private void assertContentServiceWithRetire(ClusterMembership instance) {
        assertEquals(ClusterSpec.Type.content, instance.cluster().type());
        assertEquals("id1", instance.cluster().id().value());
        assertEquals(37, instance.index());
        assertTrue(instance.retired());
        assertEquals("content/id1//37/retired/stateful", instance.stringValue());
        // Legacy form:
        assertEquals(instance, ClusterMembership.from("content/id1/37/retired/stateful", instance.cluster().vespaVersion(), Optional.empty()));
    }
Serializing a spec without a group assigned works, but not deserialization
config-provisioning/src/test/java/com/yahoo/config/provision/ClusterMembershipTest.java