In file included from /in/driver/main.cpp:5:0:
/in/hashtable.hpp: In instantiation of 'bool HashTable<Key, Value, Hash, KeyEqual>::contains(const Key&) [with Key = int; Value = int; Hash = std::hash<int>; KeyEqual = std::equal_to<int>]':
/in/driver/main.cpp:92:38:   required from here
/in/hashtable.hpp:247:10: error: unused variable 'res' [-Werror=unused-variable]
     bool res = find(key) != end();
          ^~~
cc1plus: all warnings being treated as errors