foo.cc: In function ‘int main()’:
foo.cc:21:20: warning: ‘day’ may be used uninitialized [-Wmaybe-uninitialized]
   21 |         std::cout<<day;
      |                    ^~~
foo.cc:5:17: note: ‘day’ was declared here
    5 |         int y,m,day;
      |                 ^~~