UpdateRSOopClosure is used to re-build the remembered sets after a G1 full GC. This is done after reference discovery and enqueuing (to the pending list) has been completed, so all discovered references have their next-field set. Before the full GC is started all references discovered by the concurrent-mark reference processor is clear to allow them to be discovered during the FullGC.

So at the point of applying the UpdateRSOopClosure, no references should have their discovered field set but still be active (next == null). So overriding apply_to_weak_ref_discovered field to make sure the discovered field is treated doesn't have any effect. Either the discovered and next field are both set and the gets handled the normal way, or the discovered field is not set and there is no reason to handle it specially.