I use something like this to turn __VA_ARGS__ into count/pointer:

  // Create a stack-allocated argv from N args gcc, clang, tcc, mingw, msvc, etc
  // argc is always correct, for portability argv has a dummy element when argc==0
  // This avoids problematic 0-length compound literal arrays.
  #define _ARG_CV(type, ...)  _ARG_CV1(((type []){0, __VA_ARGS__}))
  #define _ARG_CV1(array)     _ALEN(array) - 1, array + (_ALEN(array) > 1)
  #define _ALEN(array)         sizeof array / sizeof *array

Then it's just:

  #define sum(...) sum(_ARG_CV(int, __VA_ARGS__))