In an attempt to model some XML resembling

<outer>
  Hello, world!
  <inner>This is some mixed content!</inner>
  Here's some more text!
</outer>

Where the text and inner can be in any arbitrary order, of zero or more items.

I have created a class hierarchy that looks like

@Serializable
data class Outer(
  @XmlValue
  val content: List<Content>
) {
  @Serializable
  sealed interface Content;

  @Serializable
  value class Text(val content: String): Content

  @Serializable
  data class Inner(
    @XmlValue
    val innerContent: String
  ): Content
}

However, the serializer spits back `Invalid XML value at position: 11:30: Serializer for subclass 'kotlin.String' is not found in the polymorphic scope of 'Content'.

Based on https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/value-classes.md, I would expect the value class to be "flattened" into a String automatically, but it seems like this is not happening? I'm no expert in XML in general, so maybe I've missed something?