a nonlocal in class nested within a function causes the referenced var to disappear from the outer scope

Bug Report

c.f.: #12730

When a class definition appears within a function, a nonlocal in the class body causes the referenced variable to disappear from the outer scope.

To Reproduce

Analyze: [mypy-play.net]

from __future__ import annotations
from typing import (
        Optional,
        )
from typing_extensions import (
        assert_type,
        )

def in_a_function() -> None:

    some_value: Optional[int] = None

    assert_type(some_value, Optional[int])

    class SomeClass:

        nonlocal some_value
        assert_type(some_value, Optional[int])
        some_value = 42

    assert_type(some_value, Optional[int])
            # mypy: Expression is of type "Any", not "Optional[int]"
            #       [assert-type]
            # mypy: Name "some_value" is not defined  [name-defined]
    assert some_value == 42
            # mypy: Name "some_value" is not defined  [name-defined]
            # works at runtime

    print("all is well")  # is printed

if __name__ == '__main__':
    in_a_function()

Expected Behavior

This code should produce no errors. (Analogous code, outside a def, is analyzed correctly.)

Actual Behavior

See: mypy's output, inline as comments.

Your Environment

• Mypy version used: 1.8.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.8, 3.12

提供された reproducer から始め、issue にリンクされている類似のトップレベルクラスの例とその動作を比較します。関数内にネストされたクラスでの nonlocal 宣言を mypy がどのように処理するかを追跡し、クラスの後でも外側の変数が型付けされたまま使用可能であることを示す回帰テストを追加します。reproducer がエラーを生成しなければ完了です。