Virtual inheritance and method overloading

[Michiel Nauta]

L.S.

I am trying to learn how to program in C++. At the moment I work my way
trough B. Preiss his book on data structures. I run here into an
excersize which won't compile with g++. A dressed down version (in
reality both Base1 and Base2 inherit from the same class, so I have a
diamond in the inheritance diagram for example) is included below.

#include <iostream>
using namespace std;

class Base1 {
public:
  virtual ~Base1() {}
  virtual Base1& SomeFunc() =0;
};

class Base2 {
public:
  virtual ~Base2() {}
  virtual void DoSomething() {cout << "Hallo world\n";}
};

class Derived: virtual public Base1, virtual public Base2 {
public:
  Derived& SomeFunc() {return *this;}
};

int main() {
  Derived deriv;
  return 0;
}

The error message that I get is (I just use g++ -c test.cpp with version
3.3.4 of gcc on slackware 10): "test.cpp:16: sorry, unimplemented:
adjusting pointers for covariant returns". Line 16 is the line declaring
the Derived class. The error goes away if I either use ordinary
inheritance instead of virtual in line 16, or if the return type of
SomeFunc in class Derived is Base1& instead of Derived&.
I don't understand what is wrong, can someone please explain.

My reason to post on this list is that the Borland compiler doesn't
complain.

Regards,
Michiel Nauta