Add support for converters with TypeVars on generic attrs classes

[chadrik]

When creating generic classes using attrs, converters with type vars are not properly integrated into the generated __init__:

from typing import TypeVar, Generic, List, Iterable, Iterator
import attr

T = TypeVar('T')

def int_gen() -> Iterator[int]:
    yield 1

def list_converter(x: Iterable[T]) -> List[T]:
    return list(x)

@attr.s(auto_attribs=True)
class A(Generic[T]):
    x: List[T] = attr.ib(converter=list_converter)
    y: T = attr.ib()

a1 = A([1], 2)   # E: Argument 1 to "A" has incompatible type "Iterator[int]"; expected "Iterable[T]"

This MR fixes the bug by copying type vars from the field/attrib into the type extracted from the converter.

[chadrik]

@t4lz @euresti Do either of you have a moment to review this attrs-related change, please?

[euresti]

This change looks good to me.

[chadrik]

The idea behind this change is that a converter should ideally return the same type as the attrs field that it is added to. For example:

def list_converter(x: Iterable[T]) -> List[T]:
    return list(x)

@attr.s(auto_attribs=True)
class A(Generic[T]):
    x: List[T] = attr.ib(converter=list_converter)

A.x is type List[T] and list_converter returns List[T], and in theory this should always be the case for converters, since their purpose is to convert the init arg to the type of the field. In my changes I'm forgoing a complete comparison of the two types, but the assumption tells us that the number and order of TypeVars should be the same. We use that to generate a replacement map which will replace the converter's broken typevars, with typevars that work within the class's context.

[ikonst]

(I assume the def int_gen slipped in from another example?)

[chadrik]

yep. I fixed the example.

[github-actions]

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

[chadrik]

Hi @JukkaL, can you or someone else from the mypy team merge this, please?

[chadrik]

Hi all, can anyone help me get this one merged? It's approved and all ready to go!

[hauntsaninja]

Thanks!