@AutoValue
abstract class Foo {
  @AutoValue.Builder
  interface Builder {
    Nested.Builder nestedBuilder();
  }
  
  abstract Optional<Nested> nested();
}

@AutoValue
abstract class Nested {
  
  static Builder builder() {
    return new AutoValue_Nested.Builder();
  }
  
  @AutoValue.Builder
  interface Builder {
  }
}