[klodolph]

C doesn't have that version of constexpr. In C2x, constexpr is just a way to define constants, like

  constexpr unsigned long long kMyBigNum = 0x1234123412341234ull;

Previously, you had to #define. Using enum causes problems when it doesn't fit in an int. And const doesn't mean the right thing:

  const int kArraySize = 5;

  void MyFunction(void) {
    int array[kArraySize]; // NO!
  }

The above function will work if you have VLAs enabled, or if your compiler specifically allows for it. It's nice to have a standardized version that works everywhere (VLAs don't work everywhere).