1. Constructs an object that does contain a nullopt value.
     optional(dmlc::nullopt_t) noexcept : is_none(true) {}
2. constructs an optional object that contains a value, initialized as if direct-initializing:
    template <typename U, typename... Args>
3. move constructor: If other contains a value, then stored value is direct-intialized with it.
     optional(optional &&other) noexcept() { }
4. move constructor: constructs the stored value with value with `other`parameter.
    optional(U &&other) noexcept {
5. (...)
Upgrade: dmlc::optional<T> value
1. T &value() &
2. const T &value() const &
3. T &&value() &
4. const T && value() const &&nter the commit message for your changes. Lines starting

…l_dmlc_move_support