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From: "fchelnokov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/103689] New: Overriding spaceship operator when parent
 class does not define spaceship operator for itself
Date: Mon, 13 Dec 2021 19:21:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103689

            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 <=3D>(const A&) const { return
std::strong_ordering::equal; }
};

struct A : I {
    virtual std::strong_ordering operator <=3D>(const A&) const =3D default;
};

int main() {
    A a;
    I & i =3D a;
    return (i <=3D> a) =3D=3D 0;
}
```
since I::operator <=3D>(const I&) const is not defined, so A::operator <=3D=
>(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).=
=20

GCC compiles it fine, but issues a linker error=20
```
undefined reference to `A::operator=3D=3D(A const&) const'
```
Demo: https://gcc.godbolt.org/z/Geoqdo4T8

Related discussion: https://stackoverflow.com/q/70334766/7325599=