Is it expected that the following snippet gives me...

# getting-started

[Tom De Decker]

Is it expected that the following snippet gives me an error indicating that then when must be exhaustive? As far as I can tell, the when is exhaustive here due to the condition above it. Is this a limitation of the current compiler or am I overlooking something?

sealed interface Base {
    data class Foo(val foo: String) : Base
    data class Bar(val bar: String) : Base
    data class Baz(val baz: String) : Base
}

fun doThing(thing: Base) {
    if (thing is Base.Foo || thing is Base.Bar) {
        when (thing) {
            is Base.Foo -> thing.foo
            is Base.Bar -> thing.bar
            // Error: When expression must be exhaustive
        }
    }
}

[Sam]

The check for exhaustiveness is confined to the when block itself. The if statement doesn’t participate in the exhaustiveness check. It could narrow the type via a smart cast, but unfortunately the common supertype of Base.Foo and Base.Bar is still Base, so there’s nothing to narrow it to.

[Sam]

In other words, the when block has to cover all possible values of the type of its subject. And the compile-time type of the when subject is Base.

[Sam]

Maybe when we have union types, it will be able to work the way you’re hoping 🤞!

[Tom De Decker]

Alright, that makes sense, thanks!

[spleenjack]

there are several issues on both the compiler and plugin:

https://youtrack.jetbrains.com/issue/KT-8781/Consider-making-smart-casts-smart-enough-to-handle-exhaustive-value-sets

https://youtrack.jetbrains.com/issue/KTIJ-20749/Exhaustive-when-check-does-not-take-into-account-the-values-excluded-by-previous-if-conditions

[Stephan Schröder]

so is when now exhaustive even if it's a statement!?

[Sam]

Yes