[KxSerialization] Fix NPE when accessing delegated property

Kotlin 1.7.20 added optimizations for delegated properties on the JVM,
which broke serialization for optimized properties. Commit bfeff81
tried to fix that, but broke non-optimized delegated properties. This
commit restores correct serialization for optimized and non-optimized
properties, also ensuring that it only affects the JVM target.

 #KT-58954 Fixed
 #KT-59113 Fixed

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

@@ -1,5 +1,3 @@
-// TARGET_BACKEND: JVM_IR
-
 // WITH_STDLIB
 
 import kotlinx.serialization.*
@@ -69,6 +67,34 @@ class DelegatedByThis(val realProp: Int) {
     val delegatedProp by this
 }
 
+// delegating directly to another property
+@Serializable
+class DelegatedByDirectProperty(var targetProperty: Int = 5) {
+    var delegatingProperty: Int by ::targetProperty
+}
+
+// generic delegate
+@Serializable
+open class GenericDelegate<Target> {
+    private var target: Target? = null
+
+    operator fun getValue(thisRef: Any?, property: KProperty<*>): Target? = target
+
+    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Target?) {
+        target = value
+    }
+}
+
+@Serializable
+class GenericDelegateHolder(val id: String) {
+    private var _delegatingProperty = GenericDelegate<String>()
+    var delegatingProperty: String? by _delegatingProperty
+
+    constructor(id: String, delegatingProperty: String?) : this(id) {
+        this.delegatingProperty = delegatingProperty
+    }
+}
+
 fun box(): String {
     val simpleDTO = SimpleDTO(123)
     val simpleDTOJsonStr = Json.encodeToString(simpleDTO)
@@ -105,5 +131,21 @@ fun box(): String {
     if (byThisDecoded.delegatedProp != byThisExp.delegatedProp) return "DelegatedByThis Delegate is incorrect!"
     if (byThisDecoded.realProp !== 123) return "DelegatedByThis Deserialization failed"
 
+    for (original in listOf(
+        GenericDelegateHolder("#1", "stuff"),
+        GenericDelegateHolder("#2", null)
+    )) {
+        val json = Json.encodeToString(original)
+        val deserialized = Json.decodeFromString(GenericDelegateHolder.serializer(), json)
+        if (deserialized.delegatingProperty != original.delegatingProperty) return "Generic delegate fail: $json"
+    }
+
+    val byDirectPropertyExp = DelegatedByDirectProperty(123)
+    val byDirectPropertyJsonStr = Json.encodeToString(byDirectPropertyExp)
+    val byDirectPropertyDecoded = Json.decodeFromString<DelegatedByDirectProperty>(byDirectPropertyJsonStr)
+    if (byDirectPropertyJsonStr != """{"targetProperty":123}""") return simpleDTOJsonStr
+    if (byDirectPropertyDecoded.delegatingProperty != byDirectPropertyExp.delegatingProperty) return "Direct property delegation, delegatingProperty fail: $byDirectPropertyJsonStr"
+    if (byDirectPropertyDecoded.targetProperty !== 123) return "Direct property delegation, targetProperty fail: $byDirectPropertyJsonStr"
+
     return "OK"
-}
+}