foo.cc: In function ‘int main()’:
foo.cc:25:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<int>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   25 |     for (int i = 0; i < primes.size(); ++i) {
      |                     ~~^~~~~~~~~~~~~~~
foo.cc:27:15: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<int>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   27 |         if (i < primes.size() - 1) cout << " ";
      |             ~~^~~~~~~~~~~~~~~~~~~