future.then will be blocked if promise is set after the invocation of then

[Xiaoshuang LU <luxiaoshuang@…>]

int function()
{
    boost::promise<int> promise;
    boost::future<int> future = promise.get_future();

    boost::future<int> result =
    future.then
    (
        boost::launch::deferred,
        [](boost::future<int> && f)
        {
            std::cout << std::this_thread::get_id() << ": callback" << std::endl;
            std::cout << "The value is: " << f.get() << std::endl;
            return f.get();
        }
    );

    // We could not reach here.
    std::cout << std::this_thread::get_id() << ": function" << std::endl;

    promise.set_value(0);

    return 0;
}