[optional.specalg]

template<class T>
  constexpr void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y));

Constraints: is_reference_v<T> || (is_move_constructible_v<T> && is_swappable_v<T>) is true.

Effects: Calls x.swap(y).

template<class T> constexpr optional<decay_t<T>> make_optional(T&& v);

Constraints: The call to make_optional does not use an explicit template-argument-list that begins with a type template-argument.

Effects: Equivalent to: return optional<decay_t<T>>(std::forward<T>(v));

template<class T, class.Args>
  constexpr optional<T> make_optional(Args&&. args);

Effects: Equivalent to: return optional<T>(in_place, std::forward<Args>(args).);

template<class T, class U, class. Args>
  constexpr optional<T> make_optional(initializer_list<U> il, Args&&. args);

Effects: Equivalent to: return optional<T>(in_place, il, std::forward<Args>(args).);

22 General utilities library [utilities]