Why can't dagger process these kotlin generics?

Question

I'm having some weird kotlin generic issues with Dagger that I kinda fixed but the solution isn't sound.

Here's the dagger classes:

@Module class P5Module {
    @Provides fun pool(): RecyclerView.RecycledViewPool = RecyclerView.RecycledViewPool()
    @Provides
    fun adapters(fusion: P5FusionAdapter, personas: P5ListAdapter, skills: P5SkillsAdapter, info: InfoAdapter)
            : List<Pageable> = listOf(fusion, personas, skills, info)
}

@ActivityScope
@Subcomponent(modules = arrayOf(P5Module::class)) interface P5Component {
    fun adapter(): PageableAdapter
}

interface Pageable {
    fun manager(ctx: Context): LayoutManager
    fun attach()
    fun adapter(): Adapter<*>
}

class PageableAdapter
@Inject constructor(val pageables: List<Pageable>, val pool: RecyclerView.RecycledViewPool) :PagerAdapter()

When I build, I get this kapt error in the top level component:

e: C:\Users\daykm\StudioProjects\P5Executioner\app\build\tmp\kapt3\stubs\appDebug\com\daykm\p5executioner\AppComponent.java:17: error: [com.daykm.p5executioner.main.P5Component.adapter()] java.util.List<? extends com.daykm.p5executioner.view.Pageable> cannot be provided without an @Provides-annotated method.
e: 

e: public abstract interface AppComponent {
e:                 ^
e:       java.util.List<? extends com.daykm.p5executioner.view.Pageable> is injected at
e:           com.daykm.p5executioner.view.PageableAdapter.<init>(pageables, …)
e:       com.daykm.p5executioner.view.PageableAdapter is provided at
e:           com.daykm.p5executioner.main.P5Component.adapter()
e: java.lang.IllegalStateException: failed to analyze: org.jetbrains.kotlin.kapt3.diagnostic.KaptError: Error while annotation processing

I took a look at the stubs generated:

@javax.inject.Inject()
public PageableAdapter(@org.jetbrains.annotations.NotNull()
java.util.List<? extends com.daykm.p5executioner.view.Pageable> pageables, @org.jetbrains.annotations.NotNull()
android.support.v7.widget.RecyclerView.RecycledViewPool pool) {
    super();
}

@org.jetbrains.annotations.NotNull()
@dagger.Provides()
public final java.util.List<com.daykm.p5executioner.view.Pageable> adapters(@org.jetbrains.annotations.NotNull()

Apparently doesn't match with because when I modify the dagger class like so:

@Module class P5Module {
    @Provides fun pool(): RecyclerView.RecycledViewPool = RecyclerView.RecycledViewPool()
    @Provides
    fun adapters(fusion: P5FusionAdapter, personas: P5ListAdapter, skills: P5SkillsAdapter, info: InfoAdapter)
            : List<*> = listOf(fusion, personas, skills, info)
}

@ActivityScope
@Subcomponent(modules = arrayOf(P5Module::class)) interface P5Component {
    fun adapter(): PageableAdapter
}

interface Pageable {
    fun manager(ctx: Context): LayoutManager
    fun attach()
    fun adapter(): Adapter<*>
}

class PageableAdapter
@Inject constructor(val pageables: List<*>, val pool: RecyclerView.RecycledViewPool) :PagerAdapter()

The issue disappears. Am I bumping into some weird translation issue with covariance and contravariance? I'd rather not force a cast downstream.

Answer 1

Kotlin's List<out E> translates to covariance in Java's List<? extends E> by default. Dagger doesn't like that.

To get around this issue, you can either switch to MutableList<E> which is invariant, or write List<@JvmSuppressWildcards E>.