Implementation for automatically registering sealed serializers in polymorphic modules

core/api/kotlinx-serialization-core.api

@@ -1334,6 +1334,7 @@ public final class kotlinx/serialization/modules/PolymorphicModuleBuilder {
 	public final fun default (Lkotlin/jvm/functions/Function1;)V
 	public final fun defaultDeserializer (Lkotlin/jvm/functions/Function1;)V
 	public final fun subclass (Lkotlin/reflect/KClass;Lkotlinx/serialization/KSerializer;)V
+	public final fun subclassesOfSealed (Lkotlinx/serialization/KSerializer;)V
 }
 
 public abstract class kotlinx/serialization/modules/SerializersModule {

core/api/kotlinx-serialization-core.klib.api

@@ -527,9 +527,11 @@ final class <#A: in kotlin/Any> kotlinx.serialization.modules/PolymorphicModuleB
     constructor <init>(kotlin.reflect/KClass<#A>, kotlinx.serialization/KSerializer<#A>? = ...) // kotlinx.serialization.modules/PolymorphicModuleBuilder.<init>|<init>(kotlin.reflect.KClass<1:0>;kotlinx.serialization.KSerializer<1:0>?){}[0]
 
     final fun <#A1: #A> subclass(kotlin.reflect/KClass<#A1>, kotlinx.serialization/KSerializer<#A1>) // kotlinx.serialization.modules/PolymorphicModuleBuilder.subclass|subclass(kotlin.reflect.KClass<0:0>;kotlinx.serialization.KSerializer<0:0>){0§<1:0>}[0]
+    final fun <#A1: #A> subclassesOfSealed(kotlinx.serialization/KSerializer<#A1>) // kotlinx.serialization.modules/PolymorphicModuleBuilder.subclassesOfSealed|subclassesOfSealed(kotlinx.serialization.KSerializer<0:0>){0§<1:0>}[0]
     final fun buildTo(kotlinx.serialization.modules/SerializersModuleBuilder) // kotlinx.serialization.modules/PolymorphicModuleBuilder.buildTo|buildTo(kotlinx.serialization.modules.SerializersModuleBuilder){}[0]
     final fun default(kotlin/Function1<kotlin/String?, kotlinx.serialization/DeserializationStrategy<#A>?>) // kotlinx.serialization.modules/PolymorphicModuleBuilder.default|default(kotlin.Function1<kotlin.String?,kotlinx.serialization.DeserializationStrategy<1:0>?>){}[0]
     final fun defaultDeserializer(kotlin/Function1<kotlin/String?, kotlinx.serialization/DeserializationStrategy<#A>?>) // kotlinx.serialization.modules/PolymorphicModuleBuilder.defaultDeserializer|defaultDeserializer(kotlin.Function1<kotlin.String?,kotlinx.serialization.DeserializationStrategy<1:0>?>){}[0]
+    final inline fun <#A1: reified #A> subclassesOfSealed() // kotlinx.serialization.modules/PolymorphicModuleBuilder.subclassesOfSealed|subclassesOfSealed(){0§<1:0>}[0]
 }
 
