Json.serializersModule.plus does not work as expect

Describe the bug Json.serializersModule.plus a new custom module report error

Mark the class as @Serializable or provide the serializer explicitly.
kotlinx.serialization.SerializationException: Serializer for class 'Date' is not found.

To Reproduce custom serializer

object DateSerializer: KSerializer<Date> {
    private val DATE_FORMATS: List<String> = listOf(
        "yyyy-MM-dd HH:mm:ss",
        "yyyy-MM-dd",
        "yyyyMMdd'T'HHmmss"
    )
    val DEFAULTOUTPUT = SimpleDateFormat("yyyy-MM-dd HH:mm:ss")
    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("Date", PrimitiveKind.STRING)

    override fun deserialize(decoder: Decoder): Date {
        val dateStr = decoder.decodeString()
        for (dateFormat in DATE_FORMATS) {
            try {
                return SimpleDateFormat(dateFormat).parse(dateStr)
            } catch (ignore: ParseException) {
                // ignore
            }
        }
        throw SerializationException(
            "Deserializer for Date class with value ${dateStr} is not support"
        )
    }

    override fun serialize(encoder: Encoder, value: Date) {
        encoder.encodeString(DEFAULTOUTPUT.format(value))
    }
}

target object

 @Serializable
 data class DatawithDate(val haha: String, @Contextual val time: Date)

pass test

@Test
    fun `test serialize  class with date`() {
        val data = DatawithDate("haha", SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse("2021-07-01 01:02:03"))
        val customModule = SerializersModule {
            contextual(DateSerializer)
        }
        val format = Json { serializersModule = customModule }
        val result = format.encodeToString(data)
        assertNotNull(result)
        //language=JSON
        assertEquals("{\"haha\":\"haha\",\"time\":\"2021-07-01 01:02:03\"}", result)
    }

failed test

@Test
    fun `test serialize  class with date`() {
        val data = DatawithDate("haha", SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse("2021-07-01 01:02:03"))
        val customModule = SerializersModule {
            contextual(DateSerializer)
        }
        val format = Json {}
        format.serializersModule.plus(customModule)
        val result = format.encodeToString(data)
        assertNotNull(result)
        //language=JSON
        assertEquals("{\"haha\":\"haha\",\"time\":\"2021-07-01 01:02:03\"}", result)
    }

Expected behavior plus should enable the custom serializer for date Environment

• Kotlin version: [1.5.20]
• Library version: [1.2.2]
• Kotlin platforms: [JVM]
• Gradle version: [7.0.2]
• IDE version (if bug is related to the IDE) [IntellijIDEA 2021.2]
• Other relevant context [macos 10.15 jdk8]

[sandwwraith]

@oth-libs You would need to create a new instance of Json

[shanshin]

Hi! plus is a not mutating operator - it does not modify the receiver object. According to docs it Returns a combination of two serial modules. If you want to sum two different modules, you can do it in the builderAction or earlier:

    val otherModule = SerializersModule { }
    val format = Json {serializersModule = otherModule.plus(customModule)} // or serializersModule = otherModule + customModule