reproduce bug

Author: nermolov

package-lock.json

@@ -9,7 +9,7 @@
   "dependencies": {
     "@vue/cli": "^5.0.8",
     "core-js": "^3.8.3",
-    "vue": "^2.6.14"
+    "vue": "^2.7.14"
   },
   "devDependencies": {
     "@vue/cli-plugin-babel": "~5.0.0",

package.json

@@ -6,13 +6,20 @@
 </template>
 
 <script lang="ts">
-import Vue from 'vue';
+import { defineComponent, ref } from 'vue';
 import HelloWorld from './components/HelloWorld.vue';
 
-export default Vue.extend({
+export default defineComponent({
   name: 'App',
   components: {
     HelloWorld
+  },
+  setup() {
+    const helloWorld = ref<InstanceType<typeof HelloWorld> | null>(null);
+
+    return {
+      helloWorld
+    }
   }
 });
 </script>

src/App.vue

@@ -25,7 +25,8 @@
       "dom",
       "dom.iterable",
       "scripthost"
-    ]
+    ],
+    "declaration": true
   },
   "include": [
     "src/**/*.ts",