fix(slots): properly force update on forwarded slots

fix #1594

packages/runtime-core/src/vnode.ts

@@ -8,7 +8,8 @@ import {
   normalizeClass,
   normalizeStyle,
   PatchFlags,
-  ShapeFlags
+  ShapeFlags,
+  SlotFlags
 } from '@vue/shared'
 import {
   ComponentInternalInstance,
@@ -542,10 +543,22 @@ export function normalizeChildren(vnode: VNode, children: unknown) {
       return
     } else {
       type = ShapeFlags.SLOTS_CHILDREN
-      if (!(children as RawSlots)._ && !(InternalObjectKey in children!)) {
+      const slotFlag = (children as RawSlots)._
+      if (!slotFlag && !(InternalObjectKey in children!)) {
         // if slots are not normalized, attach context instance
         // (compiled / normalized slots already have context)
         ;(children as RawSlots)._ctx = currentRenderingInstance
+      } else if (slotFlag === SlotFlags.FORWARDED && currentRenderingInstance) {
+        // a child component receives forwarded slots from the parent.
+        // its slot type is determined by its parent's slot type.
+        if (
+          currentRenderingInstance.vnode.patchFlag & PatchFlags.DYNAMIC_SLOTS
+        ) {
+          ;(children as RawSlots)._ = SlotFlags.DYNAMIC
+          vnode.patchFlag |= PatchFlags.DYNAMIC_SLOTS
+        } else {
+          ;(children as RawSlots)._ = SlotFlags.STABLE
+        }
       }
     }
   } else if (isFunction(children)) {