foo.cc: In function ‘bool leap(int)’:
foo.cc:6:22: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
    6 |         return y%4==0&&y%100!=0||y%400==0;
      |                ~~~~~~^~~~~~~~~~
foo.cc: In function ‘int monthday(int, int)’:
foo.cc:16:1: warning: control reaches end of non-void function [-Wreturn-type]
   16 | }
      | ^