mypy doesn't narrow `Final` objects in nested contexts

Bug Report

I have a global constant I've marked with Final. In a class definition, I use this constant to swap out implementations of a method. While inside a type guarding block, but outside the method definition, the type of the Final object is narrowed, but inside of the method it is not narrowed.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.12&gist=2d4c97cffea1dd6710bf5cc7283f7290

from typing import *

def _init() -> None | str:
    return None

RESOLVE_X: Final = _init()

class Example:
    
    if RESOLVE_X is not None:
        
        reveal_type(RESOLVE_X)  # note: Revealed type is "builtins.str"
        
        def __str__(self) -> str:
            return RESOLVE_X  # error: Incompatible return value type (got "str | None", expected "str")  [return-value]
            
    else:
        def __str__(self) -> str:
            return "example"

Expected Behavior

Inside the __str__ definition in the if RESOLVE_X is not None: block, RESOLVE_X should stay narrowed as it cannot be reassigned and have its type re-widened in the future.

Actual Behavior

main.py:12: note: Revealed type is "builtins.str"
main.py:15: error: Incompatible return value type (got "str | None", expected "str")  [return-value]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Mypy version 1.15.0.
Python 3.12.

Inizia con la riproduzione di mypy-play collegata nell’issue ed eseguila nell’ambiente indicato con Python 3.12 e mypy 1.15.0. Traccia come viene gestito il type guard per il Final a livello di modulo nel corpo della classe rispetto alla definizione annidata di str. Il lavoro è completato quando il narrowing rivelato viene preservato all’interno di str e l’esempio passa senza l’errore relativo al valore restituito.