std::compare_weak_order_fallback

inline namespace /* unspecified */ {
    inline constexpr /* unspecified */
        compare_weak_order_fallback = /* unspecified */;
}

template< class T, class U >
    requires /* see below */
constexpr std::weak_ordering
    compare_weak_order_fallback( T&& t, U&& u ) noexcept(/* see below */);

#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
}

Вывод:

1) less
2) greater
3) greater
4) equal
5) equal