async def METHOD_NAME(subtests):
    """Ensures that `flatten` is always returning the correct type."""
    futures = [
        # Flattens:
        (Future.from_value(Future.from_value(1)), Future.from_value(1)),
        (
            FutureResult.from_value(FutureResult.from_value(1)),
            FutureResult.from_value(1),
        ),
    ]
    for container, merged in futures:
        with subtests.test(container=container, merged=merged):
            assert await flatten(container) == await merged  # type: ignore