fix(runtime-core): avoid rendering plain object as VNode (#12038)

close #12035
close vitejs/vite-plugin-vue#353

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

@@ -65,6 +65,15 @@ test('array children -> text children', () => {
   expect(inner(root)).toBe('<div>hello</div>')
 })
 
+test('plain object child', () => {
+  const root = nodeOps.createElement('div')
+  const foo = { foo: '1' }
+  // @ts-expect-error
+  render(h('div', null, [foo]), root)
+  expect('Invalid VNode type').not.toHaveBeenWarned()
+  expect(inner(root)).toBe('<div>[object Object]</div>')
+})
+
 describe('renderer: keyed children', () => {
   let root: TestElement
   let elm: TestElement

packages/runtime-core/src/vnode.ts

@@ -793,7 +793,7 @@ export function normalizeVNode(child: VNodeChild): VNode {
       // #3666, avoid reference pollution when reusing vnode
       child.slice(),
     )
-  } else if (typeof child === 'object') {
+  } else if (isVNode(child)) {
     // already vnode, this should be the most common since compiled templates
     // always produce all-vnode children arrays
     return cloneIfMounted(child)