#include <cstdio>

struct custom {
	custom(int i): i(i) { }

	friend inline custom operator + (const custom &lhs, const custom &rhs) {
		return lhs.i + rhs.i;
	}

	private:
		int i;
};

struct converted {
	inline operator custom () {
		return custom(2);
	}
};

int main()
{
	converted c;
	printf("%d\n", c + c);

	return 0;
}

In function 'int main()':
Line 23: error: no match for 'operator+' in 'c + c'
compilation terminated due to -Wfatal-errors.