This rule reports when expressions that contain an else case even though they have an exhaustive set of cases. This occurs when the subject of the when expression is either an enum class, sealed class or of type boolean. Using else cases for these expressions can lead to unintended behavior when adding new enum types, sealed subtypes or changing the nullability of a boolean, since this will be implicitly handled by the else case.

Noncompliant Code

enum class Color {
    RED,
    GREEN,
    BLUE
}
when(c) {
    Color.RED -> {}
    Color.GREEN -> {}
    else -> {}
}

Compliant Code

enum class Color {
    RED,
    GREEN,
    BLUE
}
when(c) {
    Color.RED -> {}
    Color.GREEN -> {}
    Color.BLUE -> {}
}