internal abstract class A<T>
{
    public abstract void Foo(T x);

    public abstract class B<S> : A<B<S>>
    {
        public abstract class C<U> : B<C<U>>
        {
            public abstract class D<Q> : C<D<Q>>
            {
                public class E<X> : D<E<X>> // here
                {
                }
            }
        }
    }
}