https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103689
Bug ID: 103689
Summary: Overriding spaceship operator when parent class does
not define spaceship operator for itself
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: fchelnokov at gmail dot com
Target Milestone: ---
The following program is ill-formed:
```
#include <compare>
struct A;
struct I {
virtual std::strong_ordering operator <=>(const A&) const { return
std::strong_ordering::equal; }
};
struct A : I {
virtual std::strong_ordering operator <=>(const A&) const = default;
};
int main() {
A a;
I & i = a;
return (i <=> a) == 0;
}
```
since I::operator <=>(const I&) const is not defined, so A::operator <=>(const
A&) const must be implicitly deleted, and a deleted method cannot override a
not deleted method from I. This must be a compilation error (as in Clang).
GCC compiles it fine, but issues a linker error
```
undefined reference to `A::operator==(A const&) const'
```
Demo: https://gcc.godbolt.org/z/Geoqdo4T8
Related discussion: https://stackoverflow.com/q/70334766/7325599
