#define SECOND(a, b, ...) b

#define IS_PROBE(...) SECOND(__VA_ARGS__, 0)
#define PROBE() ~, 1

#define CAT(a,b) a ## b

#define NOT(x) IS_PROBE(CAT(_NOT_, x))
#define _NOT_0 PROBE()

#define BOOL(x) NOT(NOT(x))

#define IF_ELSE(condition) _IF_ELSE(BOOL(condition))
#define _IF_ELSE(condition) CAT(_IF_, condition)

#define _IF_1(...) __VA_ARGS__ _IF_1_ELSE
#define _IF_0(...)             _IF_0_ELSE

#define _IF_1_ELSE(...)
#define _IF_0_ELSE(...) __VA_ARGS__


Should be "it was zero": IF_ELSE(0)(it was non-zero)(it was zero)
Should be "it was non-zero": IF_ELSE(1)(it was non-zero)(it was zero)
Should be "it was non-zero": IF_ELSE(123)(it was non-zero)(it was zero)