Defining Templated Derived Class Overloaded Member Functions in a Separate File

Question

I am trying to define the overloaded member functions of a templated derived class in a separate .cc file. The code structure is as follows:

//header.hh

class Base
{
public:
    virtual void visit () = 0;
};

template <class T>
class Derived : public Base
{
public:
    void visit () override;
};

//impl.cc
template <class T>
Derived<T>::visit ()
{
   return;
}

I use an instance of the derived class as a member variable in other class

//usecaseh.hh

class Usecase
{
     Derived <Usecase> d;
};

//usecase.cc

Usecase::Usecase ()
{
    d = new Derived <Usecase> ();
}

However while compilation , I exit with the following error

undefined reference to non-virtual thunk Derived<Usecase>::visit()

I am not really sure why this is happening. The code snippet shared is just a high level abstraction of the design. In practice, the Base class is a graph traversal iterator, and I plug in my Usecase class to walk a graph and do some stuff on it via the visit method.

1. Why is the compiler not able to reconcile the definition and declaration of visit method?
2. How do I resolve this