 final class <#A: kotlin/Any, #B: #A?> kotlinx.serialization.internal/ReferenceArraySerializer : kotlinx.serialization.internal/CollectionLikeSerializer<#B, kotlin/Array<#B>, kotlin.collections/ArrayList<#B>> { // kotlinx.serialization.internal/ReferenceArraySerializer|null[0]
@@ -1168,6 +1170,7 @@ final inline fun (kotlinx.serialization.encoding/Encoder).kotlinx.serialization.
 final inline fun (kotlinx.serialization.encoding/Encoder).kotlinx.serialization.encoding/encodeStructure(kotlinx.serialization.descriptors/SerialDescriptor, crossinline kotlin/Function1<kotlinx.serialization.encoding/CompositeEncoder, kotlin/Unit>) // kotlinx.serialization.encoding/encodeStructure|encodeStructure@kotlinx.serialization.encoding.Encoder(kotlinx.serialization.descriptors.SerialDescriptor;kotlin.Function1<kotlinx.serialization.encoding.CompositeEncoder,kotlin.Unit>){}[0]
 final inline fun <#A: kotlin/Any, #B: reified #A> (kotlinx.serialization.modules/PolymorphicModuleBuilder<#A>).kotlinx.serialization.modules/subclass(kotlin.reflect/KClass<#B>) // kotlinx.serialization.modules/subclass|subclass@kotlinx.serialization.modules.PolymorphicModuleBuilder<0:0>(kotlin.reflect.KClass<0:1>){0§<kotlin.Any>;1§<0:0>}[0]
 final inline fun <#A: kotlin/Any, #B: reified #A> (kotlinx.serialization.modules/PolymorphicModuleBuilder<#A>).kotlinx.serialization.modules/subclass(kotlinx.serialization/KSerializer<#B>) // kotlinx.serialization.modules/subclass|subclass@kotlinx.serialization.modules.PolymorphicModuleBuilder<0:0>(kotlinx.serialization.KSerializer<0:1>){0§<kotlin.Any>;1§<0:0>}[0]
+final inline fun <#A: kotlin/Any, #B: reified #A> (kotlinx.serialization.modules/PolymorphicModuleBuilder<#A>).kotlinx.serialization.modules/subclassesOfSealed(kotlin.reflect/KClass<#B>) // kotlinx.serialization.modules/subclassesOfSealed|subclassesOfSealed@kotlinx.serialization.modules.PolymorphicModuleBuilder<0:0>(kotlin.reflect.KClass<0:1>){0§<kotlin.Any>;1§<0:0>}[0]
 final inline fun <#A: kotlin/Any> (kotlinx.serialization.modules/SerializersModuleBuilder).kotlinx.serialization.modules/polymorphic(kotlin.reflect/KClass<#A>, kotlinx.serialization/KSerializer<#A>? = ..., kotlin/Function1<kotlinx.serialization.modules/PolymorphicModuleBuilder<#A>, kotlin/Unit> = ...) // kotlinx.serialization.modules/polymorphic|polymorphic@kotlinx.serialization.modules.SerializersModuleBuilder(kotlin.reflect.KClass<0:0>;kotlinx.serialization.KSerializer<0:0>?;kotlin.Function1<kotlinx.serialization.modules.PolymorphicModuleBuilder<0:0>,kotlin.Unit>){0§<kotlin.Any>}[0]
 final inline fun <#A: kotlin/Any?> (kotlinx.serialization.encoding/Decoder).kotlinx.serialization.encoding/decodeStructure(kotlinx.serialization.descriptors/SerialDescriptor, crossinline kotlin/Function1<kotlinx.serialization.encoding/CompositeDecoder, #A>): #A // kotlinx.serialization.encoding/decodeStructure|decodeStructure@kotlinx.serialization.encoding.Decoder(kotlinx.serialization.descriptors.SerialDescriptor;kotlin.Function1<kotlinx.serialization.encoding.CompositeDecoder,0:0>){0§<kotlin.Any?>}[0]
 final inline fun <#A: kotlin/Any?> (kotlinx.serialization.encoding/Encoder).kotlinx.serialization.encoding/encodeCollection(kotlinx.serialization.descriptors/SerialDescriptor, kotlin.collections/Collection<#A>, crossinline kotlin/Function3<kotlinx.serialization.encoding/CompositeEncoder, kotlin/Int, #A, kotlin/Unit>) // kotlinx.serialization.encoding/encodeCollection|encodeCollection@kotlinx.serialization.encoding.Encoder(kotlinx.serialization.descriptors.SerialDescriptor;kotlin.collections.Collection<0:0>;kotlin.Function3<kotlinx.serialization.encoding.CompositeEncoder,kotlin.Int,0:0,kotlin.Unit>){0§<kotlin.Any?>}[0]

