Function optional

include/tabulate/optional_lite.hpp:874–878

                                           !std::is_convertible<U &&, T>::value /*=> explicit */
                                           )>
  explicit optional(optional<U> &&other) : has_value_(other.has_value()) {
    if (other.has_value()) {
      contained.construct_value(T{std::move(other.contained.value())});
    }
  }

  // 5a (C++11) - non-explicit converting move-construct from optional
  template <typename U optional_REQUIRES_T(std::is_constructible<T, U &&>::value &&