foo.cc: In function ‘int main()’:
foo.cc:14:30: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<int>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   14 |                 for(int j=0;j<primes.size()&&i*primes[j]<=maxn;j++)
      |                             ~^~~~~~~~~~~~~~
foo.cc:23:39: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<int>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   23 |                 for(int j=0;a[i]!=1&&j<primes.size();j++)
      |                                      ~^~~~~~~~~~~~~~