Missing type narrowing with Literal[-1]

[tamird]

from collections.abc import Callable
from typing import Literal, TypeVar, Generic
from typing_extensions import assert_never

T = TypeVar("T")


def doit_if(t: T, fn: Callable[[T], Literal[1, -1]]) -> None:
    result = fn(t)
    
    if result == 1:
        pass
    elif result == -1:
        pass
    else:
        assert_never(result) # error: Argument 1 to "assert_never" has incompatible type "Literal[-1]"; expected "NoReturn"  [arg-type]
    
def doit_match(t: T, fn: Callable[[T], Literal[1, -1]]) -> None:
    result = fn(t)
    match result:
        case 1:
            pass
        case -1:
            pass
        case never:
            assert_never(never) # error: Argument 1 to "assert_never" has incompatible type "Literal[-1]"; expected "NoReturn"  [arg-type]

playground: https://mypy-play.net/?mypy=latest&python=3.12&gist=6bc7e8b2df025d7369cb79bdf4c0dcc5

Fails on the master branch as well.

[JelleZijlstra]

This is reproducible without the Callable too: https://mypy-play.net/?mypy=latest&python=3.12&gist=5c01688fc2c9bd27d67d69ea6b292f94&flags=warn-unreachable

from typing import Literal
from typing_extensions import assert_never


def doit_if(result: Literal[1, -1]) -> None:
    if result == 1:
        pass
    elif result == -1:
        reveal_type(result)
    else:
        assert_never(result)

Here mypy thinks the result == -1 branch is unreachable.