/in/foo.cc: In function 'bool runnian(int)':
/in/foo.cc:6:20: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
     if (y % 4 == 0 && y % 100 != 0 || y % 400 == 0)
         ~~~~~~~~~~~^~~~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:33:14: warning: variable 'n2' set but not used [-Wunused-but-set-variable]
  int n1 = 0, n2 = 0;
              ^~
/in/foo.cc: In function 'int monthday(int, int)':
/in/foo.cc:25:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^