Default value of keyword parameter in custom Protocol is not used

Bug Report

Consider the code below:

from typing import Protocol


class P(Protocol):
    def foo(self, n: int = 666) -> None: ...


class F:
    def foo(self, n: int = 666) -> None: ...


class S:
    def foo(self, n: int = 999) -> None: ...


def bar(x: P) -> None: ...


bar(F())
bar(S())

Expected Behavior

Warning in the last code line about incompatibility of class S and protocol P, because the default value of parameter n in P.foo is not ellipsis and the numerical value of n in S.foo is different.

Actual Behavior

Mypy does not generate any warnings.

Your Environment

• Mypy version used:

8.12
0.820+dev.8642b351cff94c471333256f9cd6867807384f01

• Mypy command-line flags:

none

• Mypy configuration options from mypy.ini (and other config files):

[mypy]

pretty = True
show_error_codes = True
strict = True
disallow_any_unimported = True
disallow_any_expr = True
disallow_any_decorated = True
disallow_any_explicit = True
no_warn_no_return = True
warn_unreachable = True

• Python version used:

3.8.10 (64-bit), 3.9.5 (64-bit), 3.10 beta (64-bit)

• Operating system and version:

Windows 10 64-bit

まず、issue の reproducer に対して mypy を実行し、foo の n パラメーターのデフォルト値に対するプロトコル互換性チェックを追跡してください。完了の条件は、mypy が S と P の非互換性を報告しつつ、F を互換性があるものとして引き続き受け入れることです。