Mypy signals error with Protocol and __rpow__ method with three arguments

Okay, this is a head-scratcher. 😕

To Reproduce

# test_case.py
from typing import Protocol, overload, runtime_checkable
@runtime_checkable
class PowableProto(Protocol):
    @overload
    def __pow__(self, other: "PowableProto") -> "PowableProto":
        ...
    @overload
    def __pow__(self, other: "PowableProto", modulo: "PowableProto") -> "PowableProto":  # <- fine
        ...
    @overload
    def __rpow__(self, other: "PowableProto") -> "PowableProto":
        ...
    @overload
    def __rpow__(self, other: "PowableProto", modulo: "PowableProto") -> "PowableProto":  # <- this (of all things) breaks
        ...
    @overload
    def __rpOw__(self, other: "PowableProto") -> "PowableProto":
        ...
    @overload
    def __rpOw__(self, other: "PowableProto", modulo: "PowableProto") -> "PowableProto":  # <- fine
        ...

% mypy --version
mypy 0.910
% mypy --config-file=/dev/null test_case.py
/dev/null: No [mypy] section in config file
test_case.py:15: error: Invalid signature "def (test_case.PowableProto, test_case.PowableProto, test_case.PowableProto) -> test_case.PowableProto"
Found 1 error in 1 file (checked 1 source file)

Expected vs. Actual Behavior

I would expect at least consistent behavior among the overridden methods. This only crops up with __rpow__ for some reason. (I have tested many others.)

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: --config-file=/dev/null (i.e., force default configuration)
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: Python 3.9.6 (from python --version)
• Operating system and version: Darwin Kernel Version 19.6.0: Thu May 6 00:48:39 PDT 2021; root:xnu-6153.141.33~1/RELEASE_X86_64 (from uname -v)

Start by running the supplied mypy command against test_case.py and compare how the three-argument pow and rpow overloads are handled. Trace the signature validation that produces the error, then make the behavior consistent and verify the reproducer no longer reports an invalid signature.