test.py

gistfile1.py

from __future__ import annotations
from typing import Any, Awaitable, Callable, TypeVar, cast, Generic
import asyncio

T = TypeVar("T", bound=int, covariant=True)
B = TypeVar("B", bound="Box[Any]")
class Box(Generic[T]): ...

async def foo(
    check: Callable[[B], bool] | None = None,
) -> B:
    return (
        await bar(check)
        if check is not None
        else cast(B, Box())
    )

def bar(check: Callable[[B], bool]) -> Awaitable[B]:
    ...

my_foo: Box[int] = asyncio.run(foo())
reveal_type(my_foo)  # revealed type is B@foo not Box[int]

non-async.py

from __future__ import annotations
from typing import Any, Awaitable, Callable, TypeVar, cast, Generic
import asyncio

T = TypeVar("T", bound=int, covariant=True)
B = TypeVar("B", bound="Box[Any]")
class Box(Generic[T]): ...

def foo(
    check: Callable[[B], bool] | None = None,
) -> B:
    return (
        bar(check)
        if check is not None
        else cast(B, Box())
    )

def bar(check: Callable[[B], bool]) -> B:
    ...

my_foo: Box[int] = foo()
reveal_type(my_foo)