Type narrowing for `LiteralString` does not work

[bryanforbes]

Describe the bug
Given the following code:

from collections.abc import Sequence
from typing import Any, TypeGuard
from typing_extensions import LiteralString, reveal_type


def _is_literal_string(obj: Any) -> TypeGuard[LiteralString]:
    return isinstance(obj, str)


def foo(s: str | Sequence[str]) -> None:
    if isinstance(s, str):
        reveal_type(s)
    else:
        reveal_type(s)


def bar(s: LiteralString | Sequence[LiteralString]) -> None:
    if _is_literal_string(s):
        reveal_type(s)
    else:
        reveal_type(s)

The output on the command line is as follows:

test.py:8:21 - information: Type of "s" is "str"
test.py:10:21 - information: Type of "s" is "Sequence[str]"
test.py:19:21 - information: Type of "s" is "LiteralString"
test.py:21:21 - information: Type of "s" is "LiteralString | Sequence[LiteralString]"

Expected behavior
The type reported on line 21 should be Sequence[LiteralString].

VS Code extension or command-line
Are you running pyright as a VS Code extension or a command-line tool?
Command line

Which version?
1.1.247

[erictraut]

The current behavior is correct. TypeGuard does not perform type narrowing in the negative ("else") case. This was intentional and is specified in PEP 647.

There has been discussion of a variant of TypeGuard that would be more restrictive (and therefore less flexible) but would allow for type narrowing in the negative case. I've proposed that we call this StrictTypeGuard, and it is prototyped in pyright. It has no runtime support currently, so you won't be able to use it in your code unless you use some hackery.

if TYPE_CHECKING:
    from typing_extensions import StrictTypeGuard
else:
    StrictTypeGuard = Any

def _is_literal_string(obj: Any) -> StrictTypeGuard[LiteralString]:
    return isinstance(obj, str)

[erictraut]

For more details about the StrictTypeGuard proposal, see this discussion.

[bryanforbes]

Thanks, @erictraut. That's exactly what I was looking for. Hopefully StrictTypeGuard gains traction.