Looking for ideas on how to do this (or anything vaguely similar) with less boilerplate:

interface HasName {
  val name: String
  fun copy(name: String = name) // boilerplate (but only once)
}

data class Person(
  override val name: String,
  val age: Int
) : HasName {
  override fun copy(name: String = name) // boilerplate (in everything that extends HasName)
}

// Because I want to do this:
val named: HasName = Person(...)
named.copy(name = "newName")

I don't see an implementation for

copy

anywhere. Is the implementation of that method supposed to be different in different implementations of

HasName

? What "boilerplate" are you trying to get rid of exactly?

Person already has a copy(name=) by virtue of being a data class. But I need to "re-" define it so that the interface can see it

I use a sealed interface and a extension

fun HasName.copy(...) = with (this) { is Person -> copy(...) }