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

["xgao at nvidia dot com" <gcc-bugzilla@gcc.gnu.org>]

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.