Kotlin when used with recent versions ( at least 2.1.0 ) and they don't even bother to document it, like here : https://kotlinlang.org/docs/kotlin-evolution-principles.html . This stinks. When trying to get a project to compile, it showed min sdk was 24 for 2.1.0. No idea where I can find which version can be used for 21, but clearly this matters.