Fix descriptor overrides for nullable values (using a synthetic marker

annotation and exposing the delegate on a negative index).

core/src/commonMain/kotlin/nl/adaptivity/xmlutil/XmlSerializer.kt

@@ -49,6 +49,13 @@ public fun SerialDescriptor.xml(
     return XmlSerialDescriptorImpl(this, xmlDescriptor, serialQName)
 }
 
+/**
+ * Marker to signify that the descriptor is an xmlSerialDescriptor and the delegate
+ * can be retrieved through getElementDescriptor with negative value.
+ */
+@XmlUtilInternal
+public annotation class XmlSerialDescriptorMarker
+
 /**
  * Serial Descriptor delegate that supports special casing by the XML format. This means
  * that the descriptor can be different for non-xml and xml serialization. (Used by the QName
@@ -73,7 +80,10 @@ public interface XmlSerialDescriptor : SerialDescriptor {
     override fun getElementAnnotations(index: Int): List<Annotation> = delegate.getElementAnnotations(index)
 
     @ExperimentalSerializationApi
-    override fun getElementDescriptor(index: Int): SerialDescriptor = delegate.getElementDescriptor(index)
+    override fun getElementDescriptor(index: Int): SerialDescriptor = when {
+        index < 0 -> xmlDescriptor
+        else -> delegate.getElementDescriptor(index)
+    }
 
     @ExperimentalSerializationApi
     override fun getElementIndex(name: String): Int = delegate.getElementIndex(name)
@@ -87,7 +97,8 @@ public interface XmlSerialDescriptor : SerialDescriptor {
     }
 
     @ExperimentalSerializationApi
-    override val annotations: List<Annotation> get() = delegate.annotations
+    override val annotations: List<Annotation> get() =
+        listOf(XmlSerialDescriptorMarker()) + delegate.annotations
 
     override val isInline: Boolean get() = delegate.isInline
 

serialization/src/commonMain/kotlin/nl/adaptivity/xmlutil/serialization/XmlSerializationPolicy.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020.
+ * Copyright (c) 2023.
  *
  * This file is part of xmlutil.
  *
@@ -22,6 +22,7 @@ package nl.adaptivity.xmlutil.serialization
 import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.KSerializer
 import kotlinx.serialization.SerializationException
+import kotlinx.serialization.builtins.nullable
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.modules.SerializersModule
 import nl.adaptivity.xmlutil.*
@@ -654,13 +655,11 @@ private constructor(
         tagParent: SafeParentInfo
     ): KSerializer<*>? =
         when (serializerParent.elementSerialDescriptor.serialName) {
-/*
             "javax.xml.namespace.QName?",
             "javax.xml.namespace.QName" -> when {
                 serializerParent.elementSerialDescriptor.isNullable -> QNameSerializer.nullable
                 else -> QNameSerializer
             }
-*/
 
             else -> null
         }

serialization/src/commonMain/kotlin/nl/adaptivity/xmlutil/serialization/structure/XmlDescriptor.kt

@@ -1653,4 +1653,8 @@ internal fun <A : Appendable> A.appendIndent(count: Int) = apply {
     }
 }
 
-internal fun SerialDescriptor.getXmlOverride() = (this as? XmlSerialDescriptor)?.xmlDescriptor ?: this
+internal fun SerialDescriptor.getXmlOverride() = when {
+    this is XmlSerialDescriptor -> xmlDescriptor
+    isNullable && annotations.any { it is XmlSerialDescriptorMarker } -> getElementDescriptor(-1).nullable
+    else -> this
+}

serialization/src/commonTest/kotlin/nl/adaptivity/xml/serialization/QNameContentContainerTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021.
+ * Copyright (c) 2023.
  *
  * This file is part of xmlutil.
  *
@@ -53,7 +53,7 @@ class QNameContentContainerTest : PlatformTestBase<QNameContentContainerTest.Con
         val name: QName,
         @XmlElement
         @XmlSerialName("value", namespace = "urn:example.org", prefix = "otherns")
-        val value: QName
+        val value: QName?
     )
 
 }