In a `when` expression over the value of some type `T ` it i

Question

In a `when` expression over the value of some type `T ` it is not enough to have the cases `is T` and `null` or `is T` and ` is T` you still need an `else` case to be exhaustive is this a known bug or is there a reason for this

Andrew · Answer

are you in an `inline reified` method

y · Answer

I don t think so it s an `override fun` of an `interface` the implementor is an `object`

Andrew · Answer

kotlin along with java uses type erasure of generics at runtime so you don t actually know what type `T` is at runtime unless you are using an inline reified function

y · Answer

so `T` unifies with `T ` at runtime

Andrew · Answer

`is T` doesn t really do what you think it does at runtime

Andrew · Answer

I d recommend googling things like kotlin type erasure generics inline reified etc

Andrew · Answer

can probably explain it better than I can

y · Answer

thank you I ll re read about `reified` anyway the compiler seems to reject this even here ```inline fun lt reified T gt theValueOf k T Int = when k is T gt 1 null gt 0 ``` and here ```inline fun theValueOf k Int Int = when k is Int gt 1 null gt 0 ```

ephemient · Answer

in this case it s because the compiler only tries keeping track of exhaustive for enums and sealed classes

ephemient · Answer

```sealed interface X class Y X class Z X fun f x X = when x is Y gt 1 is Z gt 2 null gt 0 ``` this works for example

y · Answer

indeed which is where my surprise came from my very naive expectation would be that `T ` would be treated as sealed class of `T` and `null`

ephemient · Answer

I think a more reasonable expectation is that the `when` cases cover the types `T` and `Nothing ` whose union is `T ` but the compiler isn t currently about to use that

y · Answer

it s also surprising to me that `is T` and ` is T` is not enough to be exhaustive

ephemient · Answer

<https youtrack jetbrains com issue KT 47709|https youtrack jetbrains com issue KT 47709> under current language design only enums sealed Boolean which is basically like a small enum and nullable versions of them are handled for when exhaustiveness

ephemient · Answer

checking `is T` doesn t do what you want unless ` lt reified T Any gt ` also if your `null` check is first then the subject will be smart cast to a `T amp Any` in following branches such as `else`