tests/other/test_std_cmp.cpp

#include <utility>
#include <cstdio>

// Uncommenting the next line will disable "signed/unsigned comparison" warnings:
// #pragma GCC diagnostic ignored "-Wsign-compare"

int main() {
  static_assert(sizeof(int) == 4); // precondition

  // Quite surprisingly
  static_assert(-1 > 1U ); //< warning: sign-unsign comparison
  // because after implicit conversion of -1 to the RHS type (`unsigned int`)
  // the expression is equivalent to:
  static_assert(0xFFFFFFFFU > 1U);
  static_assert(0xFFFFFFFFU == static_cast<unsigned>(-1));

  // In contrast, the cmp_* family compares integers as most expected -
  // negative signed integers always compare less than unsigned integers:
  static_assert(std::cmp_less( -1, 1U ));
  static_assert(std::cmp_less_equal( -1, 1U ));
  static_assert(!std::cmp_greater( -1, 1U ));
  static_assert(!std::cmp_greater_equal( -1, 1U));

  static_assert(-1 == 0xFFFFFFFFU); //< warning: sign-unsign comparison
  static_assert(std::cmp_not_equal(-1, 0xFFFFFFFFU ));
  printf("done\n");
  return 0;
}