fix(runtime-core): Correct update renders both old and new dynamic child node arrays are empty

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

@@ -389,24 +389,32 @@ function buildDynamicSlot(
   return createObjectExpression(props)
 }
 
-function hasForwardedSlots(children: TemplateChildNode[]): boolean {
+function hasForwardedSlots(
+  children: TemplateChildNode[],
+  hasBranches = false
+): boolean {
   for (let i = 0; i < children.length; i++) {
     const child = children[i]
     switch (child.type) {
       case NodeTypes.ELEMENT:
         if (
           child.tagType === ElementTypes.SLOT ||
-          hasForwardedSlots(child.children)
+          (child.tagType === ElementTypes.ELEMENT && hasBranches) ||
+          hasForwardedSlots(child.children, hasBranches)
         ) {
           return true
         }
         break
       case NodeTypes.IF:
-        if (hasForwardedSlots(child.branches)) return true
+        hasBranches = true
+        if (hasForwardedSlots(child.branches, hasBranches)) return true
         break
       case NodeTypes.IF_BRANCH:
+        hasBranches = true
+        if (hasForwardedSlots(child.children, hasBranches)) return true
+        break
       case NodeTypes.FOR:
-        if (hasForwardedSlots(child.children)) return true
+        if (hasForwardedSlots(child.children, hasBranches)) return true
         break
       default:
         break

packages/compiler-dom/__tests__/transforms/__snapshots__/Transition.spec.ts.snap

@@ -41,7 +41,7 @@ return function render(_ctx, _cache) {
                     ], 2112 /* STABLE_FRAGMENT, DEV_ROOT_FRAGMENT */))
               ]))
       ]),
-      _: 1 /* STABLE */
+      _: 3 /* FORWARDED */
     }))
   }
 }"

packages/compiler-ssr/__tests__/ssrComponent.spec.ts

@@ -280,7 +280,7 @@ describe('ssr: components', () => {
                 ]
               }
             }),
-            _: 1 /* STABLE */
+            _: 3 /* FORWARDED */
           }, _parent))
         }"
       `)
@@ -341,12 +341,12 @@ describe('ssr: components', () => {
                           ? (_openBlock(), _createBlock("div", { key: 0 }))
                           : _createCommentVNode("v-if", true)
                       ]),
-                      _: 1 /* STABLE */
+                      _: 3 /* FORWARDED */
                     })
                   ]
                 }
               }),
-              _: 1 /* STABLE */
+              _: 3 /* FORWARDED */
             }, _parent))
           }"
         `)

packages/runtime-core/__tests__/rendererOptimizedMode.spec.ts

@@ -23,7 +23,8 @@ import {
   createApp,
   FunctionalComponent,
   renderList,
-  onUnmounted
+  onUnmounted,
+  createElementBlock
 } from '@vue/runtime-test'
 import { PatchFlags, SlotFlags } from '@vue/shared'
 import { SuspenseImpl } from '../src/components/Suspense'
@@ -933,4 +934,60 @@ describe('renderer: optimized mode', () => {
     // should successfully unmount without error
     expect(inner(root)).toBe(`<!---->`)
   })
+
+  // #7256
+  test('When both the old and new dynamic node arrays are empty, turn off optimization and fully diff', async () => {
+    const show = ref(false)
+
+    const Child = {
+      setup(props: any, { slots }: SetupContext) {
+        return () => {
+          return renderSlot(slots, 'default', {}, () => [
+            !show.value
+              ? (openBlock(),
+                createElementBlock('div', { key: 0 }, 'Fallback Content'))
+              : createCommentVNode('v-if', true)
+          ])
+        }
+      }
+    }
+
+    const Parent = {
+      setup() {
+        return () => {
+          return (
+            openBlock(),
+            createBlock(Child, null, {
+              default: withCtx(() => [
+                show.value
+                  ? (openBlock(),
+                    createElementBlock('div', { key: 0 }, 'Provided Content'))
+                  : createCommentVNode('v-if', true)
+              ]),
+              _: 3 /* FORWARDED */
+            })
+          )
+        }
+      }
+    }
+
+    const app = createApp({
+      setup() {
+        return () => {
+          return h(Parent)
+        }
+      }
+    })
+    app.mount(root)
+    await nextTick()
+    expect(inner(root)).toBe(`<div>Fallback Content</div>`)
+
+    show.value = true
+    await nextTick()
+    expect(inner(root)).toBe(`<div>Provided Content</div>`)
+
+    show.value = false
+    await nextTick()
+    expect(inner(root)).toBe(`<div>Fallback Content</div>`)
+  })
 })

packages/runtime-core/src/renderer.ts

@@ -821,7 +821,7 @@ function baseCreateRenderer(
     }
 
     const areChildrenSVG = isSVG && n2.type !== 'foreignObject'
-    if (dynamicChildren) {
+    if (dynamicChildren && dynamicChildren.length > 0) {
       patchBlockChildren(
         n1.dynamicChildren!,
         dynamicChildren,