Add some tests for serial polymorphic numbers which fail for now

Some miscellaneous changes:
1. Add a `getSerialPolymorphicNumberByBaseClass` function in `SerialDescriptor` to throw the appropriate exception.
1. Add `defaultDeserializerForNumber` which was missing in `PolymorphicModuleBuilder`.

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

@@ -144,7 +144,7 @@ public class SealedClassSerializer<T : Any>(
     private val serialPolymorphicNumber2Serializer: Map<Int, KSerializer<out T>>? by lazy(LazyThreadSafetyMode.PUBLICATION) {
         if (descriptor.useSerialPolymorphicNumbers)
             class2Serializer.entries.groupingBy {
-                it.value.descriptor.serialPolymorphicNumberByBaseClass.getValue(baseClass)
+                it.value.descriptor.getSerialPolymorphicNumberByBaseClass(baseClass)
             }
                 .aggregate<Map.Entry<KClass<out T>, KSerializer<out T>>, Int, Map.Entry<KClass<*>, KSerializer<out T>>>
                 { key, accumulator, element, _ ->

core/commonMain/src/kotlinx/serialization/descriptors/SerialDescriptor.kt

@@ -208,6 +208,13 @@ public interface SerialDescriptor {
     @ExperimentalSerializationApi
     public val serialPolymorphicNumberByBaseClass: Map<KClass<*>, Int> get() = emptyMap()
 
+    @ExperimentalSerializationApi
+    public fun getSerialPolymorphicNumberByBaseClass(baseClass: KClass<*>): Int =
+        serialPolymorphicNumberByBaseClass.getOrElse(baseClass) {
+            throw SerializationException("The serial polymorphic number for $serialName in the scope of ${baseClass.simpleName} is not found. " +
+                "Please annotate the class with `@SerialPolymorphicNumber` with the first argument ${baseClass.simpleName}.")
+        }
+
     /**
      * Returns serial annotations of the associated class.
      * Serial annotations can be used to specify an additional metadata that may be used during serialization.

core/commonMain/src/kotlinx/serialization/internal/AbstractPolymorphicSerializer.kt

@@ -34,9 +34,7 @@ public abstract class AbstractPolymorphicSerializer<T : Any> internal constructo
         encoder.encodeStructure(descriptor) {
             if (descriptor.useSerialPolymorphicNumbers)
                 encodeIntElement(
-                    descriptor,
-                    0,
-                    actualSerializer.descriptor.serialPolymorphicNumberByBaseClass.getValue(baseClass)
+                    descriptor, 0, actualSerializer.descriptor.getSerialPolymorphicNumberByBaseClass(baseClass)
                 )
             else
                 encodeStringElement(descriptor, 0, actualSerializer.descriptor.serialName)

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

@@ -22,6 +22,7 @@ public class PolymorphicModuleBuilder<in Base : Any> @PublishedApi internal cons
     private val subclasses: MutableList<Pair<KClass<out Base>, KSerializer<out Base>>> = mutableListOf()
     private var defaultSerializerProvider: ((Base) -> SerializationStrategy<Base>?)? = null
     private var defaultDeserializerProvider: PolymorphicDeserializerProvider<Base>? = null
+    private var defaultDeserializerProviderForNumber: PolymorphicDeserializerProviderForNumber<Base>? = null
 
     /*
     // TODO implement this or remove?
@@ -75,6 +76,16 @@ public class PolymorphicModuleBuilder<in Base : Any> @PublishedApi internal cons
         this.defaultDeserializerProvider = defaultDeserializerProvider
     }
 
+    /**
+     * TODO
+     */
+    public fun defaultDeserializerForNumber(defaultDeserializerProviderForNumber: (serialPolymorphicNumber: Int?) -> DeserializationStrategy<Base>?) {
+        require(this.defaultDeserializerProviderForNumber == null) {
+            "Default deserializer provider for number is already registered for class $baseClass: ${this.defaultDeserializerProvider}"
+        }
+        this.defaultDeserializerProviderForNumber = defaultDeserializerProviderForNumber
+    }
+
     /**
      * Adds a default deserializers provider associated with the given [baseClass] to the resulting module.
      * This function affect only deserialization process. To avoid confusion, it was deprecated and replaced with [defaultDeserializer].
@@ -123,6 +134,10 @@ public class PolymorphicModuleBuilder<in Base : Any> @PublishedApi internal cons
         if (defaultDeserializer != null) {
             builder.registerDefaultPolymorphicDeserializer(baseClass, defaultDeserializer, false)
         }
+
+        defaultDeserializerProviderForNumber?.let {
+            builder.registerDefaultPolymorphicDeserializerForNumber(baseClass, it, false)
+        }
     }
 }
 

