Reject arguments that are both keyword and positional only

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.

Start by running mypy on the two Callable/Protocol reproductions in the issue and compare their assignment diagnostics. Trace the handling of __args and __arg in callable compatibility, then verify that arguments treated as both keyword and positional are rejected with a clear diagnostic.