`Literal["..."]`s incorrectly assignable to protocol with `__getitem__(slice) -> Self`

Bug Report

Values of Literal["..."] types are incorrectly assignable to a protocol where __getitem__(slice) returns Self

To Reproduce

from typing import Protocol, Self, Literal

class Slicable(Protocol):
    def __getitem__(self, arg: slice, /) -> Self:
        ...

class Parser[X: Slicable]:
    def handle(self, input: X) -> tuple[X, X]:
        return input[:2], input[2:]

parser = Parser[Literal["banana"]]()
#^ no error, but expected: Literal['banana'] does not satisfy Slicable bound

left, right = parser.handle("banana")
reveal_type(left)  # revealed as: Literal['banana'], but 'ba' at runtime
reveal_type(right) # revealed as: Literal['banana'], but 'nana' at runtime

https://mypy-play.net/?mypy=1.18.2&python=3.14&flags=strict&gist=315731e775d05e4b59aa0105b8f96893

Literal["banana"] should not be assignable to Slicable, because slicing a Literal["banana"] produces LiteralString, not necessarily the type itself (which would only have to be Literal["banana"])

Run the linked mypy-play reproduction with mypy 1.18.2, then trace how protocol bounds, Self-returning getitem, and Literal string slicing are checked. Done means the example rejects Literal["banana"] as satisfying Slicable and no longer reveals the sliced results as that literal type.