Literal["..."]s incorrectly assignable to protocol with __getitem__(slice) -> Self #20058
Description
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 runtimehttps://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"])