[Bug c++/99628] New: g++ fails to do the implicit conversion when rewritten operator<=>

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99628

            Bug ID: 99628
           Summary: g++ fails to do the implicit conversion when rewritten
                    operator<=>
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

gcc-trunk rejects this valid code:

struct S{
  operator int() { return 0; }
};

auto operator<=>(S, S) { return S{}; }

int main() {
  return S{} < S{};
}

(godbolt://godbolt.org/z/9ah1xE)


The expression S{} < S{} would be rewritten as (S{} <=> S{}) < 0, since the
(S{} <=> S{}) return S{}, we compare S{} with 0, and S{} explicit convert to 0,
so we compare 0 < 0 which is false, but gcc-trunk rejects with:

<source>:8:14: error: no match for 'operator<' (operand types are 'S' and
'int')
    8 |   return S{} < S{};