Support "sealed" keyword on interfaces and unions in Kotlin

[kbrooks]

Is your feature request related to a problem? Please describe.
Add sealed keyword` to generated interfaces and unions in order to enable exhaustive type-checking in written code. For example

sealed interface Animal {}
data class Cat(val foo: String): Animal  
data class Dog(val bar: String): Animal  

fun cases(obj: Animal) {                                
    return  when (obj) {
        is Cat -> "foo"
        is Dog -> "bar"; // Removing this line will fail compilation 
    }   
}

The Kotlin language will default to exhaustive when statements (expressions are already exhaustive) in version 1.7, which will make it easier to take advantage of this feature. https://kotlinlang.org/docs/whatsnew1530.html#exhaustive-when-statements-for-sealed-and-boolean-subjects

Describe the solution you'd like
A generateSealedInterfaces build parameter that if true will add the sealed keyword to generated interfaces in Kotlin

Describe alternatives you've considered
n/a

[kbrooks]

I noticed Scala has a sealed class option as well, but it looks like you have to make sure the classes are in the same file as the interface which seems like a non-starter

[jxnu-liguobin]

I noticed Scala has a sealed class option as well, but it looks like you have to make sure the classes are in the same file as the interface which seems like a non-starter

Yes, so I don't think Scala applies.

Note: this means that the analysis of the compiler will be more rigorous, but less flexible

[kbrooks]

It looks like Java actually added support for this feature in v15. I can add this as well https://openjdk.java.net/jeps/409

[kbrooks]

Since Java's sealed interfaces have a more complex syntax that requires listing the implementing classes in the interface definition, it is significantly harder to add support; we can track it in another issue