int fn () __attribute__ ((warn_unused_result));

int fn () {return 1;}

int main ()
{
        if (fn () < 0) return -1;
        fn ();
        return 0;
}

cc1plus: warnings being treated as errors
In function 'int main()':
Line 8: warning: ignoring return value of 'int fn()', declared with attribute warn_unused_result