--- old/src/hotspot/share/gc/shared/workgroup.hpp	2019-11-28 15:24:27.925215460 +0100
+++ new/src/hotspot/share/gc/shared/workgroup.hpp	2019-11-28 15:24:27.657206340 +0100
@@ -50,6 +50,7 @@
 
 class AbstractGangWorker;
 class Semaphore;
+class ThreadClosure;
 class WorkGang;
 
 // An abstract task to be worked on by a gang.