The future of futures

As with std::shared_mutex, the standard library's own version of std::future is only half-baked. A much more complete and useful version of future is coming, perhaps in C++20, and there are very many third-party libraries that incorporate the best features of the upcoming version. The best of these libraries include boost::future and Facebook's folly::Future.

The major problem with std::future is that it requires "touching down" in a thread after each step of a potentially multi-step computation. Consider this pathological usage of std::future:

    template<class T>
    auto pf() {
      std::promise<T> p;
      std::future<T> f = p.get_future();
      return std::make_pair(std::move(p), std::move(f));
    }

    void test() {
      auto [p1, f1] = pf<Connection>();
      auto [p2, f2] = pf<Data>();
      auto [p3, f3] = pf<Data>();

      auto t1 = std::thread([p1 = std::move(p1)]() mutable {
        Connection conn = slowly_open_connection...