data

copy()

copy()

copy

@Target(AnnotationTarget.CLASS)
annotation class Copy

@Copy
open class Parent(
    val a: Int,
) {
    open fun copy(a: Int? = null): Parent {
        return Parent(
            a = a ?: this.a
        )
    }
}

@Copy
class Child(
    a: Int,
    val b: Int,
): Parent(a) {

    override fun copy(a: Int?): Parent {
        return copy(a = a, b = this.b)
    }

    fun copy(a: Int? = null, b: Int? = null): Child {
        return Child(
            a = a ?: this.a,
            b = b ?: this.b,
        )
    }
}

val one: Child = Child(a = 1, b = 2).copy(a = 3, b = 1)  // fine
val two: Child = Child(a = 1, b = 2).copy(a = 3)  // call is surprisingly(?) fine, BUT returns a Parent, so does not compile

override fun copy(a: Int?): Child {
        return copy(a = a, b = this.b)
    }

Any

String

Parent(a)

@Copy
open class Parent(
    val a: Int
)

@Copy
class Child(
    a: Int,
    val b: Int,
): Parent(a * b)

var x: Parent = Child(..., 2)
// Later ...
x.copy(a = 2).a == 4  // if X was unchanged
x.copy(a = 2).a == 2  // if someone overwrote it with Parent(...)

x.copy(a = 2).a == 2 // why it must be 4?

x == Child(..., 2)

copy(a = q)

Child(a = q, b = 2)

Parent(a = q * 2)

Parent(a = 4)