Presence of (transient) delegated field in the serialized class breaks deserialization (JetBrains#5103)

Do not include delegated field into generated constructor even though it
might have backing field.

Test that shows issue was added. Properties that are explicitly marked
as delegated are now excluded.

Fixes Kotlin/kotlinx.serialization#2091

Author: AngryGami

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializableIrGenerator.kt

@@ -82,7 +82,7 @@ class SerializableIrGenerator(
                     it is IrProperty && it.backingField != null -> {
                         if (it in serialDescs) {
                             current = it
-                        } else if (it.backingField?.initializer != null) {
+                        } else if (it.backingField?.initializer != null && !it.isDelegated) {
                             // skip transient lateinit or deferred properties (with null initializer)
                             val expression = initializerAdapter(it.backingField!!.initializer!!)
 

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

@@ -0,0 +1,109 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.json.*
+import kotlin.reflect.KProperty
+import kotlin.properties.*
+
+@Serializable
+data class SimpleDTO(
+    val realProp: Int,
+) {
+    @Transient
+    private val additionalProperties: Map<String, Int> = mapOf("delegatedProp" to 123)
+    val delegatedProp: Int? by additionalProperties
+}
+
+// optimized properties must also work
+// https://kotlinlang.org/docs/whatsnew1720.html#more-optimized-cases-of-delegated-properties
+// A named object:
+object NamedObject {
+    operator fun getValue(thisRef: Any?, property: KProperty<*>): String = "test-string"
+}
+
+@Serializable
+data class DelegatedByObjectProperty(
+    val realProp: Int
+) {
+    val delegatedProp: String by NamedObject
+}
+
+// A final val property with a backing field and a default getter in the same module:
+val impl: ReadOnlyProperty<Any?, String> = object : ReadOnlyProperty<Any?, String> {
+    override operator fun getValue(thisRef: Any?, property: KProperty<*>): String = "test-string"
+}
+
+@Serializable
+class DelegatedByFinalVal(
+    val realProp: Int
+) {
+    val delegatedProp: String by impl
+}
+
+// A var property with a backing field and a default getter in the same module:
+var implvar: ReadWriteProperty<Any?, String> = object : ReadWriteProperty<Any?, String> {
+    private var value = "test-string"
+    override operator fun getValue(thisRef: Any?, property: KProperty<*>): String = value
+    override operator fun setValue(
+        thisRef: Any?,
+        property: KProperty<*>,
+        value: String) { this.value = value }
+}
+
+@Serializable
+class DelegatedByVar(
+    val realProp: Int
+) {
+    var delegatedProp: String by implvar
+}
+
+// delegated by this
+@Serializable
+class DelegatedByThis(val realProp: Int) {
+    operator fun getValue(thisRef: Any?, property: KProperty<*>) = "test-string"
+
+    val delegatedProp by this
+}
+
+fun box(): String {
+    val simpleDTO = SimpleDTO(123)
+    val simpleDTOJsonStr = Json.encodeToString(simpleDTO)
+    val simpleDTODecoded = Json.decodeFromString<SimpleDTO>(simpleDTOJsonStr)
+    if (simpleDTOJsonStr != """{"realProp":123}""") return simpleDTOJsonStr
+    if (simpleDTODecoded.delegatedProp != simpleDTO.delegatedProp) return "SimpleDTO Delegate is incorrect!"
+    if (simpleDTODecoded.realProp !== 123) return "SimpleDTO Deserialization failed"
+
+    val objProp = DelegatedByObjectProperty(123)
+    val objPropJsonStr = Json.encodeToString(objProp)
+    val objPropDecoded = Json.decodeFromString<DelegatedByObjectProperty>(objPropJsonStr)
+    if (objPropJsonStr != """{"realProp":123}""") return simpleDTOJsonStr
+    if (objPropDecoded.delegatedProp != objProp.delegatedProp) return "DelegatedByObjectProperty Delegate is incorrect!"
+    if (objPropDecoded.realProp !== 123) return "DelegatedByObjectProperty Deserialization failed"
+
+    val byFinal = DelegatedByFinalVal(123)
+    val byFinalJsonStr = Json.encodeToString(byFinal)
+    val byFinalDecoded = Json.decodeFromString<DelegatedByObjectProperty>(byFinalJsonStr)
+    if (byFinalJsonStr != """{"realProp":123}""") return simpleDTOJsonStr
+    if (byFinalDecoded.delegatedProp != byFinal.delegatedProp) return "DelegatedByFinalVal Delegate is incorrect!"
+    if (byFinalDecoded.realProp !== 123) return "DelegatedByFinalVal Deserialization failed"
+
+    val byVar = DelegatedByVar(123)
+    val byVarJsonStr = Json.encodeToString(byVar)
+    val byVarDecoded = Json.decodeFromString<DelegatedByObjectProperty>(byVarJsonStr)
+    if (byVarJsonStr != """{"realProp":123}""") return simpleDTOJsonStr
+    if (byVarDecoded.delegatedProp != byVar.delegatedProp) return "DelegatedByVar Delegate is incorrect!"
+    if (byVarDecoded.realProp !== 123) return "DelegatedByVar Deserialization failed"
+
+    val byThisExp = DelegatedByThis(123)
+    val byThisJsonStr = Json.encodeToString(byThisExp)
+    val byThisDecoded = Json.decodeFromString<DelegatedByObjectProperty>(byThisJsonStr)
+    if (byThisJsonStr != """{"realProp":123}""") return simpleDTOJsonStr
+    if (byThisDecoded.delegatedProp != byThisExp.delegatedProp) return "DelegatedByThis Delegate is incorrect!"
+    if (byThisDecoded.realProp !== 123) return "DelegatedByThis Deserialization failed"
+
+    return "OK"
+}