combo.cpp: In function 'char extend(std::string&)':
combo.cpp:23:28: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
23 | if(press(ans+c)==ans.size()+1) return c;
| ~~~~~~~~~~~~^~~~~~~~~~~~~~
combo.cpp:29:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
29 | if(res==ans.size()) return options[2];
| ~~~^~~~~~~~~~~~
combo.cpp:32:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
32 | if(res==ans.size()) return options[1];
| ~~~^~~~~~~~~~~~
combo.cpp:15:48: warning: control reaches end of non-void function [-Wreturn-type]
15 | vector <char> options = {'A', 'B', 'X', 'Y'};
| ^