Method conjunction

include/sol/optional_implementation.hpp:1014–1017

		/// \returns `u` if `*this` has a value, otherwise an empty optional.
		template <class U>
		constexpr optional<typename std::decay<U>::type> conjunction(U&& u) const {
			using result = optional<detail::decay_t<U>>;
			return has_value() ? result { u } : result { nullopt };
		}

		/// \returns `rhs` if `*this` is empty, otherwise the current value.
		/// \group disjunction