Generated parent component factory has an incorrect return type

[jonamireh]

Summary

When an @MergeSubcomponent is declared with an inner parent component interface, Anvil fails to generate a proper subclass of the parent component interface with the correct return type during component merging. What a mouthful, so let's look at an example 😅

Context

Consider the following, within the :sample:app module of this repository:

@SingleIn(LoggedInScope::class)
@MergeSubcomponent(LoggedInScope::class)
interface LoggedInComponent {
    @ContributesTo(AppScope::class)
    interface ParentBindings {
        fun createLoggedInComponent(): LoggedInComponent
    }
}

When Anvil generates the subcomponent, it will create the following subclass of ParentBindings where createLoggedInComponent is now overridded to the merged type MergedLoggedInComponent:

@Subcomponent(modules = [MergedLoggedInComponent.BindingModule::class])
public interface MergedLoggedInComponent : LoggedInComponent {
  @Module
  public interface BindingModule {
    @Binds
    public fun bindMergedLoggedInComponent(`impl`: MergedLoggedInComponent): LoggedInComponent
  }

  public interface ParentComponent : LoggedInComponent.ParentBindings {
    override fun createLoggedInComponent(): MergedLoggedInComponent
  }
}

When Anvil later performs component merging for AppScope, it will attempt to generate the following:

public interface MergedAppComponent : AppComponent, LoggedInComponent.ParentBindings,
    MergedLoggedInComponent.ParentComponent {
  override fun createLoggedInComponent(): LoggedInComponent = createLoggedInComponent()
}

MergedAppComponent#createLoggedInComponent cannot be overridden with a return type of LoggedInComponent because MergedLoggedInComponent.ParentComponent already overrides it to explicitly be MergedLoggedInComponent:

MergedAppComponent.kt:44:43 Return type of 'createLoggedInComponent' is not a subtype of the return type of the overridden member 'public abstract fun createLoggedInComponent(): MergedLoggedInComponent defined in com.squareup.anvil.sample.MergedLoggedInComponent.ParentComponent'

A patch is available to reproduce this example: issue.patch

Workaround

If you declare a @MergeComponent.Factory and return it from ParentBindings instead, the issue is no longer present:

@SingleIn(LoggedInScope::class)
@MergeSubcomponent(LoggedInScope::class)
interface LoggedInComponent {
    @MergeSubcomponent.Factory
    interface Factory {
        fun create(): LoggedInComponent
    }

    @ContributesTo(AppScope::class)
    interface ParentBindings {
        fun createLoggedInComponentFactory(): Factory
    }
}

[ZacSweers]

That's a tricky one, I'll try to look into it when I have time. In the meantime - PRs welcome and hopefully that workaround suits for now?

[jonamireh]

What's the purpose of the ParentComponent interface, if there's already a generated @Binds for the concrete subcomponent? It seems like if that didn't exist, everything would compile?

[ZacSweers]

It's hard to say off-hand without digging into the code (this all gets really murky with all the various vague names floating around) but it might be that we already generate it and it's a bit of the left hand not knowing about the right hand in this area of code gen