[Bug c++/111504] New: compare operator not defined for recursive data types on C++20

[xgao at nvidia dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111504

            Bug ID: 111504
           Summary: compare operator not defined for recursive data types
                    on C++20
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xgao at nvidia dot com
  Target Milestone: ---

Related bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111316

The following code works on C++17 but not C++20:

#include <cstdint>
#include <type_traits>
#include <vector>

template <typename T1, typename T2>
static auto hasLessThanHelper(int)
    -> decltype(std::declval<T1>() < std::declval<T2>(), std::true_type{});

template <typename, typename>
static auto hasLessThanHelper(long) -> std::false_type;

template <typename T1, typename T2>
struct hasLessThan : decltype(hasLessThanHelper<T1, T2>(0)) {};

struct DynamicType {
  using T1 = int64_t;
  using T2 = std::vector<DynamicType>;
};

template <
    typename DT,
    typename = std::enable_if_t<
        (hasLessThan<typename DT::T1, typename DT::T1>::value ||
         hasLessThan<typename DT::T1, typename DT::T2>::value ||
         hasLessThan<typename DT::T2, typename DT::T1>::value ||
         hasLessThan<typename DT::T2, typename DT::T2>::value)>>
inline constexpr bool operator<(const DT& x, const DT& y) {
  // implementation omitted
  return true;
}

int main() {
  using DT = DynamicType;
  // This assert works on C++17, but fails on C++20
  static_assert(hasLessThan<std::vector<DT>, std::vector<DT>>::value);
}