Composition API SFC test issue when using Vue 2.7 or the compat Mode

Subject of the issue

Hi,

this issue is created, after another issue was opened here in vue-testing-library : https://github.com/testing-library/vue-testing-library/issues/275

The core of the problem seems to be in @vue/test-utils:

When using the compat mode or the 2.7 build, the unit tests start failing when using the composition api in the SFC syntax. I tried using the composition api in a SFC synctax from the 2.7 build itself and this behaviour appears:

<script setup>
import { ref, onBeforeMount, defineProps } from 'vue'

defineProps({
  msg: {
    type: String,
    default: ''
  },
})

const label = ref('init')

onBeforeMount(() => {
  label.value = 'mounted'
})
</script>

<template>
  <div>{{ msg }} - {{ label }}</div>
</template>

results in this testing html

If we use the regular export default syntax with the composition api, it seems to be still working.

Steps to reproduce

To reproduce the behavior:

Use this minimal reproduction repo: https://github.com/lisilinhart/vue-2.7-sfc-test-bug-repo

Expected behaviour

When mounting the component with the composition API in a sfc syntax, it should render the component correctly like it does for the options api.

Actual behaviour

The asynchronous text is never rendered in the mounted component.

Possible Solution

[kingyue737]

@lmiller1990 We cannot migrate to Vue 2.7 without test😭😭

[lmiller1990]

The whole back porting of Vue 3 to Vue 2 is making a lot of work for this repo (and old versions of Vue Jest).

I have no bandwidth to work on Vue Test Utils v1/Vue 2/Vue Jest combo right now.

If there’s a bug in Vue 3/Vue Test Utils 2 (different repo, as mentioned above) we will actively try to fix/address it.