foo.cc: In function 'int gcd(int, int)':
foo.cc:5:12: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
     while(t=a%b) {   
           ~^~~~
foo.cc: In function 'int main()':
foo.cc:19:23: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
             if(x == x0&&y == y0||x == y0&&y == x0) {
                ~~~~~~~^~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。