Rather confusing error message when argument name is wrong

Bug Report

If you run Mypy on this code:

from typing import Any, Awaitable, Callable, Protocol, Union, TypeVar, Generic


class IRequest:
    pass


KleinSynchronousRenderable = Union[str, bytes]
KleinRenderable = Union[
    KleinSynchronousRenderable, Awaitable[KleinSynchronousRenderable]
]


T_co = TypeVar('T_co', contravariant=True)

class KleinRoute(Protocol[T_co]):
    def __call__(v, /, self: T_co, request: IRequest) -> KleinRenderable:
        """
        Function that, when decorated by L{Klein.route}, handles a Klein
        request.
        """


class Klein(Generic[T_co]):
    def route(self, url: str, *args: Any, **kwargs: Any) -> Callable[[KleinRoute[T_co]], KleinRoute[T_co]]:
        """
        Decorator for endpoint routing.
        """


class Application:
    klein: Klein[Application] = Klein()

    @klein.route("/foo")
    def foo(self, resource: IRequest) -> KleinRenderable:
        pass

You get this error:

main.py:34: error: Argument 1 has incompatible type "Callable[[Application, IRequest], Union[Union[str, bytes], Awaitable[Union[str, bytes]]]]"; expected "KleinRoute[Application]"

Using mypy 0.790 on Python 3.9.

If you change the resource argument to route (second to last line), this passes. The above error suggests a different problem entirely, so it had us chasing our tails a bit.

Commencez par exécuter la reproduction fournie de mypy 0.790 sur Python 3.9 et comparez le diagnostic lorsque le paramètre s’appelle resource plutôt que route. Suivez le code qui formate cette erreur d’argument incompatible ; le travail est terminé lorsqu’un nom d’argument incorrect produit un diagnostic qui pointe vers le véritable problème de nom de paramètre, au lieu de signaler uniquement un type incompatible.