Reject arguments that are both keyword and positional only

[c4f3a0ce]

I tried to use Calalble protocol like this:

from typing import Callable, Protocol

class F(Protocol):
    def __call__(self, *__args: str,  __arg: int) -> int: ...


def f(*__args: str, __arg: int) -> int: ...

f_: F = f

but it fails with quite confusing message:

error: Incompatible types in assignment (expression has type "Callable[[VarArg(str), NamedArg(int)], int]", variable has type "F")
note: "F.__call__" has type "Callable[[VarArg(str), NamedArg(int)], int]"
Found 1 error in 1 file (checked 1 source file)

If I move varargs, things pass:

class G(Protocol):
    def __call__(self, __arg: int, *__args: str) -> int: ...
    
def g(__arg: int, *__args: str) -> int: ...
g_: G = g

I cannot say if this is a bug, or expected behaviour with confusing message.

[JelleZijlstra]

Not sure it's related, but by convention mypy treats arguments prefixed with two underscores as positional-only, so that may lead to confusion here.

[msullivan]

Yeah I think this is expected behavior with a confusing message? Double underscore means positional-only and you are putting in a keyword-only position, which then doesn't match. Probably we should error on arguments that are both keyword-only and positional-only.

[c4f3a0ce]

Makes perfect sense. Thanks @JelleZijlstra and @msullivan.