[Bug c++/107325] New: Defaulted operator<=> with a non-comparison-category return type is not deleted

["de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107325

            Bug ID: 107325
           Summary: Defaulted operator<=> with a non-comparison-category
                    return type is not deleted
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

In this example, the operator<=> overload for Bar<int> is deleted according to
[class.spaceship] (1.3) and (2.2), and thus decltype(Bar<int>{} <=> Bar<int>{})
is ill-formed.

But gcc currently accepts it (Godbolt link: https://godbolt.org/z/z1bPbGPx7).

#include <compare>

template<class Ord>
struct Bar {
    friend Ord operator<=>(Bar, Bar) = default;
};

using SO = decltype(Bar<std::strong_ordering>{} <=>
Bar<std::strong_ordering>{}); // OK
using WO = decltype(Bar<std::weak_ordering>{} <=> Bar<std::weak_ordering>{});
// OK
using PO = decltype(Bar<std::partial_ordering>{} <=>
Bar<std::partial_ordering>{}); // OK

using I = decltype(Bar<int>{} <=> Bar<int>{}); // This should be rejected.