[clang] operator<=> generated too early

[kelbon]

#include <vector>

struct X;

struct Y {
    std::vector<X> x;
    std::strong_ordering operator<=>(const Y& r) const = default;
};

struct X {
    int i;

    std::strong_ordering operator<=>(const X& r) const = default;
};


int main(){
    Y y;
    // error: object of type 'Y' cannot be compared because its 'operator<=>' is implicitly deleted
    (void)(y <=> y);
}

https://godbolt.org/z/PP1as95Ks

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (kelbon)

#include <vector>

struct X;

struct Y {
    std::vector<X> x;
    std::strong_ordering operator<=>(const Y& r) const = default;
};

struct X {
    int i;

    std::strong_ordering operator<=>(const X& r) const = default;
};


int main(){
    Y y;
    // error: object of type 'Y' cannot be compared because its 'operator<=>' is implicitly deleted
    (void)(y <=> y);
}

https://godbolt.org/z/PP1as95Ks

[kelbon]

clang 21 have this issue too

[shafik]

Looks like only gcc accepts: https://godbolt.org/z/nxzscG7KM

Can you provide a rationale for why you think we should accept this?

[kelbon]

Looks like only gcc accepts: https://godbolt.org/z/nxzscG7KM

Can you provide a rationale for why you think we should accept this?

Because of this behavior, in many places where it's possible to write comparison operators manually, it becomes impossible to use = default. I have a whole set of such structures, and this is a typical situation

I see no reason why such code shouldn't be accepted

[frederick-vs-ja]

You can change your code like this:

#include <vector>

struct X;

struct Y {
    std::vector<X> x;
    constexpr std::strong_ordering operator<=>(const Y&) const;
};

struct X {
    int i;

    std::strong_ordering operator<=>(const X& r) const = default;
};

constexpr std::strong_ordering Y::operator<=>(const Y&) const = default;

int main(){
    Y y;
    (void)(y <=> y);
}

Godbolt link

I see no reason why such code shouldn't be accepted

I think there's one reason. The original code was eventually detecting constraints about three-way-comparability of X where X was incomplete, which shouldn't be valid.

[kelbon]

You can change your code like this:

#include

struct X;

struct Y {
std::vector x;
constexpr std::strong_ordering operator<=>(const Y&) const;
};

struct X {
int i;

std::strong_ordering operator<=>(const X& r) const = default;

};

constexpr std::strong_ordering Y::operator<=>(const Y&) const = default;

int main(){
Y y;
(void)(y <=> y);
}
Godbolt link

year ago .cpp file with comparsions was producing internal error on gcc or clang i dont remember

[kelbon]

https://github.com/bot-motherlib/TGBM/actions/runs/18518557306/job/52773686748?pr=124

like this one on gcc14