I’m currently working on an application that requires delegating function variables. I’ve encountered some problems when trying to do so. Here’s the code of a minimal reproducible example:

package org.prismsus.tank

import kotlin.reflect.KProperty
fun addFunc(a : Int, b : Int) : Int {
    return a + b
}

class Test{
    inner class PrintFuncCall1{
        operator fun getValue(thisRef: Any?, property: KProperty<*>): (Int, Int) -> Int {
            fun temp(a : Int, b : Int) : Int {
                println("Function ${property.name}")
                return addFunc(a, b)
            }
            return ::temp
        }
    }
    val delegatedFun1 : (Int, Int) -> Int by PrintFuncCall1()


    inner class PrintFuncCall2<F : (Array<out Any>) -> R, R>{
        operator fun getValue(thisRef : Any?, property : KProperty<*>) : (Array<out Any>) -> R{
            fun temp(vararg params : Any) : R{
                println("Function ${property.name}")
                println("Params: ${params.joinToString()}")
                return addFunc(params[0] as Int, params[1] as Int) as R
            }
            return ::temp
        }
    }

    val delegatedFun2 : (Int, Int) -> Int by PrintFuncCall2()
}

fun main(){
    val test = Test()
    println(test.delegatedFun1(1, 2))
    println(test.delegatedFun2(3, 4))
}

Here delegatedFun1 and PrintFuncCall1 worked fine. However, I tried to modify the delegate class to a generic class and found that this didn’t work. Specifically, I got this compilation error for delegatedFun2 and PrintFuncCall2:

Property delegate must have a 'getValue(Test, KProperty<*>)' method. None of the following functions is suitable: 
public final operator fun getValue(thisRef: Any?, property: KProperty<*>): (Array<out Any>) -> ??? defined in org.prismsus.tank.Test.PrintFuncCall2

I don’t quite get why getValue requires thisRef to be of type Test, which is the type of the larger class. This is especially strange as I’m delegating a variable of type (Int, Int) -> Int. Further, even if I tried to change the signature of getValue to getValue(thisRef : Test, property : KProperty<*>), I got the following error:

Property delegate must have a 'getValue(Test, KProperty<*>)' method. None of the following functions is suitable: 
public final operator fun getValue(thisRef: Test, property: KProperty<*>): (Array<out Any>) -> ??? defined in org.prismsus.tank.Test.PrintFuncCall2

Besides these, a minor question I have is about Kotlin’s constrain mechanism to type parameters. My intention for PrintFuncCall2 is to mark F as the type of the function, and R as the return value of function F. However, when changing the signature of getValue from getValue(thisRef : Test, property : KProperty<*>) : (Array<out Any>) -> R to getValue(thisRef : Test, property : KProperty<*>) : F, I got this message from IDE on the line return ::temp:

Required:
KFunction1<Array<out Any>, R>
Found:
F

Isn’t that these two supposed to be the same thing?