Nothing type

Nothing type can be used as a return type for a function that always throws an exception. When you call such a function, the compiler uses the information that the execution doesn’t continue beyond the function.

import kotlin.IllegalArgumentException

fun failWithWrongAge(age: Int?) {
    throw IllegalArgumentException("Wrong age: $age")
}

fun checkAge(age: Int?) {
    if (age == null || age !in 0..150) failWithWrongAge(age)
    println("Congrats! Next year you'll be ${age + 1}.")
}

fun main() {
    checkAge(10)
}

如果这里failWithWrongAge 方法没有标明返回值类型为Nothing ，那么 the checkAge function doesn’t compile because the compiler assumes the age can be null.你应该 Specify Nothing return type for the failWithWrongAge function.