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

Inizia con il reproducer fornito e confrontane il comportamento con quello dell’esempio analogo di classe di primo livello collegato nell’issue. Traccia il modo in cui mypy gestisce una dichiarazione nonlocal in una classe annidata all’interno di una funzione, quindi aggiungi un test di regressione che mostri che la variabile esterna rimane tipizzata e utilizzabile dopo la classe; il lavoro è completo quando il reproducer non produce errori.