Explicit Backing Fields

build.gradle

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

plugins {
    kotlin("jvm") version "1.7.0"
}

group = "org.example"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
}

dependencies {
    testImplementation(kotlin("test"))
}

tasks.test {
    useJUnitPlatform()
}

tasks.withType<KotlinCompile> {
    kotlinOptions.jvmTarget = "1.8"
    // Only works with K2 compiler!
    kotlinOptions.freeCompilerArgs += listOf("-Xuse-k2")
}

kotlin {
    sourceSets.all {
        // Must be explcitly enabled!
        languageSettings.enableLanguageFeature("ExplicitBackingFields")
    }
}

Test.kt

class Test {
    val names: List<String>
        field: MutableList<String> = mutableListOf<String>()

    fun doThing() {
        names.add("Hello!")
    }
}

fun main() {
    val test = Test()
    test.doThing()

    println(test.names)
}