The spec for the method getBounds() of javax.lang.model.element.TypeParameterElement says:

---
List<? extends TypeMirror> getBounds()
    Returns the bounds of this type parameter. These are the types given by the extends clause used to declare this type parameter. *If no explicit extends clause was used, an empty list is returned.*
    Returns:
        the bounds of this type parameter, or an empty list if there are none
---

  However, for each of {TP1,TP2,TP3,TP4} in this source the list with one TypeMirror representing java.lang.Object is returned:

public class S_TPE0002<TP1> extends TestSource {
    public <TP2> S_TPE0002(TP2 param) {}
    public <TP3> TP3 loopback(TP3 param) {
        return param;
    }
}

interface I_TPE0002_1<TP4> {
}