class Base
    {
    };

    class Derived
        : private Base
    {

    public:
        Base &get_base_type();
        Base const &get_base_type() const;
    };

    int main()
    {
        Derived d;
        Base    &b  =   d.get_base_type();

        return 0;
    }

    Base &Derived::get_base_type()
    {
        return *this;
    }

    Base const &Derived::get_base_type() const
    {
        return *this;
    }

Gives

H:\Dev\Test\compiler\dmc\privbase>dmc privbase.cpp
        return *this;
                    ^
privbase.cpp(24) : Error: member 'Base' of class 'Derived' is not accessible
        return *this;
                    ^
privbase.cpp(29) : Error: member 'Base' of class 'Derived' is not accessible
--- errorlevel 1