Kotlin data class copy method not deep copying all members

The copy method of Kotlin is not supposed to be a deep copy at all. As explained in the reference doc (https://kotlinlang.org/docs/reference/data-classes.html), for a class such as:

data class User(val name: String = "", val age: Int = 0)

the copy implementation would be:

fun copy(name: String = this.name, age: Int = this.age) = User(name, age)

So as you can see, it’s a shallow copy. The implementations of copy in your specific cases would be:

fun copy(a: Int = this.a, bar: Bar = this.bar, list: MutableList<Int> = this.list) = Foo(a, bar, list)

fun copy(x: Int = this.x) = Bar(x)