sealed interface with value class subtypes + JsonContentPolymorphicSerializer: Index -1 out of bounds for length 0

[aSemy]

Describe the bug

Requirement is to decode polymorphic JSON, where the JSON can be one of

• String
• Set<String>
• Map<String, String>

To try to solve this I created a sealed interface, AnyValue, with 3 value class subtypes. To polymorphically decode I defined a JsonContentPolymorphicSerializer.

To Reproduce

import kotlinx.serialization.DeserializationStrategy
import kotlinx.serialization.Serializable
import kotlinx.serialization.decodeFromString
import kotlinx.serialization.json.*

fun main() {

  // plain string -> works
  val anyValueString: AnyValue = Json.decodeFromString(""" "I'm a string" """)
  println(anyValueString)


  val complexJson = """ {"id": "1", "name": "object"} """

  // works
  val anyValueComplex1: AnyValue.Complex = Json.decodeFromString(complexJson)
  println(anyValueComplex1)

  try {
    // fails
    val anyValueComplex2: AnyValue = Json.decodeFromString(complexJson)
    println(anyValueComplex2)
  } catch (e: Exception) {
    println("⚠️ anyValueComplex2 failed")
  }

  val multiJson = """["list", "of", "strings"]"""

  // works
  val anyValueList1: AnyValue.Multi = Json.decodeFromString(multiJson)
  println(anyValueList1)

  try {
    // fails
    val anyValueList2: AnyValue = Json.decodeFromString(multiJson)
    println(anyValueList2)
  } catch (e: Exception) {
    println("⚠️ anyValueList2 failed")
  }
}


@Serializable(with = AnyValue.Companion.Serializer::class)
sealed interface AnyValue {

  @JvmInline
  @Serializable
  value class Single(val value: String) : AnyValue

  @JvmInline
  @Serializable
  value class Multi(val values: List<String>) : AnyValue

  @JvmInline
  @Serializable
  value class Complex(val values: Map<String, String>) : AnyValue

  @JvmInline
  @Serializable
  value class Unknown(val value: JsonElement) : AnyValue

  companion object {
    object Serializer : JsonContentPolymorphicSerializer<AnyValue>(AnyValue::class) {

      override fun selectDeserializer(element: JsonElement): DeserializationStrategy<out AnyValue> =
        when {
          element is JsonArray && element.all { it is JsonPrimitive && it.isString }         -> {
            println("selected Multi.serializer()")
            Multi.serializer()
          }

          element is JsonObject && element.values.all { it is JsonPrimitive && it.isString } -> {
            println("selected Complex.serializer()")
            Complex.serializer()
          }

          element is JsonPrimitive && element.isString                                       -> {
            println("selected Single.serializer()")
            Single.serializer()
          }

          else                                                                               -> {
            println("selected Unknown.serializer()")
            Unknown.serializer()
          }
        }
    }
  }
}

selected Single.serializer()
Single(value=I'm a string)
selected Complex.serializer()
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index -1 out of bounds for length 0
	at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
	at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
	at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:248)
	at java.base/java.util.Objects.checkIndex(Objects.java:372)
	at java.base/java.util.ArrayList.remove(ArrayList.java:536)
	at kotlinx.serialization.internal.TaggedDecoder.popTag(Tagged.kt:322)
	at kotlinx.serialization.internal.TaggedDecoder.decodeInline(Tagged.kt:213)
	at AnyValue$Complex$$serializer.deserialize-0-2muUA(main.kt:30)
	at AnyValue$Complex$$serializer.deserialize(main.kt:30)
	at kotlinx.serialization.json.internal.PolymorphicKt.decodeSerializableValuePolymorphic(Polymorphic.kt:61)
	at kotlinx.serialization.json.internal.AbstractJsonTreeDecoder.decodeSerializableValue(TreeJsonDecoder.kt:52)
	at kotlinx.serialization.json.internal.TreeJsonDecoderKt.readJson(TreeJsonDecoder.kt:25)
	at kotlinx.serialization.json.Json.decodeFromJsonElement(Json.kt:115)
	at kotlinx.serialization.json.JsonContentPolymorphicSerializer.deserialize(JsonContentPolymorphicSerializer.kt:93)
	at kotlinx.serialization.json.internal.StreamingJsonDecoder.decodeSerializableValue(StreamingJsonDecoder.kt:70)
	at kotlinx.serialization.json.Json.decodeFromString(Json.kt:95)
	at MainKt.main(main.kt:86)
	at MainKt.main(main.kt)

Expected behavior

• """ {"id": "1", "name": "object"} """ can be decoded to AnyValue.Complex
• """["list", "of", "strings"]""" can be decoded to AnyValue.Multi

Environment

• Kotlin version: 1.8.0
• Library version: 1.4.1
• Kotlin platforms: JVM
• Gradle version: 7.8

[sandwwraith]

Also #2049