Prohibited use of elements other than JsonObject in JsonTransformingSerializer with polymorphic serialization

formats/json-tests/commonTest/src/kotlinx/serialization/JsonElementPolymorphicErrorTest.kt

@@ -0,0 +1,71 @@
+/*
+ * Copyright 2017-2024 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization
+
+
+import kotlinx.serialization.json.*
+import kotlinx.serialization.modules.SerializersModule
+import kotlinx.serialization.modules.polymorphic
+import kotlinx.serialization.test.*
+import kotlin.test.*
+
+class JsonElementPolymorphicErrorTest : JsonTestBase() {
+
+    @Serializable
+    abstract class Abstract
+
+    @Serializable
+    data class IntChild(val value: Int) : Abstract()
+
+    @Serializable
+    data class CollectionChild(val value: Int) : Abstract()
+
+    @Serializable
+    data class Holder(val value: Abstract)
+
+    private val format = Json {
+        prettyPrint = false
+        serializersModule = SerializersModule {
+            polymorphic(Abstract::class) {
+                subclass(IntChild::class, IntChildSerializer)
+                subclass(CollectionChild::class, CollectionChildSerializer)
+            }
+        }
+    }
+
+    object IntChildSerializer : JsonTransformingSerializer<IntChild>(serializer()) {
+        override fun transformSerialize(element: JsonElement): JsonElement {
+            return element.jsonObject.getValue("value")
+        }
+    }
+
+    object CollectionChildSerializer : JsonTransformingSerializer<CollectionChild>(serializer()) {
+        override fun transformSerialize(element: JsonElement): JsonElement {
+            val value = element.jsonObject.getValue("value")
+            return JsonArray(listOf(value))
+        }
+    }
+
+    @Test
+    fun test() = parametrizedTest { mode ->
+        assertFailsWithMessage<SerializationException>("Class with serial name kotlinx.serialization.JsonElementPolymorphicErrorTest.IntChild cannot be serialized polymorphically because it is represented as JsonLiteral. Make sure that its JsonTransformingSerializer returns JsonObject, so class discriminator can be added to it") {
+            format.encodeToString(
+                Holder.serializer(),
+                Holder(IntChild(42)),
+                mode
+            )
+        }
+
+        assertFailsWithMessage<SerializationException>("Class with serial name kotlinx.serialization.JsonElementPolymorphicErrorTest.CollectionChild cannot be serialized polymorphically because it is represented as JsonArray. Make sure that its JsonTransformingSerializer returns JsonObject, so class discriminator can be added to it") {
+            format.encodeToString(
+                Holder.serializer(),
+                Holder(CollectionChild(42)),
+                mode
+            )
+        }
+
+    }
+
+}

formats/json/commonMain/src/kotlinx/serialization/json/internal/Polymorphic.kt

@@ -100,3 +100,7 @@ internal fun SerialDescriptor.classDiscriminator(json: Json): String {
     return json.configuration.classDiscriminator
 }
 
+internal fun throwJsonElementPolymorphicException(serialName: String?, element: JsonElement): Nothing {
+    throw JsonEncodingException("Class with serial name $serialName cannot be serialized polymorphically because it is represented as ${element::class.simpleName}. Make sure that its JsonTransformingSerializer returns JsonObject, so class discriminator can be added to it.")
+}
+

formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonEncoder.kt

@@ -54,6 +54,9 @@ internal class StreamingJsonEncoder(
     }
 
     override fun encodeJsonElement(element: JsonElement) {
+        if (polymorphicDiscriminator != null && element !is JsonObject) {
+            throwJsonElementPolymorphicException(polymorphicSerialName, element)
+        }
         encodeSerializableValue(JsonElementSerializer, element)
     }
 

formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonEncoder.kt

@@ -41,6 +41,9 @@ private sealed class AbstractJsonTreeEncoder(
         descriptor.getJsonElementName(json, index)
 
     override fun encodeJsonElement(element: JsonElement) {
+        if (polymorphicDiscriminator != null && element !is JsonObject) {
+            throwJsonElementPolymorphicException(polymorphicSerialName, element)
+        }
         encodeSerializableValue(JsonElementSerializer, element)
     }
 

formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicEncoders.kt

@@ -165,6 +165,9 @@ private class DynamicObjectEncoder(
     }
 
     override fun encodeJsonElement(element: JsonElement) {
+        if (polymorphicDiscriminator != null && element !is JsonObject) {
+            throwJsonElementPolymorphicException(polymorphicSerialName, element)
+        }
         encodeSerializableValue(JsonElementSerializer, element)
     }