Experimental RFC 436 - using `withDefaults` with generic attribute cause error

[Sengulair]

I'm trying to use withDefaults function to set the default value for the property, but some error occurs. There is no error if I define props without it.

Here is a public repo to showcase the issue. The generic component with the error is AppInput.vue

[johnsoncodehk]

See #1964 (comment)