Additional fix for Compose:

directly search for fields in IR class w/o using the symbol table.
Because Compose copies whole IR, fields got referenced incorrectly.
In the end, bytecode generator thinks that this is a field from other
class and creates a synthetic setter, which is prohibited in Java 9+
(Update to non-static final field ... attempted from a different method than <init>)

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

@@ -214,17 +214,16 @@ interface IrBuilderExtension {
     // note: this method should be used only for properties from current module. Fields from other modules are private and inaccessible.
     val SerializableProperty.irField: IrField get() = compilerContext.symbolTable.referenceField(this.descriptor).owner
 
-    val SerializableProperty.irProp: IrProperty
-        get() {
-            val desc = this.descriptor
-            // this API is used to reference both current module descriptors and external ones (because serializable class can be in any of them),
-            // so we use descriptor api for current module because it is not possible to obtain FQname for e.g. local classes.
-            return if (desc.module == compilerContext.moduleDescriptor) {
-                compilerContext.symbolTable.referenceProperty(desc).owner
-            } else {
-                compilerContext.referenceProperties(this.descriptor.fqNameSafe).single().owner
-            }
+    fun SerializableProperty.getIrPropertyFrom(thisClass: IrClass): IrProperty {
+        val desc = this.descriptor
+        // this API is used to reference both current module descriptors and external ones (because serializable class can be in any of them),
+        // so we use descriptor api for current module because it is not possible to obtain FQname for e.g. local classes.
+        return thisClass.searchForDeclaration(desc) ?: if (desc.module == compilerContext.moduleDescriptor) {
+            compilerContext.symbolTable.referenceProperty(desc).owner
+        } else {
+            compilerContext.referenceProperties(desc.fqNameSafe).single().owner
         }
+    }
 
     fun IrBuilderWithScope.getProperty(receiver: IrExpression, property: IrProperty): IrExpression {
         return if (property.getter != null)

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

@@ -99,13 +99,15 @@ class SerializableIrGenerator(
             for (index in startPropOffset until serializableProperties.size) {
                 val prop = serializableProperties[index]
                 val paramRef = ctor.valueParameters[index + seenVarsOffset]
-                // assign this.a = a in else branch
-                val assignParamExpr = setProperty(irGet(thiz), prop.irProp, irGet(paramRef))
+                // Assign this.a = a in else branch
+                // Set field directly w/o setter to match behavior of old backend plugin
+                val backingFieldToAssign = prop.getIrPropertyFrom(irClass).backingField!!
+                val assignParamExpr = irSetField(irGet(thiz), backingFieldToAssign, irGet(paramRef))
 
                 val ifNotSeenExpr: IrExpression = if (prop.optional) {
                     val initializerBody =
                         requireNotNull(transformFieldInitializer(prop.irField)) { "Optional value without an initializer" } // todo: filter abstract here
-                    setProperty(irGet(thiz), prop.irProp, initializerBody)
+                    irSetField(irGet(thiz), backingFieldToAssign, initializerBody)
                 } else {
                     // property required
                     if (useFieldMissingOptimization) {

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

@@ -138,7 +138,7 @@ open class SerializerIrGenerator(
             if (classProp.transient) continue
             +addFieldCall(classProp)
             // add property annotations
-            val property = classProp.irProp
+            val property = classProp.getIrPropertyFrom(serializableIrClass)
             copySerialInfoAnnotationsToDescriptor(
                 property.annotations,
                 localDescriptor,
@@ -268,7 +268,7 @@ open class SerializerIrGenerator(
                 irGet(
                     type = ownerType,
                     variable = objectToSerialize.symbol
-                ), irProp
+                ), getIrPropertyFrom(serializableIrClass)
             )
         }