Inherited serializable fields lead to incorrect serialization order

This Issue is producable with kotlin >=1.3.20.

Consider the following scenario with a base class:

import kotlinx.serialization.Serializable

@Serializable
abstract class TestBase {
    val b = "val b"
    val a = "val a"
}

…and one (or two) implementations

@Serializable
class Test : TestBase() {
    val c = "val c"
}

@Serializable
class Test2 : TestBase() {
    val c2 = "val c2"
}

The serialized output of a “Test” object with Json(indented = true) is not consistent: Sometimes it will be as expected, but sometimes it will be this:

{
    "a": "val b",
    "b": "val a",
    "c": "val c"
}

However, I was not able to get an absolutely clear order of file manipulation to yield the above output, but In order to reproduce the behavior, I would usually have to change something in the implementation class(es) Test or Test2, like remove or add the second implementation. It also seemed to only work, when base- and implementation classes were in separate Files.

[sandwwraith]

I can confirm that most of the bugs related to multi-module/project setups are fixed in Kotlin 1.4 (starting from 1.4-M3, to be precise). So this long and painful story is finally over.

[altavir]

The only workaround I found is to mark ancestor as non-serializeable and override all properties in the child. It does the trick, but requires a lot of duplicated code.