https://pl.kotl.in/Fa9favTpr I was getting this error and as a newbie I had a hard time determining why I could not do this (i.e. treating lambda as a function reference). Should the compiler error be more helpful?

fun fn1(x: Int): Boolean = x % 2 == 0
fun fn2(x: Int): Boolean { return x % 2 == 0 }

fun main () {
    val listNum = listOf(1, 2, 3, 4, 5, 6)
    val fn3: (Int) -> Boolean = { it % 2 == 0 }

    println(listNum.filter(::fn1)) // compiles and outputs [2, 4, 6]
    println(listNum.filter(::fn2)) // compiles and outputs [2, 4, 6]
    println(listNum.filter(fn3)) // compiles and outputs [2, 4, 6]

    // e: org.jetbrains.kotlin.util.KotlinFrontEndException: Exception while analyzing expression at (17,13)
    println(listNum.filter(::fn3)) // Error
}

::

references a function

val fn3

declares a variable not a function so you can't use ::

Thanks. I read that after I was experimenting. My question is of the error message from the compiler isn’t helpful

Yep looks like a bug 🙂

Well, that's obvious, isn't it? The message even tells you to report this as bug. 😄

Except there is not icon on the bottom right and the image link is broken 😆

And regarding

why I could not do this (i.e. treating lambda as a function reference).

Of course you can do this, or your second line would not work. Your problem is, that you already have the function reference in the variable fn3 and with

::fn3

you make a reference to the function reference, namely a reference to the variable

fn3

.

That was my mistake. Still learning the language

project KT, this error is most likely coming from the Kotlin plugin, not from IntelliJ itself

The error is more usefull if you use

::fun3.get()

Unsupported [References to variables aren't supported yet]

that's the way you should reference properties, but as this is a var is unsupported .. yet.

If you are still following along, I’ve opened two issues: https://youtrack.jetbrains.com/issue/KT-42795 https://youtrack.jetbrains.com/issue/KTIJ-257