Contextual serialization for derived classes (#1277)

Fixes #1276

The issue #1276 is happening because, in the implementation of ContextualSerializer, the serializer is looked up using the concrete class instead of the serializable class

Author: martinraison

core/commonMain/src/kotlinx/serialization/ContextualSerializer.kt

@@ -45,21 +45,20 @@ public class ContextualSerializer<T : Any>(
     private val typeParametersSerializers: Array<KSerializer<*>>
 ) : KSerializer<T> {
 
+    private fun serializer(serializersModule: SerializersModule): KSerializer<T> =
+        serializersModule.getContextual(serializableClass) ?: fallbackSerializer ?: serializableClass.serializerNotRegistered()
+
     // Used from auto-generated code
     public constructor(serializableClass: KClass<T>) : this(serializableClass, null, EMPTY_SERIALIZER_ARRAY)
 
     public override val descriptor: SerialDescriptor =
         buildSerialDescriptor("kotlinx.serialization.ContextualSerializer", SerialKind.CONTEXTUAL).withContext(serializableClass)
 
     public override fun serialize(encoder: Encoder, value: T) {
-        val clz = value::class
-        val serializer = encoder.serializersModule.getContextual(clz) ?: fallbackSerializer ?: serializableClass.serializerNotRegistered()
-        @Suppress("UNCHECKED_CAST")
-        encoder.encodeSerializableValue(serializer as SerializationStrategy<T>, value)
+        encoder.encodeSerializableValue(serializer(encoder.serializersModule), value)
     }
 
     public override fun deserialize(decoder: Decoder): T {
-        val serializer = decoder.serializersModule.getContextual(serializableClass) ?: fallbackSerializer ?: serializableClass.serializerNotRegistered()
-        return decoder.decodeSerializableValue(serializer)
+        return decoder.decodeSerializableValue(serializer(decoder.serializersModule))
     }
 }

formats/json/commonTest/src/kotlinx/serialization/features/DerivedContextualSerializerTest.kt

@@ -0,0 +1,49 @@
+package kotlinx.serialization.features
+
+import kotlin.test.*
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.modules.*
+
+class DerivedContextualSerializerTest {
+
+    @Serializable
+    abstract class Message
+
+    @Serializable
+    class SimpleMessage(val body: String) : Message()
+
+    @Serializable
+    class Holder(@Contextual val message: Message)
+
+    object MessageAsStringSerializer : KSerializer<Message> {
+        override val descriptor: SerialDescriptor =
+            PrimitiveSerialDescriptor("kotlinx.serialization.MessageAsStringSerializer", PrimitiveKind.STRING)
+
+        override fun serialize(encoder: Encoder, value: Message) {
+            // dummy serializer that assumes Message is always SimpleMessage
+            check(value is SimpleMessage)
+            encoder.encodeString(value.body)
+        }
+
+        override fun deserialize(decoder: Decoder): Message {
+            return SimpleMessage(decoder.decodeString())
+        }
+    }
+
+    @Test
+    fun testDerivedContextualSerializer() {
+        val module = SerializersModule {
+            contextual(MessageAsStringSerializer)
+        }
+        val format = Json { serializersModule = module }
+        val data = Holder(SimpleMessage("hello"))
+        val serialized = format.encodeToString(data)
+        assertEquals("""{"message":"hello"}""", serialized)
+        val deserialized = format.decodeFromString<Holder>(serialized)
+        assertTrue(deserialized.message is SimpleMessage)
+        assertEquals("hello", deserialized.message.body)
+    }
+}