#include <iostream>
#include <initializer_list>
using namespace std;

struct AddInitializerList {
	void operator+= (initializer_list<int> values) {
		// Do nothing	
	}
	
	void operator+ (initializer_list<int> values) {
		// Do nothing
	}
};

int main() {
	AddInitializerList adder;
	adder += {1, 2, 3};  // Totally legit
	adder +  {1, 2, 3};  // Not okay!
	
	return 0;
}