In file included from /in/driver/main.cpp:5:0:
/in/hashtable.hpp: In instantiation of 'Value& HashTable<Key, Value, Hash, KeyEqual>::operator[](const Key&) [with Key = int; Value = int; Hash = std::hash<int>; KeyEqual = std::equal_to<int>]':
/in/driver/main.cpp:129:94:   required from here
/in/hashtable.hpp:401:26: error: cannot bind non-const lvalue reference of type 'int&' to an rvalue of type 'int'
             return Value();
                          ^