import as Async component way with Vue.extend beforeRouteLeave throw error parentVal.concat is not a function

[Bee777]

Version

3.0.3, also 3.1.3

Reproduction link

https://codesandbox.io/s/vue-template-1148z

Steps to reproduce

To see the error:

1. Enter a page for the target route, not by $router.push but manually enter the url target and press enter on keyboard
2. See error on browser console.

What is expected?

Get into the page as usually without the error.

What is actually happening?

When entered the page got the error:

vue-router.esm.js?8c4f:2079 TypeError: parentVal.concat is not a function
at mergeHook (vue.runtime.esm.js?2b0e:1272)
at mergeField (vue.runtime.esm.js?2b0e:1552)
at mergeOptions (vue.runtime.esm.js?2b0e:1543)
at Function.Vue.extend (vue.runtime.esm.js?2b0e:5153)

---

To make its work 3 ways:

1. import as a normal not async way.
2. combine import as a normal and import as async way.
3. just remove the beforeRouteLeave.

[posva]

Please provide a boiled down runnable repro, not just the code sample. Ping when you got a working repro or open a new issue

[Bee777]

@posva Now i have created the codesandbox that you can run.
Please take a look there:
The error will show when you try to refresh the async component page which is /async.

https://codesandbox.io/s/vue-template-1148z