CV-qualified base classes in C++

While refactoring some C++11 code, I stumbled upon a strange thing. Namely, it seems to be impossible to define a CV-qualified (const, volatile or const volatile) base class, e.g:

struct A { int a = 0; };
struct B: A const {}; // Error here with Clang and GCC!

However, the following compiles without errors:

struct A { int a = 0; };
using AC = A const;
struct B: AC {};      // NO ERROR HERE!? Qualifiers are ignored.
int main() {
  B b;
  b.a = 42;           // NO ERROR modifying a field of const base.
  return b.a;
}

I have two questions:

1. What in the C++ standards prohibits defining a CV-qualified base class, if at all?
2. Why does the second example compile?

PS: Since this is a language-lawyer question, please provide references to the C++ standard.