Java forgetting parameterized return type

gistfile1.java

public class Base<T extends Base> {
  public T f1() {
    return (T) this; //Warning => Type safety: Unchecked cast from Forgetful.Base<T> to T
  }
  
  public T f2() {
    return (T) this; //Warning => Type safety: Unchecked cast from Forgetful.Base<T> to T
  }
}

public class Sub<T extends Sub> extends Base<Sub> {
  public T f3() {
    return (T) this;
  }
}

public class Test {
  public static void main(String[] args) {
    Base b0 = new Base().f1().f2();
    
    //Warning => Forgetful.Base is a raw type. References to generic type Forgetful.Base<T> should be parameterized
    //Error => Type mismatch: cannot convert from Object to Forgetful.Base<Forgetful.Base>
    //f1() is returning T (which is essentially Object) instead of Base
    Base<Base> b = new Base<Base>().f1().f2();
    
    //no errors, but multiple warnings on each line...
    Base<Base> b1 = new Base<Base>();
    Base<Base> b2 = b1.f1();
    Base<Base> b3 = b2.f2();
    
    //Error: The method f3() is undefined for the type Base
    Sub<Sub> s = new Sub<Sub>().f1().f2().f3();
  }
}