static_cast and reference to pointers

Question

Can anyone tell me why this doesn't compile:

struct A { };
struct B : public A { };

int main()
{
  B b;
  A* a = &b;
  B* &b1 = static_cast<B*&>(a);
  return 0;
}

Now, if you replace the static cast with:

B* b1 = static_cast<B*>(a);

then it does compile.

Edit: It is obvious that the compiler treats A* and B* as independent types, otherwise this would work. The question is more about why is that desirable?

Answer 1

B is derived from A, but B* isn't derived from A*. A pointer to a B is not a pointer to an A, it can only be converted to one. But the types remain distinct (and the conversion can, and often will, change the value of the pointer). A B*& can only refer to a B*, not to any other pointer type.