MissingFieldException when trying to deserialize enum field in sealed hierarchy

[msalikhov]

Describe the bug
Exception occurred when trying to deserialize object as sealed root, if it contains field with unknown enum value.
Exception in thread "main" kotlinx.serialization.MissingFieldException: Field 'status' is required for type with serial name 'org.example.Feature', but it was missing at path: $

To Reproduce

@Serializable
internal sealed class TestSealed {
    @Serializable
    @SerialName("foo")
    data class FirstSealed(@SerialName("feature") val feature: Feature) : TestSealed()
}

@Serializable
internal data class Feature(
    @SerialName("status") val status: FeatureStatus?,
)

@Serializable
internal enum class FeatureStatus {
    @SerialName("first") First,
    @SerialName("second") Second,
}

val json = """{
    "feature": {
      "status": "unknown"
    },
    "type": "foo"
}"""

fun main() {
    val parser = Json {
        coerceInputValues = true
        explicitNulls = false
        ignoreUnknownKeys = true
    }
    println(parser.decodeFromString<TestSealed>(json))
}

If try to decode as concrete impl, than all is ok parser.decodeFromString<TestSealed.FirstSealed>(json)

Also noticed that moving "type" key up - fixes the problem. Does the order of keys is important?

val json = """{
    "type": "foo",
    "feature": {
      "status": "unknown"
    }
}"""

Expected behavior
Decode success, and prints: FirstSealed(feature=Feature(status=null))

Environment

• Kotlin version: [2.1.0]
• Library version: [1.8.0]
• Kotlin platforms: [JVM]
• Gradle version: [8.10]
• IDE version [IntelliJ IDEA 2024.3.1.1 (Community Edition)]
• Other relevant context [MacOS, OpenJDK 23]

[sandwwraith]

Yes, that indeed looks like a bug

[msalikhov]

I managed to get a little closer to the problem.
That code throws exception:

val parser = Json {
      coerceInputValues = true
      explicitNulls = false
      ignoreUnknownKeys = true
    }
    val string = """{"status": "unknown"}"""
    val json = parser.parseToJsonElement(string)
    parser.decodeFromJsonElement<Feature>(json)

@Serializable
internal data class Feature(
  @SerialName("status") val status: FeatureStatus?,
)

@Serializable
internal enum class FeatureStatus {
  @SerialName("first") First,
  @SerialName("second") Second,
}

And that is ok:

val parser = Json {
      coerceInputValues = true
      explicitNulls = false
      ignoreUnknownKeys = true
    }
    val string = """{"status": "unknown"}"""
    parser.decodeFromString<Feature>(string)

When parsing JsonElement (which fails), JsonTreeDecoder is used, and StreamingJsonDecoder is used with string

[msalikhov]

If you try to manually deserialize in such manner

val composite = decoder.beginStructure(descriptor)
    val index = composite.decodeElementIndex(descriptor)
    println("index: $index")

When parsing json element index will be -1
When parsing string index will be 0

[msalikhov]

This condition inside kotlinx.serialization.json.internal.JsonTreeDecoder#decodeElementIndex seems broken?
(!configuration.coerceInputValues || !coerceInputValue(descriptor, index, name))

It looks like ! before configuration.coerceInputValues not needed. Or i missed something?