[Bug c++/103947] New: wishlist: warning if explicitly defaulted (spaceship) operator is deleted

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103947

            Bug ID: 103947
           Summary: wishlist: warning if explicitly defaulted (spaceship)
                    operator is deleted
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: f.heckenbach@fh-soft.de
  Target Milestone: ---

% cat test.cpp
#include <compare>

struct A { };

struct B: A
{
  auto operator <=> (const B &) const = default;
};

int main ()
{
  B () == B ();
}
% g++ -std=c++20 -Wall -Wextra test.cpp
test.cpp:7:8: note: 'constexpr bool B::operator==(const B&) const' is
implicitly deleted because the default definition would be ill-formed:
[...]

This error is correct, of course.

When commenting out the statement in main, though, the program compiles without
a warning.

I think a warning would be useful here, since the programmer explicitly
declared the defaulted operator, so they wouldn't expect it to be deleted.

In my actual use case, I did get the error where the code tried to use the
operator, but that was (a) far removed from that declaration and (b) buried in
lots of "candidate" notes (as is common, especially with commonly used
operators), so I actually had to grep through the compiler output to find the
place where it explained why this (intended) candidate was deleted. A warning
right at the declaration would have made it much easier to find.