Fix attrs.evolve on bound TypeVar

[ikonst]

Fixes the error on the last line of this example:

@attrs.define
class A:
  x: int

T = TypeVar('T', bound=A)

def f(t: T) -> None:
   _ = attrs.evolve(t, x=42)  # E: Argument 1 to "evolve" has incompatible type "T"; expected an attrs class

Since T is bounded by A, we know it can be treated as A.

[ikonst]

👋 @hauntsaninja @JelleZijlstra

[github-actions]

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

[hauntsaninja]

Looks great, thank you!

[ikonst]

@hauntsaninja thinking about it, is there an existing facility I can use here, so that I could also support unions? Feels like checking isinstance and accessing .upper_bound is a whack-a-mole that doesn't belong here.

[hauntsaninja]

Good question, if you can get something from mypy.checkmember to work for you that's probably the way to go

[ikonst]

That was my first thought, but checkmember is complex and probably ill-fitted for anything that's not true member access.

I thought I'd need some kind of TypeVisitor that'll list the types, then iterate over the types and "union" the signatures, e.g.

(a: int, b: int, c: object) | (a: bool, b: str) = (a: bool,)