fix(slots): properly force update on forwarded slots

fix #1594

packages/compiler-core/__tests__/transforms/vSlot.spec.ts

@@ -719,6 +719,23 @@ describe('compiler: transform component slots', () => {
     expect(generate(root, { prefixIdentifiers: true }).code).toMatchSnapshot()
   })
 
+  test('generate flag on forwarded slots', () => {
+    const { slots } = parseWithSlots(`<Comp><slot/></Comp>`)
+    expect(slots).toMatchObject({
+      type: NodeTypes.JS_OBJECT_EXPRESSION,
+      properties: [
+        {
+          key: { content: `default` },
+          value: { type: NodeTypes.JS_FUNCTION_EXPRESSION }
+        },
+        {
+          key: { content: `_` },
+          value: { content: `3` } // forwarded
+        }
+      ]
+    })
+  })
+
   describe('errors', () => {
     test('error on extraneous children w/ named default slot', () => {
       const onError = jest.fn()

packages/compiler-core/src/transforms/vSlot.ts

@@ -33,6 +33,7 @@ import {
 } from '../utils'
 import { CREATE_SLOTS, RENDER_LIST, WITH_CTX } from '../runtimeHelpers'
 import { parseForExpression, createForLoopParams } from './vFor'
+import { SlotFlags } from '@vue/shared/src'
 
 const defaultFallback = createSimpleExpression(`undefined`, false)
 
@@ -321,13 +322,19 @@ export function buildSlots(
     }
   }
 
+  const slotFlag = hasDynamicSlots
+    ? SlotFlags.DYNAMIC
+    : hasForwardedSlots(node.children)
+      ? SlotFlags.FORWARDED
+      : SlotFlags.STABLE
+
   let slots = createObjectExpression(
     slotsProperties.concat(
       createObjectProperty(
         `_`,
         // 2 = compiled but dynamic = can skip normalization, but must run diff
         // 1 = compiled and static = can skip normalization AND diff as optimized
-        createSimpleExpression(hasDynamicSlots ? `2` : `1`, false)
+        createSimpleExpression('' + slotFlag, false)
       )
     ),
     loc
@@ -354,3 +361,19 @@ function buildDynamicSlot(
     createObjectProperty(`fn`, fn)
   ])
 }
+
+function hasForwardedSlots(children: TemplateChildNode[]): boolean {
+  for (let i = 0; i < children.length; i++) {
+    const child = children[i]
+    if (child.type === NodeTypes.ELEMENT) {
+      if (
+        child.tagType === ElementTypes.SLOT ||
+        (child.tagType === ElementTypes.ELEMENT &&
+          hasForwardedSlots(child.children))
+      ) {
+        return true
+      }
+    }
+  }
+  return false
+}