ChildBindings.kt

package com.google.bugs.daggerchild

import dagger.Binds
import dagger.BindsOptionalOf
import dagger.Component
import dagger.Module
import dagger.Provides
import dagger.Subcomponent
import java.util.Optional
import javax.inject.Inject
import javax.inject.Qualifier
import javax.inject.Singleton

interface Coffee

data object Sugar

class JudgmentalCoffee @Inject internal constructor(private val sugar: Optional<Sugar>): Coffee {
  override fun toString() = if (sugar.isPresent) "unhealthy coffee" else "healthy coffee"
}

@Module
internal interface ParentModule {
  @Binds fun coffee(impl: JudgmentalCoffee): Coffee
  @BindsOptionalOf fun sugar(): Sugar
}

@Qualifier annotation class Opinion

@Module
internal object ChildModule {
  @Provides fun sugar() = Sugar
  @Provides @Opinion fun opinion(coffee: Coffee) = "you drink $coffee"
}

@Subcomponent(modules = [ChildModule::class])
interface ChildComponent {
  @Opinion fun opinion(): String
}

@Singleton
@Component(modules = [ParentModule::class])
interface ParentComponent {
  fun child(): ChildComponent
}

fun main() {
  println(DaggerParentComponent.create().child().opinion())
}