java.lang.ClassCastException: kotlin.Result cannot be cast to java.util.List

Prerequisites

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• I am running the latest version
• I checked the documentation and found no answer
• I checked to make sure that this issue has not already been filed

Expected Behavior

I’m testing a class that has a suspend fun and returns Result<List<..>>. In a unit test I’m mocking this class and want to return a Result.success(...) from that method. Expecting that my mock returns that value when called, but it crashes instead.

First I thought it’s a Kotlin bug and wanted to file an issue there, however, in production and when removing the mockk calls, the code works as expected, so I assume it’s instead a mockk bug.

This sample code snippet:

class ExampleUnitTest {

    private val testClass = mockk<TestClass>()

    @Test
    fun `test bug`() = runBlockingTest {
        coEvery { testClass.execute() } returns Result.success(listOf("1"))

        testClass.execute()
          .fold(
            onSuccess = {
                println(it)
            },
            onFailure = {
                println("fail")
            }
          )
    }
}

class TestClass {

    suspend fun execute(): Result<List<String>> {
        return suspendCoroutine {
            it.resume(Result.success(listOf("")))
        }
    }
}

crashes with

java.lang.ClassCastException: kotlin.Result cannot be cast to java.util.List

	at de.syex.resultbug.ExampleUnitTest$test bug$1.invokeSuspend(ExampleUnitTest.kt:24)
	at de.syex.resultbug.ExampleUnitTest$test bug$1.invoke(ExampleUnitTest.kt)
	at kotlinx.coroutines.test.TestBuildersKt$runBlockingTest$deferred$1.invokeSuspend(TestBuilders.kt:50)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56)
	at kotlinx.coroutines.test.TestCoroutineDispatcher.dispatch(TestCoroutineDispatcher.kt:50)
	...

The offending line 24 is the fold(). I found this issue while testing some code that actually runs fine. Instantiating TestClass instead

private val testClass = TestClass()

lets the test pass as expected.

• MockK version: 1.10.0
• Kotlin version: 1.4.0
• Coroutine version: 1.3.9
• JDK version: 1.8
• JUnit version: 4.12 & 5.6.2
• Gradle version: 6.1.1 & 6.5.1
• Type of test: unit test

[dkuteynikov]

I have reported it straight to the Kotlin team: https://youtrack.jetbrains.com/issue/KT-46477#focus=Comments-27-4952485.0-0 Because it is a language-level bug.

[Raibaz]

I’ve seen the implementation that has been done in Mockito to handle this.

It seems like we can replicate it in mockk, it’s basically about trying to unbox the underlying value contained in the Result class if possible and returning it in the answerer.

However, it’s only feasible from Kotlin 1.5.0 onwards, because it requires the @JvmInline annotation to be available and set on the Result class (or any other inline class, for that matter).

Given the fact that 1.5.0 was released very recently and using Result as a return type is not possible by default but it requires the -Xallow-result-return-type kotlin option to be set, this doesn’t look to me like a high priority issue right now.

Reopening it for now and leaving it here in case anyone wants to submit a PR.