Proposed resolution:

This wording is relative to N4713.

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

   // [alg.three.way], three-way comparison algorithms
   template<class T, class U>
   constexpr autosee below compare_3way(const T& a, const U& b);
   

2. Modify [alg.three.way] as indicated:

   template<class T, class U> constexpr autosee below compare_3way(const T& a, const U& b);
   

   -1- Effects: Compares two values and produces a result of the strongest applicable comparison category type:

   1. (1.1) — Returns a <=> b if that expression is well-formedIf the expression a <=> b is well-formed, returns a value of type decay_t<decltype(a <=> b)> initialized from a <=> b.

   2. (1.2) — Otherwise, if the expressions a == b and a < b are each well-formed and convertible to bool, returns a value of type strong_ordering equal to strong_ordering::equal when a == b is true, otherwise returns strong_ordering::less when a < b is true, and otherwise returns strong_ordering::greater.

   3. (1.3) — Otherwise, if the expression a == b is well-formed and convertible to bool, returns a value of type strong_equality equal to strong_equality::equal when a == b is true, and otherwise returns strong_equality::nonequal.

   4. (1.4) — Otherwise, the return type is void and the function is defined as deleted.