Bug#395837: g++-4.1: dynamic_cast returns different results based on the order of bases

[Matthias Klose <doko@cs.tu-berlin.de>]

tag 395837 + moreinfo
thanks

please recheck with gcc-4.2/gcc-snapshot from current unstable and/or
forward to gcc-help@gcc.gnu.org

Vladimir Grebinskiy writes:
> Package:g++-4.1
> Version:4.1.1-18
> 
> What should dynamic_cast return if we have virtual base class that is
> first derived publicly, then privately? The following example shows
> that in gcc-4.1 dynamic_cast result depends on the order of
> declarations:
> /////// cl1.cc //////////////
> #include <iostream>
> using namespace std;
> class A {
> public:
>   virtual ~A() {}
> };
> 
> class B : public virtual A {};
> class C : private virtual A {}; //! derived privately
> class T { public: virtual ~T() {}};
> class D : public C, public B, public T {};
> 
> A* f(T* p) {return dynamic_cast<A*>(p);}
> 
> int main() {
>   D* p = new D;
>   A* q = p;
>   cout << f(p) << " " << q << endl;
> }
> ////////// end of cl1.cc /////////
> 
> >g++ cl1.cc && ./a.out
> 0x804b008 0x804b008
> ^^^^^^^^^
> dynamic_cast from T* to A* is successfull
> 
> 
> Now, derive D first from B then C:
> 
> ////////////////////// cl2.cc /////////////////
> #include <iostream>
> using namespace std;
> class A {
> public:
>   virtual ~A() {}
> };
> 
> class B : public virtual A {};
> class C : private virtual A {};
> class T {
> public:
>     virtual ~T() {}
> };
> class D : public B, public C, public T {}; //! change order of B and C
> 
> 
> A* f(T* p) {return dynamic_cast<A*>(p);}
> 
> int main() {
>   D* p = new D;
>   A* q = p;
>   cout << f(p) << " " << q << endl;
> }
> ///////////////// end of cl2.cc /////////////
> 
> >g++ cl2.cc && ./a.out
> 0 0x804b008
> ^^
> dynamic_cast from T* to A* failed.
> 
> On the other hand paragraph 5.2.7.8 of the C++ standard seems to
> indicate that dynamic_cast should succeed  (T* is public base; is A
> unambiguous? -- yes, is A public base - yes (and no?))
> Same results with gcc-snapshot.
> 
> Vladimir