Never

Never is the bottom type, i.e., it is a subtype of all types. There is no value that has type Never. Never can be used as the return type for functions that never return normally. For example, it is the return type of the function panic.

// Declare a function named `crashAndBurn` which will never return,
// because it calls the function named `panic`, which never returns.
//
fun crashAndBurn(): Never {
    panic("An unrecoverable error occurred")
}

// Invalid: Declare a constant with a `Never` type, but the initial value is an integer.
//
let x: Never = 1

// Invalid: Declare a function which returns an invalid return value `nil`,
// which is not a value of type `Never`.
//
fun returnNever(): Never {
    return nil
}