template class inheritance question

example.h

// class derived_class;

class main_class {
public:
    derived_class foo();            // error: ‘derived_class’ does not name a type
};

derived_class main_class::foo() {}  // error: ‘derived_class’ does not name a type

template <typename T>
class template_class {};

class derived_class : public template_class<int> {};