Pointer-to-Member-Function Conversions and Virtual Base Classes

[JaR]

Hello!
I would like to have some input on this obscure feature of the
language. First some code to clarify the issue:

class Base {
  public:
    void member() { cout << "test" << endl; }
};

class Derived : public virtual Base {};

int main() {
  Derived obj;
  void (Derived::* pMem)() = &Derived::member;  // (*)
  (obj.*pMem)();
}

The line marked with (*) triggers an error in g++ and as far as I
know, the standard describes it as ill formed. Microsoft Visual
C++ 2010 shows an "IntelliSense" error in the editor but the
compiler itself does not even issue a warning.

My question is: If this code is ill formed, why does it seem, that
Microsoft created an extra member pointer structure (which is 12
bytes in size on a 32-bit system) to support such a program?

Greetings,
Jar