Mockito Kotlin is breaking with new type inference

I’m referring to https://youtu.be/MyljSWm0Y_k?t=1282 I turned it on this way

    tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
        kotlinOptions {
            // progressive helps detecting unstable Kotlin code earlier, see https://blog.jetbrains.com/kotlin/2018/06/kotlin-1-2-50-is-out/
            freeCompilerArgs = ["-Xjsr305=strict", "-progressive", "-XXLanguage:+NewInference", "-XXLanguage:+InlineClasses"]
            allWarningsAsErrors = true
        }
    }

The error is the following

e: /Users/rwondratschek/dev/projects/evernote/evernote/app/src/test/kotlin/com/evernote/EvernoteAppSetupRule.kt: (68, 64): Overload resolution ambiguity:
public inline fun <reified T : Any> mock(extraInterfaces: Array<KClass<out Any>>? = ..., name: String? = ..., spiedInstance: Any? = ..., defaultAnswer: Answer<Any>? = ..., serializable: Boolean = ..., serializableMode: SerializableMode? = ..., verboseLogging: Boolean = ..., invocationListeners: Array<InvocationListener>? = ..., stubOnly: Boolean = ..., @Incubating useConstructor: UseConstructor? = ..., @Incubating outerInstance: Any? = ...): TypeVariable(T) defined in com.nhaarman.mockitokotlin2
public inline fun <reified T : Any> mock(extraInterfaces: Array<KClass<out Any>>? = ..., name: String? = ..., spiedInstance: Any? = ..., defaultAnswer: Answer<Any>? = ..., serializable: Boolean = ..., serializableMode: SerializableMode? = ..., verboseLogging: Boolean = ..., invocationListeners: Array<InvocationListener>? = ..., stubOnly: Boolean = ..., @Incubating useConstructor: UseConstructor? = ..., @Incubating outerInstance: Any? = ..., stubbing: KStubbing<TypeVariable(T)>.(TypeVariable(T)) -> Unit): TypeVariable(T) defined in com.nhaarman.mockitokotlin2
@Deprecated public inline fun <reified T : Any> mock(a: Answer<Any>): TypeVariable(T) defined in com.nhaarman.mockitokotlin2

The issue should be easy to reproduce, basically every call like this fails

        val account = mock<AppAccount> {
            on { databaseHelper } doThrow IllegalStateException()
        }

[nhaarman]

This is something that breaks compilation of the library itself as well when -XXLanguage:+NewInference is enabled. Since there isn’t any (at least I haven’t found any) official documentation or discussion available on this “new type inference” system, it’s hard to tell how the Kotlin team will handle issues like these.

One specific case in the library itself is argThat(T.() -> Boolean) and argThat(ArgumentMatcher), which results in the following compilation error:

e: /home/nhaarman/dev/nhaarman/mockito-kotlin/mockito-kotlin/src/main/kotlin/com/nhaarman/mockitokotlin2/Matchers.kt: (93, 12): Overload resolution ambiguity: 
public inline fun <reified T : Any> argThat(noinline predicate: TypeVariable(T).() -> Boolean): TypeVariable(T) defined in com.nhaarman.mockitokotlin2
public inline fun <reified T : Any> argThat(matcher: ArgumentMatcher<TypeVariable(T)>): TypeVariable(T) defined in com.nhaarman.mockitokotlin2

The simplest solution is to remove the custom functions that accept a lambda and let the new type inference system handle this. However, this is a breaking change and will affect users that don’t have the new type inference system enabled. Since the -XXLanguage:+NewInference is an internal compiler argument, I’m favoring for not addressing this issue in 2.x.
I know it’s a pain for people that do have this flag enabled, but stuff like this is expected for experimental features.

3.x might be able to support his, but for support for the new type inference to have any standing at least some official documentation and stance from the Kotlin team needs to be available, and the feature needs to be stable.

[wiyarmir]

I’ve got a less cosmetic solution which is to specify the parameter name, so the overload resolution works.

val account = mock<AppAccount>(stubbing = {
// on {} etc...
})