fix(types): fix $children and $root instance types

fix #12655

types/test/v3/define-component-test.tsx

@@ -1148,6 +1148,7 @@ defineComponent({
     this.$on('foo', () => {})
     this.$ssrContext
     this.$isServer
+    this.$children[0].$root.$children
     return h('div', {}, [...this.$slots.default!])
   }
 })

types/test/vue-test.ts

@@ -16,6 +16,7 @@ class Test extends Vue {
     this.$isServer
     this.$ssrContext
     this.$vnode
+    this.$root.$children[0].$children[0]
   }
 
   // test property reification

types/vue.d.ts

@@ -35,6 +35,8 @@ export interface CreateElement {
   ): VNode
 }
 
+type NeverFallback<T, D> = [T] extends [never] ? D : T
+
 export interface Vue<
   Data = Record<string, any>,
   Props = Record<string, any>,
@@ -47,10 +49,10 @@ export interface Vue<
   // properties with different types in defineComponent()
   readonly $data: Data
   readonly $props: Props
-  readonly $parent: Parent extends never ? Vue : Parent
-  readonly $root: Root extends never ? Vue : Root
-  readonly $children: Children extends never ? Vue[] : Children
-  readonly $options: ComponentOptions<Vue>
+  readonly $parent: NeverFallback<Parent, Vue>
+  readonly $root: NeverFallback<Root, Vue>
+  readonly $children: NeverFallback<Children, Vue[]>
+  readonly $options: NeverFallback<Options, ComponentOptions<Vue>>
   $emit: Emit
 
   // Vue 2 only or shared