Proposed resolution:

This wording is relative to N4842.

1. Modify [cmp.concept] as indicated:

   template<class T, class U, class Cat = partial_ordering>
     concept three_way_comparable_with =
       weakly-equality-comparable-with<T, U> &&
       partially-ordered-with<T, U> &&
       three_way_comparable<T, Cat> &&
       three_way_comparable<U, Cat> &&
       common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
       three_way_comparable<
         common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>, Cat> &&
       weakly-equality-comparable-with<T, U> &&
       partially-ordered-with<T, U> &&
       requires(const remove_reference_t<T>& t, const remove_reference_t<U>& u) {
         { t <=> u } -> compares-as<Cat>;
         { u <=> t } -> compares-as<Cat>;
       };