Awaitable typeguards

Currently it seems impossible to have async TypeGuard or TypeIs:

from typing_extensions import TypeIs

async def foo(a: int | str) -> TypeIs[int]:
    raise NotImplementedError

async def main() -> None:
    a: int | str = 10
    if await foo(a):
        reveal_type(a)
    else:
        reveal_type(a)

Current specification is not clear about how async functions returning TypeIs or TypeGuards should be treated, but I believe this would be useful for async lazy evaluation of typestate fields, example:

from typing_extensions import TypeIs, TypeVar, Generic


T = TypeVar('T', int | None, int, None, covariant=True)


class A(Generic[T]):
    _a: T
    
    async def get_a(self: A[int | None]) -> int | None:
        a = 123  # imagine some expensive async operation, e.g. fetch value from API
        self._a = a
        return self._a
        
    def method_requires_a(self: A[int]) -> None:
        raise NotImplementedError
        
    def __init__(self):
        self._a = None

async def has_a(a: A[int | None]) -> TypeIs[A[int]]:
    return await a.get_a() is not None
 
 
async def main(a: A[int | None]):
    if await has_a(a):
        a.method_requires_a()  # currently fails as type was not narrowed

The issue names no repository files, tests, or implementation entry points. Start by reviewing the typing specification's treatment of async functions returning TypeIs or TypeGuard, then determine how the proposed narrowing should be defined; done requires an agreed specification and corresponding implementation and tests.