Inconsistent assignability of callable with optional keywords to positional-only callback

Bug Report, To Reproduce, and Actual Behaviour

A callable with an optional keyword argument can be assigned directly to a positional-only callback protocol, but not assigned to a generic function taking the callback protocol as an argument.

from typing import *

class PosOnly[*Ts, R](Protocol):
    def __call__(self, /, *args: *Ts) -> R: ...

def accepts_pos_only[*Ts, R](f: PosOnly[*Ts, R], /) -> PosOnly[*Ts, R]:
    return f

def my_func(a: int, b: str, /, *, c: int = 8) -> None: ...

b: PosOnly[int, str, None] = my_func  # OK
reveal_type(accepts_pos_only(b))  # OK; reveals `PosOnly[int, str, None]`
reveal_type(accepts_pos_only(my_func))  # Fails; reveals `PosOnly[Unpack[builtins.tuple[Never, ...]], Never]`

Expected Behavior

No errors; both reveals PosOnly[int, str, None]

Your Environment

• Mypy version used: 1.17
• Python version used: 3.12

Start by running the reproducer with mypy 1.17 and compare direct assignment of my_func with the generic accepts_pos_only(my_func) call. Trace callable protocol assignability and type inference for optional keyword arguments and positional-only callbacks. Done when both reveal_type calls succeed as PosOnly[int, str, None] without errors.