gistfile1.txt

        // Inhibit converting a future<future<T>> to a future<void> in one step.
        template <typename T>
        future(future<future<T> >&& other,
            typename std::enable_if<std::is_void<R>::value, T>::type* = nullptr
        ) = delete;

        // Inhibit converting a future<shared_future<T>> to a future<void> in
        // one step.
        template <typename T>
        future(future<shared_future<T> >&& other,
            typename std::enable_if<std::is_void<R>::value, T>::type* = nullptr
        ) = delete;