Enable Java 8 compatibility for Kotlin sources

Enabling Java 8 compatibility via the compileOptions block only works for Java sources. To do the same for Kotlin you need to use kotlinOptions on any KotlinCompile Gradle tasks. Interesting…

For Kotlin

tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
    kotlinOptions {
        jvmTarget = JavaVersion.VERSION_1_8
    }
}

For Java

android {
  compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
  }
}

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Thanks for reading!