Noticed this warning today, `Non exhaustive 'when'...

# stdlib

[ghosalmartin]

Noticed this warning today, Non exhaustive 'when' statements on sealed class/interface will be prohibited in 1.7, add 'else' branch Does that mean using a sealed class/interface will no longer be exhaustive in a when . I thought that was a feature of ``sealed`?

[ephemient]

you need to explicitly match everything or add else

[ephemient]

see https://kotlinlang.org/docs/whatsnew1530.html#exhaustive-when-statements-for-sealed-and-boolean-subjects

[Roukanken]

currently in when statements, you don't need else - there is an implicit "else -> do nothing" in statements (when's without return value)

in 1.7 this will no longer be case, and you'll need to add the else clause even in statements

[ephemient]

there is still an implicit "else -> do nothing" for non-sealed non-enum types

[Roukanken]

and yeah, this is only a topic when we're talking about sealed/enums

[ghosalmartin]

hmm okays but what about if am attaching the result of a when to a variable and the switching type is a sealed

[Roukanken]

in that case, then even in current version you need to list all the cases OR use an else
the 1.7 change is just, that it will work the same way even if you are not using the result of the when

[ephemient]

exactly. if the value is used, the when must be exhaustive, always - either by having full branch coverage or by else, regardless of the type. if the value is not used, previously Kotlin did not enforce exhaustive when, but for enums and sealed types it will.

[Richard Gomez]

@ghosalmartin What does your code look like? I ran into a strange issue last week with a when block and a `sealed interface.`It forced me to add an else statement despite having all the cases defined.

[ghosalmartin]

tbf this code is purely to flex the feature shall we say but

[ghosalmartin]

but this gives me the warning

[ghosalmartin]

sealed interface House {
    data class Semidetached(val sharedWall: String): House
    data class Detached(val noSharedWall: String): House
}

fun test(){
    val myHouse: House = House.Detached("")
    
    when(myHouse){
        is House.Detached -> myHouse.noSharedWall
        is House.Semidetached -> myHouse.sharedWall
    }
    
}

[ephemient]

IDE or command-line compiler? there's a number of IJ plugin bugs around this, such as https://youtrack.jetbrains.com/issue/KTIJ-21016 https://youtrack.jetbrains.com/issue/KTIJ-21190

[Richard Gomez]

IDE or command-line compiler? there's a number of IJ plugin bugs around this, such as

In my case it was the IDE. Good to know.