This fails to compile:

---

 interface I {
   Object m(Iterable l);
 }

 interface J<S> {
   S m(Iterable<String> l);
 }

 interface K<T> {
   T m(Iterable<String> l);
 }

 interface Functional<S,T> extends I, J<S>, K<T> {}

---

On the other hand, this compiles fine:

---

interface Parent<T> {
   T m(Iterable<String> i);
 }

 interface Child<T> extends Parent<T> {
   Object m(Iterable i);
 }

---

This compiles fine, too:

---

interface I {
   int[] m(Iterable l);
 }

 interface J<S> {
   java.io.Serializable m(Iterable<String> l);
 }

 interface K<T> {
   Cloneable m(Iterable<String> l);
 }

 interface Functional<S,T> extends I, J<S>, K<T> {}

---
 
Problem appears to be that javac is looking for pairwise return-type-substitutable when it uses the rule "R1 = |R2|", while JLS 9.4.1 just says that there needs to be one method RTS for all the others.