Proposed resolution:
This wording is relative to N4800.
Modify [refwrap] as indicated:
-1- reference_wrapper<T> is a Cpp17CopyConstructible and Cpp17CopyAssignable wrapper around a reference to an object or function of type T. If reference_wrapper is instantiated with a non-referenceable type ([defns.referenceable]) as the argument for the template parameter T, the program is ill-formed.
-2- reference_wrapper<T> is a trivially copyable type ([basic.types]).
-3- The template parameter T of reference_wrapper may be an incomplete type.