C++ method that converts object of base class to derived class

Question

How can I write a method toDerived() (instead or additionally to a constructor) that converts a base class object to a derived class object?

With the following setup I get the error "Incomplete result type 'Derived' in function definition of 'toDerived()'".

// base.h
#ifndef BASE_H
#define BASE_H

#include "derived.h"

class Derived;
class Base { 
public:
  // constructor etc.

  Derived toDerived() const { /* ... */ };
};

#endif

// derived.h
#ifndef DERIVED_H
#define DERIVED_H

#include "base.h"

class Derived : public Base { 
public:
  // constructor etc.
};