(inference) underconstrained type variable cannot be dereferenced

The following program is correct but rejected by javac:

X.java:3: X cannot be dereferenced
       String s = new X().foo().bar();
                  ^

/////////////////////////////////////////
public class X {
   public static void main(String[] args) {
       String s = new X().foo().bar();
   }
   <V extends X> V foo() {
       return null;
     }
   String bar() {
       return "";
   }
}