Generic callback protocols break with kwargs

I'm getting an unexpected error when using callback protocols with kwargs

Normal callback with no args or kwargs works fine:

from typing import *

T1 = TypeVar('T1')
T2 = TypeVar('T2')

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

class Callback(Protocol[T_contra, T_co]):
  def __call__(self, __element: T_contra) -> T_co:
    pass

def applyit(func: Callback[T1, T2], y: T1) -> T2:
    return func(y)

def callback(x: str) -> int:
    pass

applyit(callback, 'foo')  # OK!

callback with args works fine:

from typing import *

T1 = TypeVar('T1')
T2 = TypeVar('T2')

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

class CallbackWithArgs(Protocol[T_contra, T_co]):
  def __call__(self, __element: T_contra, *args: Any) -> T_co:
    pass

def applyit_with_args(func: CallbackWithArgs[T1, T2], y: T1) -> T2:
    return func(y)

def callback_with_args(x: str, *args: Any) -> int:
    pass

applyit_with_args(callback_with_args, 'foo')  # OK!

But the same thing with kwargs produces an error:

from typing import *

T1 = TypeVar('T1')
T2 = TypeVar('T2')

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

class CallbackWithKwargs(Protocol[T_contra, T_co]):
  def __call__(self, __element: T_contra, **kwargs: Any) -> T_co:
    pass

def applyit_with_kwargs(func: CallbackWithKwargs[T1, T2], y: T1) -> T2:
    return func(y)

def callback_with_kwargs(x: str, **kwargs: Any) -> int:
    pass

# Argument 1 to "applyit_with_kwargs" has incompatible type "Callable[[str, KwArg(Any)], int]"; expected "CallbackWithKwargs[str, <nothing>]"
applyit_with_kwargs(callback_with_kwargs, 'foo')  # Error!

Perhaps this is expected behavior and I'm just overlooking some subtle interplay between the args in the last case, but it seems like a bug.

mypy 0.720

提供された callback protocol の再現コードを、報告された mypy 0.720 の挙動で実行し、引数なし、*args、**kwargs のケースを比較します。診断の原因となっている型推論と callback 互換性の経路を追跡し、その後、kwargs callback が戻り値の型を失わずに受け入れられることを示す回帰テストを追加します。