Reduce two (Data) Classes in Kotlin

ClazzesExt.kt

inline infix fun <reified T : Any> T?.reduce(other: T?): T? = when {
  this == null && other == null -> null
  this != null && other == null -> this
  this == null && other != null -> other
  else -> {
    val nameToProperty = T::class.declaredMemberProperties.associateBy { property -> property.name }
    val primaryConstructor = T::class.primaryConstructor!!
    val args = primaryConstructor.parameters.associate { parameter ->
      val property = nameToProperty[parameter.name]!!

      val revertPrivateVisibility = if (property.visibility == PRIVATE || property.visibility == PROTECTED) {
        property.isAccessible = true
        true
      } else false

      parameter to (property.get(other!!) ?: property.get(this!!)).also {
        if (revertPrivateVisibility) property.isAccessible = false
      }
    }
    primaryConstructor.callBy(args)
  }
}

ClazzesExtKtTest.kt

class ClazzesExtKtTest {
  companion object {
    private const val DEFAULT_STRING = "TEST"
  }

  @Test fun `Reduce two classes should return the correct values`() {
    var mutable: TestClassToMerge? = TestClassToMerge()

    mutable = mutable reduce TestClassToMerge(_booleanValue = true)

    mutable.`should not be null`()
    mutable.booleanValue.`should be true`()
    mutable.list.`should equal`(DEFAULT_LIST)

    mutable = mutable reduce TestClassToMerge(
      _booleanValue = false,
      _stringValue = DEFAULT_STRING,
      _listValue = listOf(DEFAULT_STRING)
    )

    mutable.`should not be null`()
    mutable.booleanValue.`should be false`()
    mutable.stringValue.`should be equal to`(DEFAULT_STRING)
    mutable.list.`should not be empty`()
  }
}

@Suppress("MemberVisibilityCanBePrivate")
open class TestClassToMerge(
  private val _booleanValue: Boolean? = null,
  internal val _stringValue: String? = null,
  protected val _listValue: List<String>? = null
) {

  companion object {
    const val DEFAULT_BOOLEAN = false
    const val DEFAULT_STRING = ""
    val DEFAULT_LIST = listOf<String>()
  }

  val booleanValue: Boolean
    get() = _booleanValue ?: DEFAULT_BOOLEAN

  val stringValue: String
    get() = _stringValue ?: DEFAULT_STRING

  val list: List<String>
    get() = _listValue ?: DEFAULT_LIST
}

Based on this awesome gist by @josdejong https://gist.github.com/josdejong/fbb43ae33fcdd922040dac4ffc31aeaf