foo.cc: In function ‘int main()’:
foo.cc:15:10: warning: statement has no effect [-Wunused-value]
   15 |     for (k1; k1<=k2; k1++)
      |          ^~
foo.cc:17:16: warning: ‘sum’ may be used uninitialized [-Wmaybe-uninitialized]
   17 |             sum+=k1;
      |             ~~~^~~~
foo.cc:13:16: note: ‘sum’ was declared here
   13 |     int k1, k2,sum;
      |                ^~~