Kotlin: How to inherit property in data class

I have abstract Token class declared like this:

abstract class Token(var index: Int = 0) {
open fun merge(toMerge: Token): Token? {
    return null
    }
}

I want to inherit index property in data class, like this:

data class CloseLoop(index: Int, var openLoopIndex: Int = 0) : Token(index)

But it gives me error Data class primary constructor must have only property (val / var) parameters

What i have to do to fix this?

Solution

There are at least two workarounds:

Make the property open and override it in the data class primary constructor declaration:

abstract class Token(open var index: Int = 0)

data class CloseLoop(
    override var index: Int, 
    var openLoopIndex: Int = 0
) : Token(index)

Declare a property with another name and initialize the base class with it:
```
data class CloseLoop(val theIndex: Int, var openLoopIndex: Int = 0) : Token(theIndex)
```
^{Make it private if you find it appropriate.}