### Version 2.5.17 ### Reproduction link [https://stackblitz.com/edit/typescript-bc9ywj](https://stackblitz.com/edit/typescript-bc9ywj) ### Steps to reproduce 1. open `index.ts`, you will see:  2. change `x: [String, Number, Array]` to `x: [String, Number]`, then errors disappear:  ### What is expected? no typing errors. ### What is actually happening? typing errors exist. <!-- generated by vue-issues. DO NOT REMOVE -->