Prohibited use of elements other than JsonObject is When polymorphic …

…serialization for JsonTransformingSerializer

If JsonTransformingSerializer is used as a serializer for the descendant of a polymorphic class, then we do not know how to add a type discriminator to the returned result of a primitive or array type.

Since there is no general solution to this problem on the library side, the user must take care of the correct processing of such types and, possibly, manually implement polymorphism.

Resolves #2164

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>("Json element JsonLiteral cannot be serialized polymorphous, for serial name 'kotlinx.serialization.JsonElementPolymorphicErrorTest.IntChild'") {
+            format.encodeToString(
+                Holder.serializer(),
+                Holder(IntChild(42)),
+                mode
+            )
+        }
+
+        assertFailsWithMessage<SerializationException>("Json element JsonArray cannot be serialized polymorphous, for serial name 'kotlinx.serialization.JsonElementPolymorphicErrorTest.CollectionChild'") {
+            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("Json element ${element::class.simpleName} cannot be serialized polymorphous, for serial name '$serialName'. Make sure that all JsonTransformingSerializer serializers return JsonObject")
+}
+

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)
     }