struct nullopt_t{see below};
constexpr nullopt_t nullopt(unspecified);
The struct nullopt_t is an empty structure type used as a unique type to indicate the state of not containing a value for optional objects. In particular, optional<T> has a constructor with nullopt_t as a single argument; this indicates that an optional object not containing a value shall be constructed.
Type nullopt_t shall not have a default constructor or an initializer-list constructor, and shall not be an aggregate.