HRESULT XUserSignOutAsync(
         XUserHandle user,
         XAsyncBlock* async
) noexcept

HRESULT SignOutUser(XUserHandle user, XTaskQueueHandle queue)
{
    if (!XUserIsSignOutPresent())
    {
        // Sign-out is unavailable through this API.
        return S_FALSE;
    }

    auto async = std::make_unique<XAsyncBlock>();
    async->queue = queue;
    async->callback = [](XAsyncBlock* ab)
    {
        std::unique_ptr<XAsyncBlock> owned{ ab };
        HRESULT hr = XUserSignOutResult(ab);
        if (FAILED(hr))
        {
            // Report hr. Use XUserGetState and user change events,
            // not this result, to decide how to treat the user.
        }
    };

    HRESULT hr = XUserSignOutAsync(user, async.get());
    if (SUCCEEDED(hr))
    {
        // The completion callback now owns the XAsyncBlock.
        async.release();
    }
    return hr;
}