[JVM IR] Interface Delegation by Property should use existing field

When a property backed by a final field is the delegate for interface
implementation by delegation, the backend should reuse the backing
field rather than introduce a new, extraneous `$$delegate_n` field.

compiler/ir/ir.psi2ir/src/org/jetbrains/kotlin/psi2ir/generators/ClassGenerator.kt

@@ -16,6 +16,7 @@
 
 package org.jetbrains.kotlin.psi2ir.generators
 
+import org.jetbrains.kotlin.backend.common.CodegenUtil
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.ir.declarations.*
@@ -25,6 +26,8 @@ import org.jetbrains.kotlin.ir.expressions.mapValueParameters
 import org.jetbrains.kotlin.ir.expressions.putTypeArguments
 import org.jetbrains.kotlin.ir.expressions.typeParametersCount
 import org.jetbrains.kotlin.ir.util.declareSimpleFunctionWithOverrides
+import org.jetbrains.kotlin.ir.util.fields
+import org.jetbrains.kotlin.ir.util.properties
 import org.jetbrains.kotlin.ir.util.referenceFunction
 import org.jetbrains.kotlin.psi.KtClassOrObject
 import org.jetbrains.kotlin.psi.KtDelegatedSuperTypeEntry
@@ -176,14 +179,21 @@ class ClassGenerator(
         val superClass = superTypeConstructorDescriptor as? ClassDescriptor
             ?: throw AssertionError("Unexpected supertype constructor for delegation: $superTypeConstructorDescriptor")
 
-        val delegateDescriptor = IrImplementingDelegateDescriptorImpl(irClass.descriptor, delegateType, superType, delegateNumber)
-        val irDelegateField = context.symbolTable.declareField(
-            ktDelegateExpression.startOffsetSkippingComments, ktDelegateExpression.endOffset,
-            IrDeclarationOrigin.DELEGATE,
-            delegateDescriptor, delegateDescriptor.type.toIrType(),
-            createBodyGenerator(irClass.symbol).generateExpressionBody(ktDelegateExpression)
-        )
-        irClass.addMember(irDelegateField)
+        val propertyDescriptor = CodegenUtil.getDelegatePropertyIfAny(ktDelegateExpression, irClass.descriptor, context.bindingContext)
+
+        val irDelegateField: IrField = if (CodegenUtil.isFinalPropertyWithBackingField(propertyDescriptor, context.bindingContext)) {
+            irClass.properties.first { it.descriptor == propertyDescriptor }.backingField!!
+        } else {
+            val delegateDescriptor = IrImplementingDelegateDescriptorImpl(irClass.descriptor, delegateType, superType, delegateNumber)
+            context.symbolTable.declareField(
+                ktDelegateExpression.startOffsetSkippingComments, ktDelegateExpression.endOffset,
+                IrDeclarationOrigin.DELEGATE,
+                delegateDescriptor, delegateDescriptor.type.toIrType(),
+                createBodyGenerator(irClass.symbol).generateExpressionBody(ktDelegateExpression)
+            ).apply {
+                irClass.addMember(this)
+            }
+        }
 
         val delegatesMap = DelegationResolver.getDelegates(irClass.descriptor, superClass, delegateType)
         val delegatedMembers = delegatesMap.keys.toList().sortedByRenderer()

compiler/testData/codegen/box/delegation/delegationToVal.kt

@@ -1,5 +1,4 @@
 // IGNORE_BACKEND_FIR: JVM_IR
-// IGNORE_BACKEND: JVM_IR
 // TARGET_BACKEND: JVM
 
 // WITH_RUNTIME
@@ -37,7 +36,7 @@ fun box(): String {
 
     try {
         Test2::class.java.getDeclaredField("\$\$delegate_0")
-        return "\$\$delegate_0 field generated for class Test but should not"
+        return "\$\$delegate_0 field generated for class Test2 but should not"
     }
     catch (e: NoSuchFieldException) {
         // ok