CRTP with Protected Derived Member

In the CRTP pattern, we run into problems if we want to keep the implementation function in the derived class as protected. We must either declare the base class as a friend of the derived class or use something like this (I have not tried the method on the linked article). Is there some other (simple) way that allows keeping the implementation function in the derived class as protected?

Edit: Here is a simple code example:

template<class D> 
class C {
public:
    void base_foo()
    {
        static_cast<D*>(this)->foo();
    }
};


class D:  public C<D> {
protected: //ERROR!
    void foo() {
    }   
};

int main() {
    D d;
    d.base_foo();
    return 0;
}

The above code gives error: ‘void D::foo()’ is protected with g++ 4.5.1 but compiles if protected is replaced by public.