foo(BAR, 42); foo(QUX, "hallo")

exemple.c

#include <stdio.h>

union zig {
  int num;
  char *str;
};

enum pop { BAR, QUX };

void foo_ (enum pop p, union zig z){
  putchar( p == BAR ? z.num : z.str[0] );
}

#define foo(p, z) foo_(p, (union zig) z)

int main(){
  foo(BAR, 90);
  foo(QUX, "\n");
}