Proposed resolution:

This wording is relative to N4529.

1. Edit [propagate_const.algorithms] as indicated:

   -1- template <class T>
   constexpr void swap(propagate_const<T>& pt1, propagate_const<T>& pt2) noexcept(see below)
   

   -2- The constant-expression in the exception-specification is noexcept(swap(pt1.t_, pt2.t_)pt1.swap(pt2)).

   -3- Effects: swap(pt1.t_, pt2.t_)pt1.swap(pt2).