c++ std::cmp_less misbehaving for __int128 depending on standard compiler switch

Question

The following short program

#include <cstdint>
#include <utility>
#include <iostream>
int main() {
__int128 a = -1; 
__int128 b = 1; 
if (std::cmp_less(a, b)) { 
    std::cout << "less" << std::endl;
}
}

compiles if I use -std=gnu++2b (or even -std=gnu++20) but fails with -std=c++20 or -std=c++23:

In file included from <source>:2:
/opt/compiler-explorer/gcc-13.2.0/include/c++/13.2.0/utility: In instantiation of 'constexpr bool std::cmp_less(_Tp, _Up) [with _Tp = __int128; _Up = __int128]':
<source>:11:18:   required from here
/opt/compiler-explorer/gcc-13.2.0/include/c++/13.2.0/utility:141:49: error: static assertion failed
  141 |       static_assert(__is_standard_integer<_Tp>::value);
      |                                                 ^~~~~
/opt/compiler-explorer/gcc-13.2.0/include/c++/13.2.0/utility:141:49: note: 'std::integral_constant<bool, false>::value' evaluates to false
/opt/compiler-explorer/gcc-13.2.0/include/c++/13.2.0/utility:142:49: error: static assertion failed
  142 |       static_assert(__is_standard_integer<_Up>::value);
      |                                                 ^~~~~
/opt/compiler-explorer/gcc-13.2.0/include/c++/13.2.0/utility:142:49: note: 'std::integral_constant<bool, false>::value' evaluates to false
Compiler returned: 1

Can someone shed some light why this happens?

https://gcc.godbolt.org/z/WYMjjsenP

Answer 1

__int128 is not a standard type, so when you're in strict standard mode (-std=c++20) it won't work. The only suprise is that it doesn't give an error on the declarations of a and b.

You need to enable GNU extensions (with -std=gnu++20 or similar) to make it work, as you have already discovered.