[KxSerialization] Fixed access private custom serializer on property

When a custom serializer is specified on a type and this type is used in a property of another serializable class, then on the JVM this leads to an error accessing the custom serializer class - because it is private and located in another package.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/2495


Merge-request: KT-MR-12877
Merged-by: Sergei Shanshin <Sergey.Shanshin@jetbrains.com>

plugins/kotlinx-serialization/testData/boxIr/privateCustomSerializer.kt

@@ -0,0 +1,105 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+// FILE: serializer.kt
+
+package a
+
+import kotlinx.serialization.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.json.*
+import kotlin.reflect.KClass
+import kotlin.test.*
+
+@Serializable(DataSerializer::class)
+data class Data(
+    val i: Int
+)
+
+
+@Serializer(forClass = Data::class)
+private object DataSerializer
+
+
+@Serializable
+object SerializableObject
+
+private object CustomSerializer : KSerializer<SerializableObject> {
+    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("My.SerializableObject", PrimitiveKind.STRING)
+
+    override fun serialize(encoder: Encoder, value: SerializableObject) {
+        encoder.encodeString("custom.serializer")
+    }
+
+    override fun deserialize(decoder: Decoder): SerializableObject {
+        TODO()
+    }
+}
+
+@Serializable
+data class DataWithObject(
+    // property is object and have custom private serializer
+    @Serializable(CustomSerializer::class) val obj: SerializableObject
+)
+
+class Outer {
+    private object CustomPrimitiveSerializer : KSerializer<JsonPrimitive> {
+        override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("My.CustomPrimitiveSerializer", PrimitiveKind.STRING)
+
+        override fun serialize(encoder: Encoder, value: JsonPrimitive) {
+            encoder.encodeString("custom.primitive.serializer")
+        }
+
+        override fun deserialize(decoder: Decoder): JsonPrimitive {
+            TODO()
+        }
+    }
+
+    fun serialize(): String {
+        @Serializable
+        data class Wrapper(
+            // property in local class with private custom serializer
+            @Serializable(CustomPrimitiveSerializer::class)
+            val value: JsonPrimitive,
+        )
+
+        return Json.encodeToString(Wrapper(JsonNull))
+    }
+}
+
+
+// FILE: holder.kt
+
+package b
+
+import kotlinx.serialization.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.json.*
+import kotlin.reflect.KClass
+import kotlin.test.*
+import a.Data
+import a.DataWithObject
+import a.SerializableObject
+import a.Outer
+
+@Serializable
+data class Holder(
+    val data: Data
+)
+
+
+fun box(): String {
+    val json = Json.encodeToString(Holder(Data(1)))
+    if (json != "{\"data\":{\"i\":1}}") return json
+
+    val jsonWithObject = Json.encodeToString(DataWithObject(SerializableObject))
+    if (jsonWithObject != "{\"obj\":\"custom.serializer\"}") return jsonWithObject
+
+    val jsonFromOuter = Outer().serialize()
+    if (jsonFromOuter != "{\"value\":\"custom.primitive.serializer\"}") return jsonFromOuter
+
+    return "OK"
+}