Consider this example where we currently emit a false positive.

from typing import Self
from enum import Enum

class Answer(Enum):
    YES = "yes"
    NO = "no"

    def is_yes(self: Self) -> bool:    # ty: Function can implicitly return `None`, which is not assignable to return type `bool`
        match self:
            case Answer.YES:
                return True
            case Answer.NO:
                return False

When the annotation is changed to self: Answer, everything works as expected. But we do not recognize that Self@is_yes with an upper bound of Answer can be narrowed down.

Similar problems certainly exist for other final types and other narrowing patterns.

It seems like we can generally simplify a non-inferable typevar T with a final upper bound F to that upper bound F? Or let T share more properties of F (like whether or not we consider it to be final).