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

@@ -230,7 +230,7 @@ public class SerializersModuleBuilder @PublishedApi internal constructor() : Ser
             // Remove previous serializers from name mapping
             if (previousSerializer != null) {
                 names.remove(previousSerializer.descriptor.serialName)
-                numbers.remove(previousSerializer.descriptor.serialPolymorphicNumberByBaseClass[baseClass])
+                previousSerializer.descriptor.serialPolymorphicNumberByBaseClass[baseClass]?.let { numbers.remove(it) }
             }
             // Update mappings
             baseClassSerializers[concreteClass] = concreteSerializer
@@ -245,7 +245,7 @@ public class SerializersModuleBuilder @PublishedApi internal constructor() : Ser
             } else {
                 // Cleanup name mapping
                 names.remove(previousSerializer.descriptor.serialName)
-                numbers.remove(previousSerializer.descriptor.serialPolymorphicNumberByBaseClass[baseClass])
+                previousSerializer.descriptor.serialPolymorphicNumberByBaseClass[baseClass]?.let { numbers.remove(it) }
             }
         }
         val previousByName = names[name]

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

@@ -0,0 +1,74 @@
+/*
+ * 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.*
+import kotlinx.serialization.test.*
+import kotlin.test.*
+
+class SerialPolymorphicNumberTest {
+    @Serializable
+    @UseSerialPolymorphicNumbers
+    sealed class Sealed1 {
+        @Serializable
+        @SerialPolymorphicNumber(Sealed1::class, 1)
+        class Case : Sealed1()
+    }
+
+    @Serializable
+    sealed class Sealed2 {
+        @Serializable
+        @SerialPolymorphicNumber(Sealed2::class, 1)
+        class Case : Sealed2()
+    }
+
+    @Serializable
+    @UseSerialPolymorphicNumbers
+    sealed class Sealed3 {
+        @Serializable
+        class Case : Sealed3()
+    }
+
+    @Test
+    fun testSealed() {
+        testConversion<Sealed1>(Sealed1.Case(), """{"type":1}""")
+        testConversion<Sealed2>(Sealed2.Case(), """{"type":"kotlinx.serialization.SerialPolymorphicNumberTest.Case"}""")
+        assertFailsWith(SerializationException::class) {
+            Json.encodeToString<Sealed3>(Sealed3.Case())
+        }
+        assertFailsWith(SerializationException::class) {
+            Json.decodeFromString<Sealed3>("{}")
+        }
+    }
+
+    @Serializable
+    @UseSerialPolymorphicNumbers
+    sealed class Abstract {
+        @Serializable
+        @SerialPolymorphicNumber(Abstract::class, 1)
+        class Case : Abstract()
+
+        @Serializable
+        class Default(val type: Int?):Abstract()
+    }
+
+    val json = Json {
+        serializersModule = SerializersModule {
+            polymorphic(Abstract::class) {
+                subclass(Abstract.Case::class)
+                defaultDeserializerForNumber {
+                    Abstract.Default.serializer()
+                }
+            }
+        }
+    }
+
+    @Test
+    fun testPolymorphicModule() {
+        testConversion<Abstract>(json, Abstract.Case(), """{"type":1}""")
+        testConversion<Abstract>(json, Abstract.Default(0), """{"type":0}""")
+    }
+}

formats/json-tests/commonTest/src/kotlinx/serialization/test/JsonTestHelpers.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2017-2024 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.test
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlin.test.*
+
+inline fun <reified T> testConversion(json: Json, data: T, expectedHexString: String) {
+    val string = json.encodeToString(data)
+    assertEquals(expectedHexString, string)
+    assertEquals(data, json.decodeFromString(string))
+}
+
+inline fun <reified T> testConversion(data: T, expectedHexString: String) =
+    testConversion(Json, data, expectedHexString)