Specialized methods on generic classes

Bug Report

Let's say I have a generic class G.

from __future__ import annotations

from collections.abc import Sized
from typing import Generic, TypeVar

T = TypeVar("T")

class G(Generic[T]):
    pass

I want to have a single method on this class that's only legal for a subset of T.

If I want to limit it to, say, int, it works:

class G(Generic[T]):
    def test(self: G[int]) -> G[int]:
        return self

G[int]().test()  # Works, as it should
G[str]().test()  # Error, as it should

But if I want to limit it to, say, a protocol like Sized, I get a false negative:

S = TypeVar("S", bound=Sized)

class G(Generic[T]):
    def test(self: G[S]) -> G[S]:
        return self


G[int]().test()  # False negative - works, but it shouldn't

This is a pity, since it'd unlock some cool features I was working on vis-a-vis using attrs attributes. Are there any workarounds? Would this be handled by intersection types maybe?

Your Environment

• Mypy version used: mypy 1.0.0+dev.b8c03ab6809aab56928f3cd865edb44944a600a2
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: Python 3.10.6

まず、提供された Python 再現プログラムを報告された mypy バージョンで実行し、int 特殊化と Sized 制約付き特殊化の動作を比較します。次に、ジェネリックな self-type のチェック経路を追跡し、プロトコル制約付きの特殊化メソッドが意図した動作に従って受け入れられるか拒否されるかを確認する回帰テストを追加または更新します。