Kotlin will do exhaustive checks in a `when` block when it is an expression. You can add a `.let { }` at the end of the when statement if you want to enforce this and not really return anything.

    sealed class Communication
    data class Email(val emailAddress: String) : Communication()
    data class Shouting(val preferredName: String) : Communication()
    object HiveMind : Communication()

    fun exhaustiveWhen(comm: Communication) {
        // not exhaustively checked (will compile)
        when (comm) {
            is Email -> println("sending email to ${comm.emailAddress}")
        }

        // *is* exhaustively checked (error at compile time)
        //  must add HiveMind or 'else' branch
        val result = when (comm) {
            is Email -> println("sending email to ${comm.emailAddress}")
            is Shouting -> println("HELLO, ${comm.preferredName}!")
        }

        // *is* exhaustively checked
        when (comm) {
            is Email -> println("sending email to ${comm.emailAddress}")
            is Shouting -> println("HELLO, ${comm.preferredName}!")
        }.let {  }
    }