Mockito cannot mock/spy because : VM does not support modification of given type

[MastaP]

Hi,

After switching kotlin's api/language from 1.4 to 1.5, the following code:

    @Test
    fun test() {
        val deferredSupplier = spy(Supplier { Flux.empty<String>() })
        Flux.defer(deferredSupplier).blockLast()
        verify(deferredSupplier, times(1)).get()
    }

fails with

Cannot mock/spy class com.example.Test$$Lambda$318/0x00000008001dc840
Mockito cannot mock/spy because :
 - VM does not support modification of given type

I'm using org.mockito.kotlin:mockito-kotlin:3.2.0 and org.mockito:mockito-inline:3.12.4
The issue is reproduced in the following project, where the only difference between two commits is changing Kotlin's api from 1.4 to 1.5
https://github.com/MastaP/mockito-kotlin15-bug/actions
Kotlin version: 1.5.30

[abelkov]

The reason for the behavior change is that since Kotlin 1.5, the Kotlin compiler uses the JVM invokedynamic instruction for SAM adapters (lambdas which are passed where Java interface is expected) by default, as javac does. See https://youtrack.jetbrains.com/issue/KT-47307#focus=Comments-27-4994817.0-0.

As a workaround, you could change the code to use an anonymous object instead of a SAM lambda:

val deferredSupplier = spy(object : java.util.function.Supplier<Any> {
    override fun get(): Any = Flux.empty<String>()
})

Or you can compile the test source set with this compiler argument:

compileTestKotlin {
    kotlinOptions.freeCompilerArgs += ['-Xsam-conversions=class']
}

cc @udalov can something be improved on the Kotlin side or this is purely a Mockito issue?

[MastaP]

@abelkov thanks, the workaround helped