--- old/src/hotspot/share/gc/z/zHeap.hpp	2019-11-28 15:24:30.285295767 +0100
+++ new/src/hotspot/share/gc/z/zHeap.hpp	2019-11-28 15:24:30.016286613 +0100
@@ -39,6 +39,8 @@
 #include "gc/z/zUnload.hpp"
 #include "gc/z/zWorkers.hpp"
 
+class ThreadClosure;
+
 class ZHeap {
   friend class VMStructs;