Protocols with self break super #12344

Bug Report

Sometimes there's a need to use a typing.Protocol to specify self typing for cases like mixin classes, where the methods aren't there at class definition, but will be there at runtime, once the mixin is inherited from. This generally works fine, except it breaks in one silly case, a short repro of which is available below.

In short, it looks like super doesn't like protocols as it's second argument, which isn't even explicitly passed here.

To Reproduce

from typing import Protocol

class MixinProt(Protocol):
    def func(self) -> None:
        ...

class Mixin:
    def func(self: MixinProt) -> None:
        return super().func()

Expected Behavior

No error, duh.

Actual Behavior

From the very last line of the code above:

Argument 2 for "super" not an instance of argument 1

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.8.10
• Operating system and version: Win7x64