c++ compile-time bitmasks

bitmasks.cpp

// https://stackoverflow.com/questions/54786278/how-do-i-write-a-maintainable-fast-compile-time-bit-mask-in-c
#include <bitset>

enum Flags { A = 1, B = 2, C = 3, D = 5,
             E = 8, F = 13, G = 21, H,
             I, J, K, L, M, N, O };

#define CPP_VERSION 14

#if CPP_VERSION==11
constexpr unsigned long long mask(){
  return 0;
}
template<class...Tail>
constexpr unsigned long long mask(unsigned char b0, Tail...tail){
  return (1ull<<b0) | mask(tail...);
}
template< unsigned char... indexes >
constexpr unsigned long long mask(){
  return mask(indexes...);
}
#endif
#if CPP_VERSION==14
template< unsigned char... indexes >
constexpr unsigned long long mask(){
  auto r = 0ull;
  using discard_t = int[]; // data never used
  // value never used:
  discard_t discard = {0,(void(
    r |= (1ull << indexes) // side effect, used
  ),0)...};
  (void)discard; // block unused var warnings
  return r;
}
#endif
#if CPP_VERSION==17
template< unsigned char... indexes >
constexpr unsigned long long mask(){
  return ((1ull<<indexes)|...|0ull);
}
#endif
void apply_known_mask(std::bitset<64> &bits) {
  constexpr auto m = mask<B,D,E,H,K,M,L,O>();
  bits &= m;
}