`self` annotation not checked in `__index__`?

Bug Report

To Reproduce

from __future__ import annotations
from typing import Callable, TypeVar, Any, Protocol, Self, Any

T = TypeVar('T', covariant=True)

class Array(Protocol[T]):
    def __index__(self: Array[int], /) -> int: ...
    def dtype(self) -> T: ...
    def __rmul__(self, other: Any) -> Self: ...

def func(a: Array[str], b: list[bool]) -> None:
    result = b * a
    reveal_type(result)

https://mypy-play.net/?mypy=latest&python=3.12&gist=596075fa56c1bd714ec44b224f5ed97c

Expected Behavior

t.py:13: note: Revealed type is "Array[str]"

(which is what Pyright does)

This is because list.__mul__ is defined as

https://github.com/python/typeshed/blob/7aad2554355b55e70e4137fadf48f294ee79efaf/stdlib/builtins.pyi#L1124

    def __mul__(self, value: SupportsIndex, /) -> list[_T]: ...

and SupportsIndex checks for __index__(self) -> int, and if value is Array[str], then Array.__index__ is not matched

Actual Behavior

t.py:13: note: Revealed type is "builtins.list[builtins.bool]"

Your Environment

• Mypy version used: 1.16.1
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

Python 3.12 と mypy 1.16.1 で issue の例を再現し、b * a に対して明らかになる型を確認します。Array.index の self アノテーションが SupportsIndex および list.mul とどのように照合されるかを追跡します。例で期待どおり Array[str] が明らかになり、回帰テストのカバレッジがあることを完了条件とします。