Is there anyway I can delegate to a getter 🙂 ? Something like

class ReadLockFreeMutableMap<K, V>(
) : MutableMap<K, V> by mutableMap {
    private val mutableMap: MutableMap<K, V>
        get() {
            TODO()
        }
}

Nope!

Only if you move the getter to the constructor:

class ReadLockFreeMutableMap<K, V>(
    private val mutableMap: MutableMap<K, V>
) : MutableMap<K, V> by mutableMap {
}

(Not really a getter at that point 😛 )

Any particular reason this is not implemented? Seems rather useful...