Bug: Scoped slots always undefined when shallow mounting and `renderStubDefaultSlot` is enabled

[tom751]

Describe the bug
When renderStubDefaultSlot is enabled, if you shallowMount a component that renders another component that has a scoped slot, then an error is thrown stating that the slot is undefined. This used to work on vue test utils v1, and is making my current Vue 2.7 > 3 migration quite difficult.

To Reproduce
TLDR Here's an example repo https://github.com/tom751/vtu-slot-issue - The HelloWorld.spec.ts will fail

Have a component that renders a child component that has a scoped slot:

<template>
  <div class="greetings">
    <h1 class="green">{{ msg }}</h1>
    <ComponentWithSlot v-slot="{ count }">
      <h2>The count is {{ count }}</h2>
    </ComponentWithSlot>
  </div>
</template>

Render the component with shallowMount and renderStubDefaultSlot enabled

describe('HelloWorld', () => {
  beforeAll(() => {
    config.global.renderStubDefaultSlot = true
  })

  it('renders properly', () => {
    const wrapper = shallowMount(HelloWorld, { props: { msg: 'Hello Vitest' } })
    expect(wrapper.text()).toContain('Hello Vitest')
  })
})

The following error is thrown after running the test:

FAIL  src/components/__tests__/HelloWorld.spec.ts > HelloWorld > renders properly
TypeError: Cannot destructure property 'count' of 'undefined' as it is undefined.
❯ src/components/HelloWorld.vue:12:34
     10|   <div class="greetings">
     11|     <h1 class="green">{{ msg }}</h1>
     12|     <ComponentWithSlot v-slot="{ count }">
       |                                  ^
     13|       <h2>The count is {{ count }}</h2>
     14|     </ComponentWithSlot>

Expected behavior
An error shouldn't be thrown, the behaviour should be the same as it is in Vue 2 + vue test utils v1

Related information:

Additional context
This used to work in Vue 2.7 and vue test utils v1

[cexbrayat]

Hi @tom751

Thanks for the repro.
I suppose that we could call the default slot with an empty object by default to avoid the error. I'll open a PR.