Retain smartcast when creating custom extension

Question

I currently have to write

val myList: List<Int>? = listOf()
if(!myList.isNullOrEmpty()){
    // myList manipulations
}

Which smartcasts myList to no non null. The below do not give any smartcast:

if(!myList.orEmpty().isNotEmpty()){
    // Compiler thinks myList can be null here
    // But this is not what I want either, I want the extension fun below
}

if(myList.isNotEmptyExtension()){
    // Compiler thinks myList can be null here
}

private fun <T> Collection<T>?.isNotEmptyExtension() : Boolean {
    return !this.isNullOrEmpty()
}

Is there a way to get smartCasts for custom extensions?

Answer 1

This is solved by contracts introduced in Kotlin 1.3.

Contracts are a way to inform the compiler certain properties of your function, so that it can perform some static analysis, in this case enable smart casts.

import kotlin.contracts.ExperimentalContracts
import kotlin.contracts.contract

@ExperimentalContracts
private fun <T> Collection<T>?.isNotEmptyExtension() : Boolean {
    contract {
       returns(true) implies (this@isNotEmptyExtension != null)
    }
    return !this.isNullOrEmpty()
}

You can refer to the source of isNullOrEmpty and see a similar contract.

contract {
    returns(false) implies (this@isNullOrEmpty != null)
}