Accessibility of virtual functions

[PredicateNormative]

Does anyone know if there are there technical reasons for these prohibitions or are they just arbitrary?

I'm not sure of the technical reasons as to why it is the case, but the standard (or, at least the 2005 draft that I have) in section 14.7.18 says:

In an explicit specialization declaration for a member of a class template or a member template that appears in namespace
scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration
shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized
as well. In such explicit specialization declaration, the keyword template followed by a template-parameter-list shall
be provided instead of the template<> preceding the explicit specialization declaration of the member. The types of
the template-parameters in the template-parameter-list shall be the same as those specified in the primary template
definition. [ Example:

template < class T1 > class A {
  template < class T2 > class B {
    template < class T3 > void mf1(T3 );
    void mf2 ();
  };
};
template <> template < class X>
class A<int >::B {
  template < class T > void mf1(T);
};
template <> template <> template < class T>
void A<int >::B<double >:: mf1(T t ) { }
template < class Y > template <>
void A<Y >::B<double >:: mf2 () { } / / ill-formed; B<double> is specialized but
                                    / / its enclosing class template A is not

I did once hear that the reason why the behaviour was prevented, was because it is technically difficult for the compilers to allow such behaviour, and therefore the standard disallowed it. But that explanation seems a little odd to me and I don't trust it. If anyone can give a good technical explanation as to why the behaviour was prevented, then I'd be happy to hear it.

Incidently, although there have been rumours that C++0x will make the above legal, it doesn't look like this will be the case as the Oct 2008 version of the C++0x draft keeps the above quotation unchanged. Therefore, VS allows you to get away with ill-formed code, which is a bit annoying.