Nondeterministic type-var error with LiteralString-bounded TypeVars

After upgrading from mypy 1.19.0 to 1.19.1, we're experiencing nondeterministic type checking behavior:

1. Without type: ignore[type-var]: Sometimes reports "Value of type variable 'S' of 'Response' cannot be 'object'" [type-var]
2. With type: ignore[type-var]: Sometimes reports "Unused 'type: ignore[type-var]' comment" [unused-ignore]

The error state is inconsistent and appears related to PR #20345 (which fixed issue #20344 about nondeterministic behavior with tuple type vars).

Environment

• Mypy version: 1.19.1
• Python version: 3.13
• Upgraded from: mypy 1.19.0 (no errors)

Example

Granted, this is a simplified, sanitized example I have extracted from our code base but I believe it highlights the case where the issue is happening

from typing import LiteralString


class Container[S: LiteralString, T]:
    """A container with a LiteralString key."""

    def __init__(self, key: S, value: T) -> None:
        self.key = key
        self.value = value


class Response[S: LiteralString, T]:
    """A response parameterized by LiteralString."""

    def __init__(self, container: Container[S, T]) -> None:
        self.container = container


def process[S: LiteralString, S1: LiteralString](
    containers: list[Container[S, int] | Container[S1, str]],
) -> dict[int, Response]:
    """Create responses from a union of containers with different LiteralString keys."""
    return {
        i: Response(container)  # May error: type-var or unused-ignore
        for i, container in enumerate(containers)
    }

Scenario 1: With type: ignore[type-var] comment

example.py:28: error: Unused "type: ignore[type-var]" comment  [unused-ignore]

Scenario 2: Without the comment (in different contexts)

Sometimes passes with no errors, sometimes shows:

example.py:28: error: Value of type variable "S" of "Response" cannot be "object"  [type-var]

I'm not able to nail down exactly when each scenario surfaces but hopefully this is enough to go on as it otherwise presents as a regression since 1.19.0

Commencez par exécuter l’exemple fourni avec mypy 1.19.0 et 1.19.1, avec et sans le commentaire type: ignore[type-var], puis comparez les diagnostics. Consultez PR #20345 et issue #20344 concernant le comportement non déterministe associé des type-var de tuple ; le travail est terminé lorsque le cas de TypeVar borné par LiteralString produit un résultat déterministe et correct.