I have a Custom Post Type that uses the default category, the same as the posts, and some other post types in my site. How can I list all non-empty categories *from the custom post type*?

For instance, if I have categories “Cat 1”, “Cat 2" and “Cat 3” and I have a *default* post with a category of “Cat 1", and a *custom post* with categories “Cat 2” and “Cat 3". How would I write a function that would return “Cat 2” and “Cat 3"?

It seems like the `get_terms()` functions don’t support a  `post_type` argument. Is there a way I can add some other query to a `get_terms()` function?