Return from lambdas or Kotlin: ‘return’ is not allowed here

You can label lambda and then use labeled return:

fun String.isNice(): Boolean {
    val hasRepeat = hasRepeat@ {
        for (i in 0 .. (length - 2)) {
            if (subSequence(i, i + 2).toSet().size == 1) {
                return@hasRepeat true
                println(subSequence(i, i + 2)) // <-- note that this line is unreachable
            }
        }
        false
    }

    return hasRepeat()
}

or you can use named local function, if you do not need hasRepeat to be function reference:

fun String.isNice(): Boolean {
    fun hasRepeat(): Boolean {
        for (i in 0 .. (length - 2)) {
            if (subSequence(i, i + 2).toSet().size == 1) {
                return true
            }
        }
        return false
    }

    return hasRepeat()
}