Proposed resolution:

This wording is relative to N4885.

1. Modify [optional.syn], header <optional> synopsis, as indicated:

   […]
   
   // [optional.nullopt], no-value state indicator
   struct nullopt_t{see below explicit nullopt_t() = default; };
   inline constexpr nullopt_t nullopt{}(unspecified);
   
   […]
   

2. Modify [optional.nullopt] as indicated:

   struct nullopt_t{see below explicit nullopt_t() = default; };
   inline constexpr nullopt_t nullopt{}(unspecified);
   

   -1- The struct nullopt_t is an empty 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 shall not have a default constructor or an initializer-list constructor, and shall not be an aggregate.