Re: Why does the C++ spec. prohibit downcasting through non-public inheritance?

If we take the code above and add the headers <iostream> and <typeinfo>,
correct the return on main and add a couple of debug print statements
there, we get:

   21:23:34 Paul Bibbings@JIJOU
   /cygdrive/d/CPPProjects/CLCPP $cat dyn_cast_ex1.cpp
   // file: dyn_cast_ex1.cpp

   #include <iostream>
   #include <typeinfo>

   struct B {
      virtual ~B() {};
   };

   struct D : protected B {
      void foo() {
         B &b = dynamic_cast< B & >( *this ); // upcast ok here
         D &d = dynamic_cast< D & >( b ); // runtime error
      }
      friend class F;
   };

   struct F {
      void foo( D &dref ) {
         B &b = dynamic_cast< B & >( dref ); // upcast ok here
         D &d = dynamic_cast< D & >( b ); // runtime error
      }
   };

   int main() {
      D d;
      try {
         d.foo();
      } catch (std::bad_cast) {
         std::cout << "std::bad_cast from d.foo()\n";
      }
      F f;
      try {
         f.foo( d );
      } catch (std::bad_cast) {
         std::cout << "std::bad_cast from f.foo(D)\n";
      }
   }

   21:23:42 Paul Bibbings@JIJOU
   /cygdrive/d/CPPProjects/CLCPP $i686-pc-cygwin-g++-4.4.3 -o
      dyn_cast_ex1 dyn_cast_ex1.cpp |
                                                       |
   21:24:27 Paul Bibbings@JIJOU |
   /cygdrive/d/CPPProjects/CLCPP $./dyn_cast_ex1 |
   std::bad_cast from d.foo() // fail <---+
   std::bad_cast from f.foo(D) // fail <---+

   21:24:33 Paul Bibbings@JIJOU
   /cygdrive/d/CPPProjects/CLCPP $i686-pc-cygwin-g++-4.5.0 -o
      dyn_cast_ex1 dyn_cast_ex1.cpp |
                                                       |
   21:25:01 Paul Bibbings@JIJOU |
   /cygdrive/d/CPPProjects/CLCPP $./dyn_cast_ex1 |
                                           // OK? <---+
   21:25:07 Paul Bibbings@JIJOU
   /cygdrive/d/CPPProjects/CLCPP $

So. gcc-4.5.0 doesn't regard there to be a problem here at all, and I'm
... confused now.

Regards

Paul Bibbings