When decodeFromJsonElement, enum parameters with no default value throw NoSuchElementException

[dahyun1226]

Describe the bug
The decodeFromJsonElement function uses the JsonTreeDecoder when element is a JsonObject.
However, the decodeElementIndex function in the JsonTreeDecoder can reach the coerceInputValue function with the explicitNulls = false option and coerceInputValues = true option provided by Json, and the situation of enum parameters with no default value.

In coerceInputValue function, currentElement(tag) can throw NoSuchElementException because currentElement assume that value with tag exist.

I don't think this was intended.

To Reproduce
It's a bit of a contrived situation test, but I think it's good enough to reproduce the situation.
The following tests will fail.

    @Serializable
    enum class TestEnum {
        ENUM_A
    }

    @OptIn(ExperimentalSerializationApi::class)
    private val json = Json {
        prettyPrint = false
        ignoreUnknownKeys = true
        encodeDefaults = true
        explicitNulls = false
        coerceInputValues = true
    }

    object TestClassSerializer : KSerializer<TestClass> {
        override val descriptor: SerialDescriptor
            get() = TestClassSurrogate.serializer().descriptor

        override fun deserialize(decoder: Decoder): TestClass {
            val surrogate = decoder.decodeSerializableValue(TestClassSurrogate.serializer())
            return TestClass(testEnum = surrogate.testEnum)
        }

        override fun serialize(encoder: Encoder, value: TestClass) {}
    }

    @Serializable(with = TestClassSerializer::class)
    data class TestClass(val testEnum: TestEnum?)

    @Serializable
    data class TestClassSurrogate(val testEnum: TestEnum?)

    @Test
    fun test() {
        val jsonString = """{}""".trimIndent()

        val jsonElement = json.decodeFromString(JsonElement.serializer(), jsonString)

        val actualTestClass = json.decodeFromJsonElement<TestClass>(jsonElement)

        Truth.assertThat(actualTestClass.testEnum).isNull()
    }

Environment

• Kotlin version: [1.7.10]
• Library version: [1.4.1]
• Kotlin platforms: [JVM]
• Gradle version: [7.4]
• IDE version (if bug is related to the IDE) [Android Studio Electric Eel 2022.1.1 Patch 2]

[sandwwraith]

Please check 1.5.0 — it has fix for #2170 which looks similar

[dahyun1226]

The error still occurs in version 1.5.0.
It is still the same to use currentElement when accessing peekNull, and this is where the error occurs.
A simpler example:

    @Serializable
    enum class TestEnum {
        ENUM_A, DEFAULT
    }

    @Test
    fun `test`() {
        @Serializable
        data class TestClass(val testEnum: TestEnum?)

        val jsonString = """{}""".trimIndent()

        val jsonElement = Json {
            prettyPrint = false
            ignoreUnknownKeys = true
            encodeDefaults = true
            explicitNulls = false
            coerceInputValues = true
        }.decodeFromString<JsonElement>(jsonString)
        val actualTestClass = deserializeJsonElement<TestClass>(jsonElement)

        assertThat(actualTestClass.testEnum).isNull()
    }

[sandwwraith]

I've checked on the current dev, and the issue is not present anymore. It was likely fixed by #2530. Also, linking #2586 as a reference — it may be related.