Properties marked with @EncodeDefault in data objects are ignored

[RaySmith-ttc]

To Reproduce

@Serializable
data object Foo {

    @EncodeDefault
    val bar: String = "value"
}

@Test
fun testFoo() {
    val json = Json.encodeToString(Foo)
    assertEquals("""{"bar":"value"}""", json) // failed, result is {}
}

Expected behavior
All properties annotated with @EncodeDefault must be present in the json, just as they are in the data classes:

@Serializable
data class FooDataClass(val bar1: String) {

    @EncodeDefault
    val bar2: String = "value2"
}

@Test
fun testFooDataClass() {
    val json = Json.encodeToString(FooDataClass("value1"))
    assertEquals("""{"bar1":"value1","bar2":"value2"}""", json) // passed
}

Environment

• Kotlin version: 2.3.10
• Library version: 1.11.0
• Kotlin platforms: JVM
• Gradle version: 9.6.0

[sandwwraith]

Hi! It is by design that objects are always serialized as an empty {} value, because one cannot reconstruct them back with different values, as objects always have a single instance. See here: https://kotlinlang.org/docs/serialization-serialize-builtin-types.html#unit-and-singleton-objects

[sandwwraith]

@daniCsorbaJB Perhaps we should add this documentation section to 'Serialize classes' section as well, somewhere after here: https://kotlinlang.org/docs/serialization-customization-options.html#serialize-generic-classes

[RaySmith-ttc]

That's unfortunate, it looked convenient for objects that only need to be serialized (especially considering that in my case, the object inherits from a sealed interface, and deserialization was handled by JsonContentPolymorphicSerializer).

Does this mean the only way to keep it as an object is to provide a custom KSerializer?