is also a test case):

from typing import Union, List

class A: pass
class B: pass

U = Union[A, B]

class Base:
    variable1 : List[U] = []
    variable2 : List[U] = []

class Derived(Base):
    variable1 = [A()]
    variable2 = variable1

before this PR, although variable1 is type-checked against List[U], it is inferred as List[A], which makes the last line an error. This PR instead uses the base type as a hint for the inference, if possible.

Note that this hinting is already done, in order to infer [A()] as List[U], so that that line doesn't throw.

Since the issue is open, I think it's fair to say that this would fix #12268

Sorry for a bit of spam, normally I'd force push, I can't force github to link the issue, for some reason.

I give up. Maybe a maintainer can help.

Hey, I think this PR is ready for someone to look over. It's been around two weeks, and I don't want it to get lost in the sea of PRs. Does anyone mind taking a look?