Generic functions are not subtypes of generic protocols

[ndmitchell]

Describe the Bug

from typing import *

_C = TypeVar("_C")

class _Decorator(Protocol):
    def __call__(self, fn: _C) -> _C: ...


def api() -> _Decorator:
    def _impl(fn: _C) -> _C:
        ...
    return _impl # E: Returned type `[_C](fn: _C) -> _C` is not assignable to declared return type `_Decorator`

Sandbox Link

https://pyrefly.org/sandbox/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoBUAUEQPoDCUAvFACqICmAagIYgAUARBZwJQkBjADYsAzqKikAIgwG4WMXOwAK4RXKG8AXESh6oAEwbBJpASyFDSpdqIZDgAGijAUWyeV5QAtAD4P7gB0wSRERiYsyOxefpIyciAKuDr6hsaSWAhC7K7uFDH%2BFCmp%2BsGBuvogDDAAriAoGdhCREA&version=3.12

(Only applicable for extension issues) IDE Information

No response