synth-three-way-result(C++20)

constexpr auto synth-three-way =                           // (1) C++20
  []<class T, class U>(const T& t, const U& u)
    requires requires {
      { t < u } -> boolean-testable ;
      { u < t } -> boolean-testable ;
    }
  {
    if constexpr (three_way_comparable_with<T, U>) {
      return t <=> u;
    } else {
      if (t < u) return weak_ordering::less;
      if (u < t) return weak_ordering::greater;
      return weak_ordering::equivalent;
   }
};

template <class T, class U=T>
using synth-three-way-result =
  decltype(synth-three-way(declval<T&>(), declval<U&>())); // (2) C++20