foo.cc: In function ‘int mon_day(int, int)’:
foo.cc:7:44: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
    7 |                 case 2:return 28+(year%4==0&&year%100!=0||year%400==0);
      |                                   ~~~~~~~~~^~~~~~~~~~~~~
foo.cc:19:1: warning: control reaches end of non-void function [-Wreturn-type]
   19 | }
      | ^