16 Library introduction [library]

16.4 Method of description [description]

16.4.2 Other conventions [conventions] Exposition-only functions [expos.only.func]

Several function templates defined in [language.support] through [thread] and [depr] are only defined for the purpose of exposition.
The declaration of such a function is followed by a comment ending in exposition only.
The following function is defined for exposition only to aid in the specification of the library:
template<class T> constexpr decay_t<T> decay-copy(T&& v)
    noexcept(is_nothrow_convertible_v<T, decay_t<T>>)           // exposition only
  { return std::forward<T>(v); }