foo.cc: In function 'int main()':
foo.cc:11:40: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
      case 2:day=(year%4==0||year%400==0&&year%100!=0?29:28);break;
                             ~~~~~~~~~~~^~~~~~~~~~~~~
foo.cc:13:11: warning: 'day' may be used uninitialized in this function [-Wmaybe-uninitialized]
     cout<<day<<endl;        
           ^~~

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