[Bug libstdc++/78723] New: [variant] P0393r3: "Making variant greater equal again" is unimplemented

["eric at efcs dot ca" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78723

            Bug ID: 78723
           Summary: [variant] P0393r3: "Making variant greater equal
                    again" is unimplemented
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric at efcs dot ca
  Target Milestone: ---

Link to paper:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0393r3.html

Reproducer:
// g++ -std=c++1z test.cpp
#include <variant>
#include <type_traits>
#include <cassert>

struct MakeEmptyT {
  MakeEmptyT() = default;
  MakeEmptyT(MakeEmptyT &&) { throw 42; }
  MakeEmptyT &operator=(MakeEmptyT &&) { throw 42; }
};
inline bool operator<(const MakeEmptyT &, const MakeEmptyT &) {
  assert(false);
  return false;
}

template <class Variant> void makeEmpty(Variant &v) {
  Variant v2(std::in_place_type<MakeEmptyT>);
  try {
    v = std::move(v2);
    assert(false);
  } catch (...) {
    assert(v.valueless_by_exception());
  }
}

int main() {
    using V = std::variant<int, MakeEmptyT>;
    V v1;
    V v2;
    makeEmpty(v2);
    assert((v1 < v2) == false);
}