if i have a few functions that have a default argument of

onComplete: () -> Unit = {}

would it be best to put this no-op function in a companion object somewhere or does kotlin automatically do any optimizations ?

A quick look at the generated bytecode for

fun foo(block: () -> Unit) {
    block()
}

fun main() {
    foo {}
    foo {}
}

shows that there's not much optimization happening here, two separate classes are created to represent the

{}

. You can try looking at your exact case too.

@karelpeeters how do you look at the generated bytecode like that?

Tools > Kotlin > Show Kotlin Bytecode (> Decompile, kind of buggy)