From 52a59790a5b6e16c9f8cc737fcd784413dda282d Mon Sep 17 00:00:00 2001
From: Evan You <yyx990803@gmail.com>
Date: Fri, 15 Jul 2022 16:52:27 +0800
Subject: [PATCH] fix(types): fix $children and $root instance types

fix #12655
---
 types/test/v3/define-component-test.tsx |  1 +
 types/test/vue-test.ts                  |  1 +
 types/vue.d.ts                          | 10 ++++++----
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/types/test/v3/define-component-test.tsx b/types/test/v3/define-component-test.tsx
index 2d6fca73d75..fde804c400b 100644
--- a/types/test/v3/define-component-test.tsx
+++ b/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.$slots.default!])
   }
 })
diff --git a/types/test/vue-test.ts b/types/test/vue-test.ts
index 45e7bc7a719..8489f3ae8ed 100644
--- a/types/test/vue-test.ts
+++ b/types/test/vue-test.ts
@@ -16,6 +16,7 @@ class Test extends Vue {
     this.$isServer
     this.$ssrContext
     this.$vnode
+    this.$root.$children[0].$children[0]
   }
 
   // test property reification
diff --git a/types/vue.d.ts b/types/vue.d.ts
index b57c5016dd3..a86e2eebf3f 100644
--- a/types/vue.d.ts
+++ b/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