Incorrectly narrowed Literal in while loops

Bug Report

The following code snippet:

from typing import Literal


class Node:
    color: Literal["red", "black"]
    next: Node


def walk(node: Node) -> None:
    while node.color == "red":
        node = node.next
    reveal_type(node.color)
    while True:
        reveal_type(node.color)
        if node.color == "red":
            return
        node = node.next
        reveal_type(node.color)

Results in a comparison-overlap false positive under mypy --strict:

test.py:12: note: Revealed type is "Literal['black']"
test.py:14: note: Revealed type is "Literal['black']"
test.py:15: error: Non-overlapping equality check (left operand type: "Literal['black']", right operand type: "Literal['red']")  [comparison-overlap]
test.py:18: note: Revealed type is "Union[Literal['red'], Literal['black']]"
Found 1 error in 1 file (checked 1 source file)

As you can see, mypy correctly concludes that at line 12, node.color must be "black", and at line 18, it may be "red" or "black". But at line 14, it still thinks that it must be "black" even though it may not be on subsequent iterations. This causes the false positive on line 15.

To Reproduce

https://mypy-play.net/?mypy=master&python=3.12&flags=strict&gist=3207b91810da9f447ff090d86df689a5

Your Environment

• Mypy version used: tested on 1.10.0+dev.80190101f68b52e960c22572ed6cc814de078b9c and 1.8.0
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.12.2

Riproduci lo snippet con mypy --strict e confronta il narrowing nei tre punti reveal_type. Traccia il flusso di controllo e la logica di narrowing del ciclo while; il lavoro è completato quando il secondo ciclo non mantiene il narrowing del primo ciclo, l’errore di sovrapposizione nel confronto scompare e i tipi rivelati corrispondono al report.