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

Reproduziere das Snippet mit mypy --strict und vergleiche die Narrowing-Ergebnisse an den drei reveal_type-Stellen. Verfolge den Kontrollfluss und die Type-Narrowing-Logik der while-Schleife; fertig, wenn die zweite Schleife das Narrowing der ersten Schleife nicht beibehält, der Fehler wegen überlappender Vergleiche verschwindet und die aufgedeckten Typen mit dem Bericht übereinstimmen.