What did you mean by "inline classes" here? My only understanding of that term applies to something like what Java is still developing for Valhalla but has not delivered yet, whereas Kotlin is arguably a little farther along with its own feature with that exact name.
Would it be the classes Java has always had that sit inside a parent class and can access the instance fields/properties of the parent? These are very Java, and hard to recreate in a language that doesn't support that type of embedding. C# really struggles with it. Using the Android API via Xamarin was always very much harder to do anything with because of this symbiotic relationship. I have done a little Kotlin, but I don't remember if it support that or not.
