`LiteralString` should not be compatible with `str` in an invariant context

[erictraut]

Consider the following code sample.

from typing import LiteralString

def func1(a: list[LiteralString]):
    v1: list[str] = a   # Should generate a type error but does not currently

I just fixed a related bug in pyright.

Mypy version 1.5.1

[JelleZijlstra]

This is because internally mypy currently treats LiteralString as an alias of str as a stopgap until we get full support (#12554). Closing as a duplicate of that issue.