Why is the overload for both the == and <=> operator required to overload all comparison operators? [duplicate]

Question

I'm running into a strange behavior with the new spaceship operator <=> in C++20. I'm using Visual Studio 2019 compiler with /std:c++latest.

This code compiles fine, as expected:

#include <compare>

struct X
{
    int Dummy = 0;
    auto operator<=>(const X&) const = default; // Default implementation
};

int main()
{
    X a, b;

    a == b; // OK!

    return 0;
}

However, if I change X to this:

struct X
{
    int Dummy = 0;
    auto operator<=>(const X& other) const
    {
        return Dummy <=> other.Dummy;
    }
};

I get the following compiler error:

error C2676: binary '==': 'X' does not define this operator or a conversion to a type acceptable to the predefined operator

I tried this on clang as well, and I get similar behavior.

I would appreciate some explanation on why the default implementation generates operator== correctly, but the custom one doesn't.

Answer 1

This is by design.

[class.compare.default] (emphasis mine)

4 If the member-specification does not explicitly declare any member or friend named operator==, an == operator function is declared implicitly for each three-way comparison operator function defined as defaulted in the member-specification, with the same access and function-definition and in the same class scope as the respective three-way comparison operator function, except that the return type is replaced with bool and the declarator-id is replaced with operator==.

Only a defaulted <=> allows a synthesized == to exist. The rationale is that classes like std::vector should not use a non-defaulted <=> for equality tests. Using <=> for == is not the most efficient way to compare vectors. <=> must give the exact ordering, whereas == may bail early by comparing sizes first.

If a class does something special in its three-way comparison, it will likely need to do something special in its ==. Thus, instead of generating a potentially non-sensible default, the language leaves it up to the programmer.