I'm assuming based on the "Future improvements" section on this KEEP https://github.com/Kotlin/KEEP/blob/master/proposals/data-class-inheritance.md that the following isn't possible? Any suggestions for the most elegant alternative?

sealed class TimedEvent(val eventTime: Long)
data class FxRateEvent(val currency: String, val rate: Double, eventTime: Long) : TimedEvent(eventTime)

Try

sealed class A(open val f: String)
data class B(val foo: String, override val f: String): A(f)

Ah nice, thanks!

That's weird, we've been using this in production a lot. Maybe some problem with reflection in Javaland? I don't know what these value types compile to.

Well looking at the bytecode, looks like the "override val" creates a new field, getter and setter, so the field is duplicated rather than reused. I've changed to using normal (non-data) classes instead which behaves as expected

You'll have to implement

.equals

and

.hashCode

though 😕 I'm surprised the byte code is this simple.

I often end up having to implement

.equals

and

.hashCode

on data classes anyway (or just not use them) because I tend to only want to test for equality based on a single ID field rather than all of them (which in some cases can end up quite expensive)