I recently started using `Result` as a return type as of Kot

Question

I recently started using `Result` as a return type as of Kotlin 1 5 I m having seemingly random crashes with suspending functions that return `Result` I haven t checked with other inline value classes it s happening in the generated `invokeSuspend` ```Fatal Exception java lang ClassCastException java util ArrayList cannot be cast to kotlin Result``` I don t have reliable repro steps for it it seems to happen on some android devices at different places but all the same crash but not always cry I m really stumped with it I ve been looking at the difference in the decompiled bytecode between suspending and non suspending functions that return `Result` I can see casting of some `Object` to `Result` assuming that s what s failing confused

stojan · Answer

`suspend fun foo A` basically desugards to `fun foo Result lt A gt ` it s a bit more complicated because of async stuff but it s the basic idea this means it can fail with `Throwable` or succeed with `A` that means that `suspend fun bar lt Result lt A gt gt ` desugars to `fun bar Result lt Result lt A gt gt ` meaning it can fail with `Throwable` outer result fail with `Throwable` or succeed with `A` The recommended solution merge the two `Throwable` failures and change the return type to ``suspend fun bar A`` You can then use `runCatching` or `try catch` when launching your coroutine to handle the failure

elizarov · Answer

Any chance you could reproduce it

aidanvii · Answer

Yea I considered unwrapping the Result s type as the return type and might be my only option Ideally though I d like to express that it could fail in the method signature

rkeazor · Answer

This is probably related to this issue <https youtrack jetbrains com issue KT 46477 focus=Comments 27 4952485 0 0>

rkeazor · Answer

< elizarov> I was also seeing a similar issue and had to downgrade Its mention that it should be resolved in kotlin 1 5 30

aidanvii · Answer

I m gonna have to replace all usages of `Result` with a non inlined version until it s resolved It s less hassle than unwrapping all return types Ideally we could somehow force it not to be inlined until it s fixed I m using Jetpack Compose which seems to lag behind kotlin versions so I imagine I ll need to wait a bit longer smiling face with tear

aidanvii · Answer

If there was a way to easily replace all usages like with the deprecation replace with functionality that can import the replaced APIs that d be great

aidanvii · Answer

I m not really fussed on losing performance here it s more about safety I care about with the `Result` API

rkeazor · Answer

You could just wait till 1 5 30 is out It s already on its Release candidate

aidanvii · Answer

I hope it fixes it In my case it was a class cast from T to Result lt T gt not the other way around as this bug states

rkeazor · Answer

try the release canaidate and see if the issue is resolve

rkeazor · Answer

or you can opt out of the IR backend

aidanvii · Answer

Trying with the IR backend disabled now yea

aidanvii · Answer

Ok ok just to clear this up it s 100 this bug here <https youtrack jetbrains com issue KT 47206>

aidanvii · Answer

I can see that my code is doing the same `suspend` function that returns a `Result` but it has a parameter with a default value removing the default value or removing the `suspend` operator works though in my case I can t remove the `suspend`

aidanvii · Answer

Here is the minimal repro steps I found simpler than the bug

aidanvii · Answer

```suspend fun main args Array lt String gt getResult true onSuccess println it A OK getResult onSuccess println it ClassCastException suspend fun getResult arg Boolean = true Result lt Boolean gt = Result success arg ```

aidanvii · Answer

And yes I ve tested 1 5 21 and 1 5 30 RC it is indeed gone in the latter