Bug: Optional props are typed as `| undefined`

[DesselBane]

Describe the bug
When using the .props() method on a VueWrapper for a prop which is not required but has a default value the returned type includes undefined.

To Reproduce
https://github.com/DesselBane/test-utils-reproduction/blob/master/src/components/foo.spec.ts

Expected behavior
I would argue that the type should not include undefined since it has a default value and this default value is returned at runtime.

Related information:
This happened on an update from v2.4.1 to v2.4.2, most likely somewhere in this PR: #2137 (but I'm not sure)

[cexbrayat]

I'm not sure about that.

@pikax What's your take on this?

[pikax]

I'm actually writing this in the advanced typescript docs here :D

That's caused because the props inferred are the wrong ComponentPublicInstance props, it should be the internal ComponentPublicInstance instead of the rendered props.

Is a bug, at runtime:

    const a = mount(
      defineComponent({
        props: {
          a: {
            type: String,
            default: 'a'
          }
        },
        setup(props) {
          return () => props.a
        }
      })
    )

    expect(a.props('a')).toBe('a')

    a.setProps({ a: undefined })
    expect(a.props('a')).toBe('a')

That's passing.