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

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

[xgao at nvidia dot com]

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);
}

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

[pinskia at gcc dot gnu.org]

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fails for the same reason with clang (both with libstdc++ and libc++) ....

Are you sure this is valid C++ 20 code?

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

[xgao at nvidia dot com]

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

--- Comment #2 from Xiang Gao <xgao at nvidia dot com> ---
(In reply to Andrew Pinski from comment #1)
> Fails for the same reason with clang (both with libstdc++ and libc++) ....
> 
> Are you sure this is valid C++ 20 code?

I am not 100% sure, but my understanding is, for the SFINAE in
  inline constexpr bool operator<(const DT& x, const DT& y)
The first condition
  hasLessThan<typename DT::T1, typename DT::T1>::value
is just hasLessThan<int64_t, int64_t>::value, which should be true, and (true
|| anything) is always true.

So the condition in SFINAE should be easily evaluated as true. So the operator<
for DynamicType should be defined.

And if operator< is defined, then the operator<=> of std::vector<DynamicType>
should also be defined. This can be validated by changing the definition of
operator< by only keeping the first condition:

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

and then both g++ and clang++ will pass.

I do observe the same error on clang, but this https://cpp.sh/ seems to compile
without problem on C++20.

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

[xgao at nvidia dot com]

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

--- Comment #3 from Xiang Gao <xgao at nvidia dot com> ---
Cross posted at: https://github.com/llvm/llvm-project/issues/67056