use base type as hint when inferring

[Riolku]

Would help with #12268. Modified a test because the two behaviours conflict right now.

[Riolku]

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.

[Riolku]

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

[Riolku]

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

[Riolku]

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

[Riolku]

I give up. Maybe a maintainer can help.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[Riolku]

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?

[hauntsaninja]

Thank you for the PR! I believe this was fixed in #13494