GCC nested varargs error

bad.c

#include <stdio.h>
#include <stdarg.h>

int sum(int count, ...) {
	int result = 0;
	va_list args;
	va_start(args, count);
	while (count--) {
		result += va_arg(args, int);
	}
	va_end(args);
	return result;
}

#define SUM(C, ...) sum(C, ##__VA_ARGS__)

int main() {
	printf("Hello %d!", SUM(2, 13, SUM(2, 14, 15)));
	return 0;
}

compile.log

> gcc -c -std=c11 bad.c
bad.c: In function 'main':
bad.c:19:2: warning: implicit declaration of function 'SUM' [-Wimplicit-function-declaration]
  printf("Hello %d!", SUM(2, 13, SUM(2, 14, 15)));
  ^

> g++ -c -xc++ -std=c++11 bad.c
bad.c: In function 'int main()':
bad.c:19:46: error: 'SUM' was not declared in this scope
  printf("Hello %d!", SUM(2, 13, SUM(2, 14, 15)));
                                              ^
bad.c:16:30: note: in definition of macro 'SUM'
 #define SUM(C, ...) sum(C, ##__VA_ARGS__)
                              ^

> gcc -std=c11 good.c && good
Hello 42!

> cl /nologo good && good
Hello 42!

> cl /nologo bad && bad
Hello 42!

good.c

#include <stdio.h>

#define ADD(x, y) ((x) + (y))

int main() {
	printf("Hello %d!", ADD(13, ADD(14, 15)));
	return 0;
}