Correctly handle situation where different serializers can be provided (#2113)

for the same KClass in SealedClassSerializer.

Previous fix supported only equal serializers.

Fixes #2110
Fixes #1937

Author: sandwwraith

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

@@ -105,9 +105,9 @@ public class SealedClassSerializer<T : Any>(
             element("type", String.serializer().descriptor)
             val elementDescriptor =
                 buildSerialDescriptor("kotlinx.serialization.Sealed<${baseClass.simpleName}>", SerialKind.CONTEXTUAL) {
-                    subclassSerializers.distinct().forEach {
-                        val d = it.descriptor
-                        element(d.serialName, d)
+                    // serialName2Serializer is guaranteed to have no duplicates — checked in `init`.
+                    serialName2Serializer.forEach { (name, serializer) ->
+                        element(name, serializer.descriptor)
                     }
                 }
             element("value", elementDescriptor)
@@ -123,6 +123,9 @@ public class SealedClassSerializer<T : Any>(
             throw IllegalArgumentException("All subclasses of sealed class ${baseClass.simpleName} should be marked @Serializable")
         }
 
+        // Note: we do not check whether different serializers are provided if the same KClass duplicated in the `subclasses`.
+        // Plugin should produce identical serializers, although they are not always strictly equal (e.g. new ObjectSerializer
+        // may be created every time)
         class2Serializer = subclasses.zip(subclassSerializers).toMap()
         serialName2Serializer = class2Serializer.entries.groupingBy { it.value.descriptor.serialName }
             .aggregate<Map.Entry<KClass<out T>, KSerializer<out T>>, String, Map.Entry<KClass<*>, KSerializer<out T>>>
@@ -137,7 +140,10 @@ public class SealedClassSerializer<T : Any>(
             }.mapValues { it.value.value }
     }
 
-    override fun findPolymorphicSerializerOrNull(decoder: CompositeDecoder, klassName: String?): DeserializationStrategy<T>? {
+    override fun findPolymorphicSerializerOrNull(
+        decoder: CompositeDecoder,
+        klassName: String?
+    ): DeserializationStrategy<T>? {
         return serialName2Serializer[klassName] ?: super.findPolymorphicSerializerOrNull(decoder, klassName)
     }
 

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

@@ -279,7 +279,7 @@ public class ClassSerialDescriptorBuilder internal constructor(
         annotations: List<Annotation> = emptyList(),
         isOptional: Boolean = false
     ) {
-        require(uniqueNames.add(elementName)) { "Element with name '$elementName' is already registered" }
+        require(uniqueNames.add(elementName)) { "Element with name '$elementName' is already registered in $serialName" }
         elementNames += elementName
         elementDescriptors += descriptor
         elementAnnotations += annotations

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

@@ -20,10 +20,19 @@ class SealedDiamondTest : JsonTestBase() {
     @SerialName("X")
     data class X(val i: Int) : B, C
 
+    @Serializable
+    @SerialName("Y")
+    object Y : B, C
+
+    @SerialName("E")
+    enum class E : B, C {
+        Q, W
+    }
+
     @Test
     fun testMultipleSuperSealedInterfacesDescriptor() {
         val subclasses = A.serializer().descriptor.getElementDescriptor(1).elementDescriptors.map { it.serialName }
-        assertEquals(listOf("X"), subclasses)
+        assertEquals(listOf("E", "X", "Y"), subclasses)
     }
 
     @Test
@@ -32,8 +41,8 @@ class SealedDiamondTest : JsonTestBase() {
         data class Carrier(val a: A, val b: B, val c: C)
         assertJsonFormAndRestored(
             Carrier.serializer(),
-            Carrier(X(1), X(2), X(3)),
-            """{"a":{"type":"X","i":1},"b":{"type":"X","i":2},"c":{"type":"X","i":3}}"""
+            Carrier(X(1), X(2), Y),
+            """{"a":{"type":"X","i":1},"b":{"type":"X","i":2},"c":{"type":"Y"}}"""
         )
     }