foo.cc: In function ‘bool pr(int)’:
foo.cc:6:19: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
    6 |     return y%4==0 && y%100!=0 || y%400==0;
      |            ~~~~~~~^~~~~~~~~~~
foo.cc: In function ‘int mon(int, int)’:
foo.cc:16:1: warning: control reaches end of non-void function [-Wreturn-type]
   16 | }
      | ^