Reports delegation when the delegate object is an instance of a class that overrides (in the case of final and sealed classes) or may override (in the case of open and abstract classes) Java default methods.

By design, Kotlin delegation does not override Java default methods, which can lead to subtle bugs, especially in cases of deep inheritance hierarchies.

Example:

// Base.java:
public interface Base {
  default void print() {
    System.out.println("Base");
  }
}

// BaseImpl.kt:
class BaseImpl : Base {
  override fun print() {
    println("BaseImpl")
  }
}

// Main.kt:
fun main() {
  val inherited = BaseImpl()
  val delegated = object : Base by inherited {}
  inherited.print() // Outputs: BaseImpl
  delegated.print() // Outputs: Base
}

Developers typically expect that all methods, including default methods overridden in the delegate, are forwarded. Instead, Java default methods are called, which can lead to unexpected results.

One quick-fix overrides Java default methods by delegating their implementation to the delegate object:

// Main.kt:
fun main() {
  val inherited = BaseImpl()
  val delegated = object : Base by inherited {
    override fun print() {
      inherited.print()
    }
  }
  inherited.print() // Outputs: BaseImpl
  delegated.print() // Outputs: BaseImpl
}

Another quick-fix overrides Java default methods by explicitly delegating their implementation to the superclass:

// Main.kt:
fun main() {
  val inherited = BaseImpl()
  val delegated = object : Base by inherited {
    override fun print() {
      super.print()
    }
  }
  inherited.print() // Outputs: BaseImpl
  delegated.print() // Outputs: Base
}

New in 2025.1