Java 8 ConcurrentHashMap.keySet() bytecode incompatibility

javap_output

// In Java 8, ConcurrentHashMap backward compatibility was broken by changing to a covariant return type

    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public static void main(java.lang.String...);
    Code:
       0: new           #2                  // class java/util/TreeSet
       3: dup           
       4: invokespecial #3                  // Method java/util/TreeSet."<init>":()V
       7: new           #4                  // class java/util/concurrent/ConcurrentHashMap
      10: dup           
      11: invokespecial #5                  // Method java/util/concurrent/ConcurrentHashMap."<init>":()V
      14: invokevirtual #6                  // Method java/util/concurrent/ConcurrentHashMap.keySet:()Ljava/util/concurrent/ConcurrentHashMap$KeySetView;
      17: invokevirtual #7                  // Method java/util/TreeSet.addAll:(Ljava/util/Collection;)Z
      20: pop           
      21: return        

While Java 8 typically does a good job of creating bytecode that is compatible with Java 7, this is an example of where it does not.