Is inline keyword valid in Kotlin class constuctors?

Question

Suppose you have 2 Kotlin classes

class Battery(
    val carbon: Any,
    val zinc: Any
)

class RemoteControl(
    /* inline */ val battery: Battery,
    val buttons: Any
)

Notice the commented out inline keyword in RemoteControl. Uncommenting it doesn't produce an error in Intellij.

Is this valid Kotlin code and what does it do?

My expectations is that it is equivalent to

class RemoteControl(
    val carbon: Any,
    val zinc: Any,
    val buttons: Any
)

But it doesn't appear to do anything.

I'm using Kotlin 1.3.72 and Android Studio 4.0.1

Answer 1

The inline modifier can be used on the property (on the property of primary constructor as well):

// var property
inline var battery: Battery
    get() = Battery("carbon", "zinc")
    set(v) {  }

// val property
inline val battery: Battery
    get() = Battery("carbon", "zinc")

// primary constructor
class RemoteControl(inline  val battery: Battery)

In such a case, all accessors are marked inline automatically. At the call site the accessors are inlined as normal functions.

Applying inline to a property that has a backing field, or its accessor, results in a compile-time error:

// error: "Inline property cannot have backing field"
inline var battery: Battery
    get() = Battery("carbon", "zinc")
    set(v) { field = v } // we use backing field here

There is more info.