Miss to check the compatibility of wildcard types

import java.util.*

class Main {
  static final void test() {
    List<? extends Number> list = new LinkedList<Number>();
    List<Number> x = list; // types are not compatible.
    List<Number> y = (List<? extends Object>) null // types are not compatible;
    list.add(1); // method call is not permitted.
  }
}