如何验证Kotlin中是否调用了类扩展功能？

Question

假设我要验证ShopCart.itemUpdated()是否被调用：

data class ShopCart(val item: Item)

data class Item(val price: Int)

class ShopCartController(private val cart: ShopCart) {
    fun updateCart() {
        cart.itemUpdated()
    }

    fun ShopCart.itemUpdated() = copy(
        item = item.copy(price = 20)
    )
}

class ShopCartControllerTest {
    @Test
    fun test() {
        val cart = ShopCart(Item(0))
        val cartSpy = spy(cart)
        val controller = ShopCartController(cartSpy)

        controller.updateCart()

        with(controller) {
            verify(cartSpy).itemUpdated()
        }
    }

但是，此测试失败并抛出java.lang.NullPointerException。

java.lang.NullPointerException
  at com.test.ShopCartController.itemUpdated(Test.kt:28)
  at com.test.ShopCartControllerTest.testMe(Test.kt:17)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:498)
  at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
  at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
  at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
  at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
  at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
  at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
  at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
  at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
  at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

是否可以验证是否调用了类扩展功能？我已经尝试了几种模拟框架（例如mockito，mockk），但是没有运气。你们有什么主意吗？