foo.cc: In function ‘int main()’:
foo.cc:23:17: error: ‘sum’ was not declared in this scope
   23 |                 sum++;
      |                 ^~~
foo.cc:9:21: warning: unused variable ‘t’ [-Wunused-variable]
    9 |                 int t=i;
      |                     ^
foo.cc:25:15: error: ‘sum’ was not declared in this scope
   25 |         cout<<sum;
      |               ^~~