Saw this error streaming from one module `'when' expression must be exhaustive. Add an 'else' branch...

# k2-adopters

[ritesh]

Saw this error streaming from one module
'when' expression must be exhaustive. Add an 'else' branch.
• I don't see any warning or lint error in AS IDE
• I can confirm, the when statement is exhuastive

[dmitriy.novozhilov]

It would be really helpful if you share some code

[ritesh]

Sure.

when (either) {
  is Either.Success -> {

  }
  is Either.Failure -> {

  }
}

sealed class Either<out S, out F> {
  data class Success<S, F> internal constructor(val success: S) : Either<S, F>()
  data class Failure<S, F> internal constructor(val failure: F) : Either<S, F>()
}

[dmitriy.novozhilov]

What is the type of either?
Is it Either<*, *>?

[ritesh]

It's basically gets wrapped in Rx Single..
Something like

Single<Either<Object1, Object2>>

[gildor]

Are you sure that either is not nullable?

[dmitriy.novozhilov]

sealed class Either<out S, out F> {
    data class Success<S, F> internal constructor(val success: S) : Either<S, F>()
    data class Failure<S, F> internal constructor(val failure: F) : Either<S, F>()
}

fun test(either: Either<Int, String>) {
    val x = when (either) {
        is Either.Success -> 1
        is Either.Failure -> 2
    }
}

This code compiles completely fine with 2.0.0

[dmitriy.novozhilov]

A side note: second type parameter for Success and Failure is redundant
Such hierarchy will be much more useful

sealed class Either<out S, out F> {
    data class Success<S>(val success: S) : Either<S, Nothing>()
    data class Failure<F>(val failure: F) : Either<Nothing, F>()
}

[dmitriy.novozhilov]

Wait a minute
Type of either is Single<Either<Object1, Object2>>, not Either<Object1, Object2>?
And Single is not part of Either hierarchy?
In this case the compiler is entirely correct

[ritesh]

Are you sure that either is not nullable?

I can cross check - 👀
In either case, the IDE should be warning for exhaustive when statement pre kotlin 2.0.0 as well right? I don't see any AS IDE error.

[gildor]

K2 compiler is not used in IDE, so IDE may show different results comparing to compiler

[ritesh]

https://kotlinlang.slack.com/archives/C03PK0PE257/p1721030806477059?thread_ts=1721029809.291729&cid=C03PK0PE257

Sure, i will cross check once again, from my end.

Even though it's wrapped in Rx Single. Subscribing to RxSingle gives EitherObject1, Object2 type

Something like

Single<Either<Object1, Object2>>.subscribe { either ->
when(either) {
}
}

[ritesh]

Code structure looks like this

val single:Single<Either<Object1,Object2>> = if (url == null) {
        api.getSingleObject()
      } else {
        Single.just(Either.success(Object1()))
      }

single.subscribe { either ->
  when(either) {
    Either.Success -> {}
    Either.Failiure -> {}
  }
}

• I can confirm it's not null, as the single declaration is non-nullable.
If i don't use if statements and directly use one of the calls from if block or else block it compiles well.

[dmitriy.novozhilov]

What is the signature of api.get?

[ritesh]

it returns Single<Either<Object1, Object2>>

[dmitriy.novozhilov]

It is java method or kotlin?

[ritesh]

The api.get() is in kotlin and all it's surrounding classes, however Single is from Rx-java a third party library, a java class

[dmitriy.novozhilov]

Still compiles fine with 2.0.0

import rx.Single

sealed class Either<out S, out F> {
    data class Success<S, F> internal constructor(val success: S) : Either<S, F>()
    data class Failure<S, F> internal constructor(val failure: F) : Either<S, F>()

    companion object {
        fun <S, F> success(x: S): Success<S, F> = Success(x)
    }
}

class Object1
class Object2

class Api {
    fun getSingleObject(): Single<Either<Object1, Object2>> = null!!
}

fun test(api: Api, cond: Boolean) {
    val single: Single<Either<Object1, Object2>> = if (cond) {
        api.getSingleObject()
    } else {
        Single.just(Either.success(Object1()))
    }

    single.subscribe { either ->
        val x = when(either) {
            is Either.Success -> 1
            is Either.Failure -> 2
        }
    }
}

[ritesh]

Running like this
api.getSingleObject().subscribe(...)
or
Single.just(Either.success(Object1())).subscribe(..)

gets through the compiler, but not when using single - 😕

I understand, the code you shared mimicks the same behaviour but.. 👀

[dmitriy.novozhilov]

Please create an issue if you can provide a self-contained example of code which doesn't work (ideally if it would be working gradle project)

[ritesh]

Sure

[ritesh]

On other note - it would have been helpful, if KGP lint in Android Studio showed same error in code.
Probably when K2 is enabled for IDE in future releases.

[dmitriy.novozhilov]

Yes, it will be fixed when IDE fully migrate to K2

[ritesh]

single.subscribe { either: Either<Object1, Object2> ->
        val x = when(either) {
            is Either.Success -> 1
            is Either.Failure -> 2
        }
    }

Adding explicit type to the either in lambda, worked for us.

Still believe, it's an issue - will open a ticket with reproducing or sample project. Thanks for the help.

[dmitriy.novozhilov]

Thank you too

[gildor]

I think I know why, because subscibe lambda param has platform type, so adding null check would help (or adding explicit non-nullable type)
But it's indeed not how old compiler works, and platform type is fine for this case

[ritesh]

Ohh, i thought by default kotlin treats platform types as non nullable. Probably, it got changed in k2 - as it became more strict towards erroring out nullability.

[gildor]

Kotlin treats platform types both as nullable and non-nullable, I honestly wouldn't expect k2 to change behaviour here