Proposed resolution:

This wording is relative to N5054.

1. Modify [optional.nullopt] as indicated:

   [Drafting note: It will become redundant to specify that `nullopt_t` is empty, non-aggregate, and modeling certain concepts. In current implementations, `nullopt_t` does have some member type, so perhaps we shouldn't specify lack of member types.]

   struct nullopt_t{see below
     // unspecified constructors
   
     friend strong_ordering operator<=>(nullopt_t, nullopt_t) = default;
   };
   inline constexpr nullopt_t nullopt(unspecified);
   

   -1- The struct `nullopt_t` is an empty a class 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.

   -2- Type `nullopt_t` does not have a default constructor or an initializer-list constructor, and is not an aggregate. `nullopt_t` models `copyable` and three_way_comparable<strong_ordering> any base class, non-static data member, default constructor, initializer-list constructor, or explicitly declared non-static member function that is not a constructor. All special member functions of `nullopt_t` are implicitly declared.

LWG 4497 specified that `std::nullopt_t` is copyable and three-way comparable. However, during implementing and testing its resolution in libc++, it was found that almost no intended operation is specified to have desirable properties - being `noexcept` or trivial, or even `constexpr` for some special member functions. Maybe we should specify the details of `std::nullopt_t` more precisely.