How to set up Kotlin’s byte code version in Gradle project to Java 8?

As Mark pointed out on Debop’s answer, you have to configure both compileKotlin and compileTestKotlin. You can do it without duplication this way:

tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
  kotlinOptions {
    jvmTarget = "1.8"
  }
}

For a pure Kotlin project, I don’t think the options sourceCompatibility and targetCompatibility do anything, so you may be able to remove them.

Ref: https://kotlinlang.org/docs/reference/using-gradle.html#compiler-options