Covariant return types and interdependent classes

I'm having a problem trying to get two classes that refer to each
other take a covariant return type. While this is occurring in the
Visual C++ 2010 compiler (and I'm posting a modified version here of
my original post in the C++ forums at MS), I have a more general
question, namely if there is a way for a compiler to pass what would
be, in my opinion, valid code for covariant return types, or if I'm at
the mercy of the compiler vendor :-)

In base1.h:

struct base2; // fwd decl

struct base1 {
  virtual base2* getBase2() const = 0;
};

- - -

In base2.h:

struct base1; // fwd decl

struct base2 {
  virtual base1* getBase1() const = 0;

};

- - -

In derived1.h:

#include "base1.h"

// necessary for compiler to see inheritance relationship
// between base2 and derived2 and allow covariant return type.

#include "derived2.h"

struct derived1: public base1 {
 derived2* getBase2() const { ... }
};

- - -

In derived2.h:

#include "base2.h"

// necessary for compiler to see inheritance relationship
// between base1 and derived1 and allow covariant return type.
#include "derived1.h"

struct derived2: public base2 {
 derived1* getBase1() const { ... }

};

In my opinion, this should be valid C++ code and in the case of the
compiler mentioned above, it fails to compile because of a
"technicality": to see the inheritance relationship one must include
derived2.h in derived1.h so it can see derived2 is-a base2, and
include the derived1.h in derived2.h so it can see derived1 is-a
base1, since a simple forward declaration for derived 2 and derived1
does not work. AS LONG AS I keep it simple and don't have
interdependent classes, covariant return types work as advertised.

If this is valid C++ code, my question is if there is a compiler that
works with this code (perhaps EDG's), and if not, is it a problem in C+
+ itself in specifying something that may not be practically met
(albeit a counter argument would be that a multi-pass compiler should
be smart enough to detect the inheritance relationship :-)

Cheers,

Javier

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