Virtual constructor & covariant return types

Here is a code from
http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.8

--------------------------------------
class Shape {
 public:
   virtual ~Shape() { } // A virtual destructor
   virtual void draw() = 0; // A pure virtual function
   virtual void move() = 0;
   ...
   virtual Shape* clone() const = 0; // Uses the copy constructor
   virtual Shape* create() const = 0; // Uses the default constructor
 };

 class Circle : public Shape {
 public:
   Circle* clone() const; // Covariant Return Types; see below
   Circle* create() const; // Covariant Return Types; see below
   ...
 };

 Circle* Circle::clone() const { return new Circle(*this); }
 Circle* Circle::create() const { return new Circle(); }

 void userCode(Shape& s)
 {
   Shape* s2 = s.clone();
   Shape* s3 = s.create();
   ...
   delete s2; // You need a virtual destructor here
   delete s3;
 }

--------------------------------------

It seems that behavior of the program will be the same one if we don't
use covariant return types, for instance

class Circle : public Shape {
 public:
   Shape* clone() const; // Non-Covariant Return Types
   Shape* create() const; // Non-Covariant Return Types
   ...
 };

Why do we need covariant return types in the "virtual constructor"
design pattern?

--
 Alex Vinokur
     email: alex DOT vinokur AT gmail DOT com
     http://mathforum.org/library/view/10978.html
     http://sourceforge.net/users/alexvn