/in/foo.cc: In function 'std::string f(int)':
/in/foo.cc:6:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    6 |         for(int i=0;i<s.size();i++){
      |                     ~^~~~~~~~~
/in/foo.cc:18:11: error: return-statement with no value, in function returning 'std::string' {aka 'std::__cxx11::basic_string<char>'} [-fpermissive]
   18 |           return;
      |           ^~~~~~