fix(slots): properly force update on forwarded slots

fix #1594

packages/shared/src/index.ts

@@ -3,6 +3,7 @@ import { makeMap } from './makeMap'
 export { makeMap }
 export * from './patchFlags'
 export * from './shapeFlags'
+export * from './slotFlags'
 export * from './globalsWhitelist'
 export * from './codeframe'
 export * from './mockWarn'

packages/shared/src/slotFlags.ts

@@ -0,0 +1,21 @@
+export const enum SlotFlags {
+  /**
+   * Stable slots that only reference slot props or context state. The slot
+   * can fully capture its own dependencies so when passed down the parent won't
+   * need to force the child to update.
+   */
+  STABLE = 1,
+  /**
+   * Slots that reference scope variables (v-for or an outer slot prop), or
+   * has conditional structure (v-if, v-for). The parent will need to force
+   * the child to update because the slot does not fully capture its dependencies.
+   */
+  DYNAMIC = 2,
+  /**
+   * <slot/> being forwarded into a child component. Whether the parent needs
+   * to update the child is dependent on what kind of slots the parent itself
+   * received. This has to be refined at runtime, when the child's vnode
+   * is being created (in `normalizeChildren`)
+   */
+  FORWARDED = 3
+}