Simply calling

new ConcurrentSkipListSet().clone();

fails with UnsupportedOperationException.
Sadly, we have no existing jdk tests that call ConcurrentSkipListSet#clone

     [java] 1) testClone(ConcurrentSkipListSetTest)java.lang.UnsupportedOperationException
     [java] at java.base/java.lang.invoke.VarForm.getMemberName(VarForm.java:92)
     [java] at java.base/java.lang.invoke.VarHandleGuards.guard_LL_V(VarHandleGuards.java:49)
     [java] at java.base/java.util.concurrent.ConcurrentSkipListSet.setMap(ConcurrentSkipListSet.java:482)
     [java] at java.base/java.util.concurrent.ConcurrentSkipListSet.clone(ConcurrentSkipListSet.java:147)