Type checking fails when registering a generic structure hook

[tdsmith]

• cattrs version: 1.1.1
• Python version: 3.8.6, mypy 0.790
• Operating System: macOS 10.15.7

Description

I might be misspecifying it, but I'm not sure how to register a generic structure hook function that can return more than one type.

mypy didn't complain about cattrs 1.0 and it continues to execute fine.

What I Did

from typing import Any, Protocol, Type, TypeVar

import attr
import cattr


class Nameable(Protocol):
    def __init__(self, name: str) -> None: ...


@attr.s(auto_attribs=True)
class A:
    name: str

@attr.s(auto_attribs=True)
class B:
    name: str


@attr.s(auto_attribs=True)
class Widget:
    spam: A
    eggs: B


T = TypeVar("T", bound=Nameable)


def instantiate_from_name(value: Any, klass: Type[T]) -> T:
    return klass(name=value)


cattr.register_structure_hook(A, instantiate_from_name)
cattr.register_structure_hook(B, instantiate_from_name)

print(cattr.structure({"spam": "Hormel", "eggs": "Farm Fresh"}, Widget))

$ python demo3.py
Widget(spam=A(name='Hormel'), eggs=B(name='Farm Fresh'))

$ mypy demo3.py
demo3.py:33: error: Argument 2 has incompatible type "Callable[[Any, Type[T]], T]"; expected "Callable[[Any, Type[T]], A]"
demo3.py:34: error: Argument 2 has incompatible type "Callable[[Any, Type[T]], T]"; expected "Callable[[Any, Type[T]], B]"
Found 2 errors in 1 file (checked 1 source file)

Changing the signature on register_structure_hook from self, cl: Type[T], func: Callable[[Any, Type[V]], T] to self, cl: Type[T], func: Callable[[Any, Type[T]], T] resolves this and I'm not sure why the latter is wrong but I'm probably missing some subtlety.

[Tinche]

Hm, I think we would be ok with changing the type signature to self, cl: Type[T], func: Callable[[Any, Type[T]], T]. I don't remember the reason for using a different type var here.

Feel free to submit a PR :)

[tdsmith]

Will do! Thanks for the quick feedback.