std::compare_weak_order_fallback(3)

NAME
std::compare_weak_order_fallback - std::compare_weak_order_fallback

Synopsis
   Defined in header <compare>
   inline namespace /* unspecified */ {
       inline constexpr /* unspecified */ (since C++20)
           compare_weak_order_fallback = /* unspecified */;
   }
   Call signature
   template< class T, class U >
       requires /* see below */
   constexpr std::weak_ordering (since C++20)
       compare_weak_order_fallback( T&& t, U&& u ) noexcept(/* see below
   */);
   Performs three-way comparison on subexpressions t and u and produces a result of
   type std::weak_ordering, even if the operator <=> is unavailable.
   If std::decay_t<T> and std::decay_t<U> are the same type,
   std::compare_weak_order_fallback(t, u) is expression-equivalent to:
     * std::weak_order(t, u), if it is a well-formed expression; otherwise,
     * t == u ? std::weak_ordering::equivalent :
       t < u ? std::weak_ordering::less :
       std::weak_ordering::greater, if the expressions t == u and t < u are
       both well-formed and each of decltype(t == u) and decltype(t < u) models
       boolean-testable, except that t and u are evaluated only once.
   In all other cases, std::compare_weak_order_fallback(t, u) is ill-formed, which can
   result in substitution failure when it appears in the immediate context of a
   template instantiation.

Example
// Run this code
 #include <compare>
 #include <iostream>
 // does not support <=>
 struct Rational_1
 {
     int num;
     int den; // > 0
 };
 inline constexpr bool operator<(Rational_1 lhs, Rational_1 rhs)
 {
     return lhs.num * rhs.den < rhs.num * lhs.den;
 }
 inline constexpr bool operator==(Rational_1 lhs, Rational_1 rhs)
 {
     return lhs.num * rhs.den == rhs.num * lhs.den;
 }
 // supports <=>
 struct Rational_2
 {
     int num;
     int den; // > 0
 };
 inline constexpr std::weak_ordering operator<=>(Rational_2 lhs, Rational_2 rhs)
 {
     return lhs.num * rhs.den <=> rhs.num * lhs.den;
 }
 inline constexpr bool operator==(Rational_2 lhs, Rational_2 rhs)
 {
     return lhs <=> rhs == 0;
 }
 void print(int id, std::weak_ordering value)
 {
     std::cout << id << ") ";
     if (value == 0)
         std::cout << "equal\n";
     else if (value < 0)
         std::cout << "less\n";
     else
         std::cout << "greater\n";
 }
 int main()
 {
     Rational_1 a{1, 2}, b{3, 4};
 // print(0, a <=> b); // does not work
     print(1, std::compare_weak_order_fallback(a, b)); // works, defaults to < and ==
     Rational_2 c{6, 5}, d{8, 7};
     print(2, c <=> d); // works
     print(3, std::compare_weak_order_fallback(c, d)); // works
     Rational_2 e{2, 3}, f{4, 6};
     print(4, e <=> f); // works
     print(5, std::compare_weak_order_fallback(e, f)); // works
 }

Output:
 1) less
 2) greater
 3) greater
 4) equal
 5) equal
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2114 C++20 the fallback mechanism only required constraints strengthened
   (P2167R3) return types to be convertible to bool

See also
   weak_order performs 3-way comparison and produces a result of type
   (C++20) std::weak_ordering
              (customization point object)