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

@@ -115,7 +115,7 @@ public class SealedClassSerializer<T : Any>(
         }
     }
 
-    private val class2Serializer: Map<KClass<out T>, KSerializer<out T>>
+    internal val class2Serializer: Map<KClass<out T>, KSerializer<out T>>
     private val serialName2Serializer: Map<String, KSerializer<out T>>
 
     init {

core/commonMain/src/kotlinx/serialization/modules/PolymorphicModuleBuilder.kt

@@ -5,6 +5,7 @@
 package kotlinx.serialization.modules
 
 import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.internal.*
 import kotlin.reflect.*
 
@@ -19,15 +20,87 @@ public class PolymorphicModuleBuilder<in Base : Any> @PublishedApi internal cons
     private val baseClass: KClass<Base>,
     private val baseSerializer: KSerializer<Base>? = null
 ) {
-    private val subclasses: MutableList<Pair<KClass<out Base>, KSerializer<out Base>>> = mutableListOf()
+    private val subclasses: MutableMap<KClass<out Base>, KSerializer<out Base>> = mutableMapOf()
     private var defaultSerializerProvider: ((Base) -> SerializationStrategy<Base>?)? = null
     private var defaultDeserializerProvider: ((String?) -> DeserializationStrategy<Base>?)? = null
 
+
+    /**
+     * Registers the child serializers for the sealed [subclass] [serializer] in the resulting module under
+     * the [base class][Base]. Please note that type `T` must be sealed, if not a runtime error will
+     * be thrown at registration time. This function is a convenience function for the version that
+     * receives a serializer.
+     *
+     *
+     * Example:
+     * ```kotlin
+     * interface Base
+     *
+     * @Serializable
+     * sealed interface Sub
+     *
+     * @Serializable
+     * class Sub1: Sub
+     *
+     * serializersModule {
+     *   polymorphic(Base::class) {
+     *      subclassesOfSealed<Sub>()
+     *   }
+     * }
+     * ```
+     */
+    @ExperimentalSerializationApi
+    public inline fun <reified T : Base> subclassesOfSealed(): Unit =
+        subclassesOfSealed(serializer<T>())
+
+
+    /**
+     * Registers the subclasses of the given class as subclasses of the outer class. This currently
+     * requires `serializer` to be sealed. If not a runtime error will be thrown at registration time.
+     *
+     * Example:
+     * ```kotlin
+     * interface Base
+     *
+     * @Serializable
+     * sealed interface Sub
+     *
+     * @Serializable
+     * class Sub1: Sub
+     *
+     * serializersModule {
+     *   polymorphic(Base::class) {
+     *      subclassesOfSealed(Sub.serializer())
+     *   }
+     * }
+     * ```
+     *
+     * It is an error if th
+     */
+    @ExperimentalSerializationApi
+    public fun <T: Base> subclassesOfSealed(serializer: KSerializer<T>) {
+        // Note that the parameter type is `KSerializer` as `SealedClassSerializer` is an internal type
+        // not available to users
+        require(serializer is SealedClassSerializer) {
+            "subClassesOf only supports automatic adding of subclasses of sealed types."
+        }
+        for ((subsubclass, subserializer) in serializer.class2Serializer.entries) {
+            // This error would be caught by the Json format in its validation, but this is format specific
+            require (subserializer.descriptor.kind != PolymorphicKind.OPEN) {
+                "It is not possible to register subclasses of sealed types when those subclasses " +
+                    "themselves are (open) polymorphic, as this would represent an incomplete hierarchy"
+            }
+            @Suppress("UNCHECKED_CAST")
+            // We don't know the type here, but it matches if correct in the sealed serializer.
+            subclass(subsubclass as KClass<T>, subserializer as KSerializer<T>)
+        }
+    }
+
     /**
      * Registers a [subclass] [serializer] in the resulting module under the [base class][Base].
      */
     public fun <T : Base> subclass(subclass: KClass<T>, serializer: KSerializer<T>) {
-        subclasses.add(subclass to serializer)
+        subclasses[subclass] = serializer
     }
 
