}
}

// in plugin
existingThing = objects.newInstance(DefaultExistingThing.class)
existingThing.convention("some-value")
```

It's not acceptable to treat an unset Provider as if the convention is requested when the convention could be set elsewhere:

```groovy
public interface NewThing {
    Property<String> getSomeProperty()
}

// Unacceptable
String value