‘Unresolved reference’ errors for android library module referenced in app module

Question

Found the problem, my android library module was missing the kotlin configuration:

apply plugin: 'kotlin-android'

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlinVersion:<version>"
}

Although I used kotlin .kt files in it, it could build without and also

Tools -> Kotlin -> 'Configure Kotlin in projects'

had told me ‘All modules with Kotlin files are configured’