Protocol restriction in generics do not work in methods. But work on functions.

Bug Report

Protocol restriction in generics do not work in methods. But work on functions.

To Reproduce

from typing import *

CanCompare = TypeVar("CanCompare", bound='Comparable')


class Comparable(Protocol):
    def __lt__(self: CanCompare, other: CanCompare) -> bool:
        pass

    def __gt__(self: CanCompare, other: CanCompare) -> bool:
        pass

    def __le__(self: CanCompare, other: CanCompare) -> bool:
        pass

    def __ge__(self: CanCompare, other: CanCompare) -> bool:
        pass


A = TypeVar('A', covariant=True)


class CustomList(list, Generic[A]):

    def getMax(self: 'CustomList'[CanCompare]) -> Optional[CanCompare]:
        return functionGetMax(self)


def functionGetMax(aList: 'CustomList'[CanCompare]) -> Optional[CanCompare]:
    pass


canCompareMe: CustomList[int] = CustomList([1, 2, 3])
canCompareMe.getMax()  # ok
functionGetMax(canCompareMe)  # ok


class NotComparable:
    pass  # no comparisons implemented


impossibleToCompare: CustomList[NotComparable] = CustomList([NotComparable(), NotComparable()])
impossibleToCompare.getMax() # Bug: No error raised
functionGetMax(impossibleToCompare)  # Correctly raised error: Value of type variable "CanCompare" of "functionGetMax" cannot be "NotComparable"

Expected Behaviou
Mypy should raise an error on the .getMax() method when called on a CustomList[NotComparable].

Actual Behavior
Instead, no error was raised.
Curiously, the functionGetMax function correctly raises the error.

(Write what happened.)

Your Environment

• Mypy version used: Both mypy 0.910 and mypy-0.920+dev.58fb493b04c61f6202ac8ba0811d3ff8546a8e60
• Mypy command-line flags: No flags
• Mypy configuration options from mypy.ini (and other config files): no config options
• Python version used: Python 3.9.6 (v3.9.6:db3ff76da1, Jun 28 2021, 11:49:53)
• Operating system and version: Macosx 10.15.7

Start with the supplied reproducer and compare how mypy checks the generic protocol restriction in getMax versus functionGetMax. Trace the method-call type-checking path and its handling of CustomList[NotComparable]; done means the method call produces the same type-variable error as the function call.