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

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlinx.serialization.compiler.backend.ir
 
 import org.jetbrains.kotlin.backend.common.lower.irThrow
+import org.jetbrains.kotlin.backend.jvm.lower.isJvmOptimizableDelegate
 import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.descriptors.DescriptorVisibilities
 import org.jetbrains.kotlin.descriptors.ValueParameterDescriptor
@@ -21,6 +22,7 @@ import org.jetbrains.kotlin.ir.types.*
 import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.platform.jvm.isJvm
 import org.jetbrains.kotlin.util.OperatorNameConventions
 import org.jetbrains.kotlin.utils.getOrPutNullable
 import org.jetbrains.kotlinx.serialization.compiler.extensions.SerializationPluginContext
@@ -82,8 +84,10 @@ class SerializableIrGenerator(
                     it is IrProperty && it.backingField != null -> {
                         if (it in serialDescs) {
                             current = it
-                        } else if (it.backingField?.initializer != null && !it.isDelegated) {
-                            // skip transient lateinit or deferred properties (with null initializer)
+                        } else if (it.backingField?.initializer != null &&
+                            !(it.isJvmOptimizableDelegate() && compilerContext.platform.isJvm())
+                        ) {
+                            // skip transient lateinit or deferred properties (with null initializer) and optimized delegations
                             val expression = initializerAdapter(it.backingField!!.initializer!!)
 
                             statementsAfterSerializableProperty.getOrPutNullable(current, { mutableListOf() })

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"
-}
+}

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrJsBoxTestGenerated.java

@@ -31,6 +31,12 @@ public class SerializationIrJsBoxTestGenerated extends AbstractSerializationIrJs
         runTest("plugins/kotlinx-serialization/testData/boxIr/constValInSerialName.kt");
     }
 
+    @Test
+    @TestMetadata("delegatedProperty.kt")
+    public void testDelegatedProperty() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/delegatedProperty.kt");
+    }
+
     @Test
     @TestMetadata("excludedFromExport.kt")
     public void testExcludedFromExport() throws Exception {