     /**
@@ -116,3 +189,10 @@ public inline fun <Base : Any, reified T : Base> PolymorphicModuleBuilder<Base>.
  */
 public inline fun <Base : Any, reified T : Base> PolymorphicModuleBuilder<Base>.subclass(clazz: KClass<T>): Unit =
     subclass(clazz, serializer())
+
+/**
+ * Registers the child serializers for the sealed class [T] in the resulting module under the [base class][Base].
+ */
+@ExperimentalSerializationApi
+public inline fun <Base : Any, reified T : Base> PolymorphicModuleBuilder<Base>.subclassesOfSealed(clazz: KClass<T>): Unit =
+    subclassesOfSealed(clazz.serializer())

docs/polymorphism.md

@@ -414,6 +414,13 @@ fun main() {
 
 > Note: On Kotlin/Native, you should use `format.encodeToString(PolymorphicSerializer(Project::class), data))` instead due to limited reflection capabilities.
 
+### Registering sealed children as subclasses
+A sealed parent interface or class can be used to directly register all its children using `subclassesOfSealed`.
+This will expose all children that would be available when serializing the parent directly, but now as sealed. Please
+note that this is will remain open serialization, and the sealed parent serializer will not be used in serialization.
+In addition, it is not valid if the hierarchy contains open (not sealed) polymorphic children (this will result in
+an error at runtime).
+
 <!--- TEST LINES_START -->
 
 ### Property of an interface type

formats/json-tests/commonTest/src/kotlinx/serialization/features/PolymorphicSealedChildTest.kt

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2017-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.features
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.modules.*
+import kotlinx.serialization.test.*
+import kotlin.test.*
+
+class PolymorphicSealedChildTest {
+
+    @Serializable
+    data class FooHolder(
+        val someMetadata: Int,
+        val payload: List<@Polymorphic FooBase>
+    )
+
+    @Serializable
+    abstract class FooBase
+
+    @Serializable
+    @SerialName("Foo")
+    sealed class Foo: FooBase() {
+        @Serializable
+        @SerialName("Bar")
+        data class Bar(val bar: Int) : Foo()
+        @Serializable
+        @SerialName("Baz")
+        data class Baz(val baz: String) : Foo()
+    }
+
+    @Serializable
+    sealed class FooOpen: FooBase() {
+        @Serializable
+        data class Bar(val bar: Int) : FooOpen()
+        @Serializable
+        abstract class Baz: FooOpen()
+        @Serializable
+        data class BazChild1(val baz: String) : Baz()
+        @Serializable
+        data class BazChild2(val baz: String) : Baz()
+    }
+
+    val sealedModule = SerializersModule {
+        polymorphic(FooBase::class) {
+            subclassesOfSealed<Foo>()
+        }
+    }
+
+    val json = Json { serializersModule = sealedModule }
+
+    @Test
+    fun testOpenGrandchildIsInvalid() {
+        assertFailsWith<IllegalArgumentException> {
+            SerializersModule {
+                polymorphic(FooBase::class) {
+                    subclassesOfSealed<FooOpen>()
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testSaveSealedClassesList() {
+        assertStringFormAndRestored(
+            """{"someMetadata":42,"payload":[
+            |{"type":"Bar","bar":1},
+            |{"type":"Baz","baz":"2"}]}""".trimMargin().replace("\n", ""),
+            FooHolder(42, listOf(Foo.Bar(1), Foo.Baz("2"))),
+            FooHolder.serializer(),
+            json,
+            printResult = true
+        )
+    }
+
+    @Test
+    fun testCanSerializeSealedClassPolymorphicallyOnTopLevel() {
+        assertStringFormAndRestored(
+            """{"type":"Bar","bar":1}""",
+            Foo.Bar(1),
+            PolymorphicSerializer(FooBase::class),
+            json
+        )
+    }
+}