Fix: Hocon polymorphic serialization (#2151)

Fixes #1581

Author: LichtHund

formats/hocon/src/main/kotlin/kotlinx/serialization/hocon/Hocon.kt

@@ -145,7 +145,7 @@ public sealed class Hocon(
 
     }
 
-    private inner class ConfigReader(val conf: Config) : ConfigConverter<String>() {
+    private inner class ConfigReader(val conf: Config, private val isPolymorphic: Boolean = false) : ConfigConverter<String>() {
         private var ind = -1
 
         override fun decodeElementIndex(descriptor: SerialDescriptor): Int {
@@ -161,8 +161,10 @@ public sealed class Hocon(
         private fun composeName(parentName: String, childName: String) =
             if (parentName.isEmpty()) childName else "$parentName.$childName"
 
-        override fun SerialDescriptor.getTag(index: Int): String =
-            composeName(currentTagOrNull.orEmpty(), getConventionElementName(index, useConfigNamingConvention))
+        override fun SerialDescriptor.getTag(index: Int): String {
+            val conventionName = getConventionElementName(index, useConfigNamingConvention)
+            return if (!isPolymorphic) composeName(currentTagOrNull.orEmpty(), conventionName) else conventionName
+        }
 
         override fun decodeNotNullMark(): Boolean {
             // Tag might be null for top-level deserialization
@@ -206,6 +208,27 @@ public sealed class Hocon(
         }
     }
 
+    private inner class PolymorphConfigReader(private val conf: Config) : ConfigConverter<String>() {
+        private var ind = -1
+
+        override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder =
+            when {
+                descriptor.kind.objLike -> ConfigReader(conf, isPolymorphic = true)
+                else -> this
+            }
+
+        override fun SerialDescriptor.getTag(index: Int): String = getElementName(index)
+
+        override fun decodeElementIndex(descriptor: SerialDescriptor): Int {
+            ind++
+            return if (ind >= descriptor.elementsCount) DECODE_DONE else ind
+        }
+
+        override fun <E> getValueFromTaggedConfig(tag: String, valueResolver: (Config, String) -> E): E {
+            return valueResolver(conf, tag)
+        }
+    }
+
     private inner class ListConfigReader(private val list: ConfigList) : ConfigConverter<Int>() {
         private var ind = -1
 
@@ -216,6 +239,7 @@ public sealed class Hocon(
 
         override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder =
             when {
+                descriptor.kind is PolymorphicKind -> PolymorphConfigReader((list[currentTag] as ConfigObject).toConfig())
                 descriptor.kind.listLike -> ListConfigReader(list[currentTag] as ConfigList)
                 descriptor.kind.objLike -> ConfigReader((list[currentTag] as ConfigObject).toConfig())
                 descriptor.kind == StructureKind.MAP -> MapConfigReader(list[currentTag] as ConfigObject)
@@ -256,6 +280,7 @@ public sealed class Hocon(
 
         override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder =
             when {
+                descriptor.kind is PolymorphicKind -> PolymorphConfigReader((values[currentTag / 2] as ConfigObject).toConfig())
                 descriptor.kind.listLike -> ListConfigReader(values[currentTag / 2] as ConfigList)
                 descriptor.kind.objLike -> ConfigReader((values[currentTag / 2] as ConfigObject).toConfig())
                 descriptor.kind == StructureKind.MAP -> MapConfigReader(values[currentTag / 2] as ConfigObject)

formats/hocon/src/test/kotlin/kotlinx/serialization/hocon/HoconPolymorphismTest.kt

@@ -23,6 +23,12 @@ class HoconPolymorphismTest {
         data class AnnotatedTypeChild(@SerialName("my_type") val type: String) : Sealed(3)
     }
 
+    @Serializable
+    data class SealedCollectionContainer(val sealed: Collection<Sealed>)
+
+    @Serializable
+    data class SealedMapContainer(val sealed: Map<String, Sealed>)
+
     @Serializable
     data class CompositeClass(var sealed: Sealed)
 
@@ -102,4 +108,46 @@ class HoconPolymorphismTest {
             serializer = Sealed.serializer(),
         )
     }
+
+    @Test
+    fun testCollectionContainer() {
+        objectHocon.assertStringFormAndRestored(
+            expected = """
+                sealed = [ 
+                    { type = annotated_type_child, my_type = override, intField = 3 }
+                    { type = object }
+                    { type = data_class, name = testDataClass, intField = 1 }
+                ]
+            """.trimIndent(),
+            original = SealedCollectionContainer(
+                listOf(
+                    Sealed.AnnotatedTypeChild(type = "override"),
+                    Sealed.ObjectChild,
+                    Sealed.DataClassChild(name = "testDataClass"),
+                )
+            ),
+            serializer = SealedCollectionContainer.serializer(),
+        )
+    }
+
+    @Test
+    fun testMapContainer() {
+        objectHocon.assertStringFormAndRestored(
+            expected = """
+                sealed = { 
+                    "annotated_type_child" = { type = annotated_type_child, my_type = override, intField = 3 }
+                    "object" = { type = object }
+                    "data_class" = { type = data_class, name = testDataClass, intField = 1 }
+                }
+            """.trimIndent(),
+            original = SealedMapContainer(
+                mapOf(
+                    "annotated_type_child" to Sealed.AnnotatedTypeChild(type = "override"),
+                    "object" to Sealed.ObjectChild,
+                    "data_class" to Sealed.DataClassChild(name = "testDataClass"),
+                )
+            ),
+            serializer = SealedMapContainer.serializer(),
+        )
+    }
 }