Properties without accessors are generated as

IrGetBackingField / IrSetBackingField expressions.

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/IrSlots.kt

@@ -22,6 +22,7 @@ const val ARGUMENT0_SLOT = 0
 const val ARGUMENT1_SLOT = 1
 const val DISPATCH_RECEIVER_SLOT = -1
 const val EXTENSION_RECEIVER_SLOT = -2
+const val BACKING_FIELD_RECEIVER_SLOT = -1
 const val FUNCTION_BODY_SLOT = -1
 const val ANONYMOUS_INITIALIZER_BODY_SLOT = -1
 const val MODULE_SLOT = 0

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/IrBackingFieldExpression.kt

@@ -16,6 +16,7 @@
 
 package org.jetbrains.kotlin.ir.expressions
 
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.PropertyDescriptor
 import org.jetbrains.kotlin.ir.*
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
@@ -24,6 +25,8 @@ import org.jetbrains.kotlin.types.typeUtil.builtIns
 
 interface IrBackingFieldExpression : IrDeclarationReference {
     override val descriptor: PropertyDescriptor
+    val superQualifier: ClassDescriptor?
+    var receiver: IrExpression?
     val operator: IrOperator?
 }
 
@@ -38,20 +41,54 @@ abstract class IrBackingFieldExpressionBase(
         endOffset: Int,
         descriptor: PropertyDescriptor,
         type: KotlinType,
-        override val operator: IrOperator? = null
-) : IrDeclarationReferenceBase<PropertyDescriptor>(startOffset, endOffset, type, descriptor), IrBackingFieldExpression
+        override val operator: IrOperator? = null,
+        override val superQualifier: ClassDescriptor? = null
+) : IrDeclarationReferenceBase<PropertyDescriptor>(startOffset, endOffset, type, descriptor), IrBackingFieldExpression {
+    override final var receiver: IrExpression? = null
+        set(value) {
+            value?.assertDetached()
+            field?.detach()
+            field = value
+            value?.setTreeLocation(this, BACKING_FIELD_RECEIVER_SLOT)
+        }
+
+    override fun getChild(slot: Int): IrElement? =
+            when (slot) {
+                BACKING_FIELD_RECEIVER_SLOT -> receiver
+                else -> null
+            }
+
+    override fun replaceChild(slot: Int, newChild: IrElement) {
+        when (slot) {
+            BACKING_FIELD_RECEIVER_SLOT -> receiver = newChild.assertCast()
+            else -> throwNoSuchSlot(slot)
+        }
+    }
+}
 
 class IrGetBackingFieldImpl(
         startOffset: Int,
         endOffset: Int,
         descriptor: PropertyDescriptor,
-        operator: IrOperator? = null
-) : IrBackingFieldExpressionBase(startOffset, endOffset, descriptor, descriptor.type, operator), IrGetBackingField {
+        operator: IrOperator? = null,
+        superQualifier: ClassDescriptor? = null
+) : IrBackingFieldExpressionBase(startOffset, endOffset, descriptor, descriptor.type, operator, superQualifier), IrGetBackingField {
+    constructor(
+            startOffset: Int,
+            endOffset: Int,
+            descriptor: PropertyDescriptor,
+            receiver: IrExpression?,
+            operator: IrOperator? = null,
+            superQualifier: ClassDescriptor? = null
+    ) : this(startOffset, endOffset, descriptor, operator, superQualifier) {
+        this.receiver = receiver
+    }
+
     override fun getChild(slot: Int): IrElement? =
-            null
+            super.getChild(slot)
 
     override fun replaceChild(slot: Int, newChild: IrElement) {
-        throwNoSuchSlot(slot)
+        super.replaceChild(slot, newChild)
     }
 
     override fun <R, D> accept(visitor: IrElementVisitor<R, D>, data: D): R {
@@ -59,7 +96,7 @@ class IrGetBackingFieldImpl(
     }
 
     override fun <D> acceptChildren(visitor: IrElementVisitor<Unit, D>, data: D) {
-        // no children
+        receiver?.accept(visitor, data)
     }
 }
 
@@ -67,15 +104,19 @@ class IrSetBackingFieldImpl(
         startOffset: Int,
         endOffset: Int,
         descriptor: PropertyDescriptor,
-        operator: IrOperator? = null
-) : IrBackingFieldExpressionBase(startOffset, endOffset, descriptor, descriptor.type.builtIns.unitType, operator), IrSetBackingField {
+        operator: IrOperator? = null,
+        superQualifier: ClassDescriptor? = null
+) : IrBackingFieldExpressionBase(startOffset, endOffset, descriptor, descriptor.type.builtIns.unitType, operator, superQualifier), IrSetBackingField {
     constructor(
             startOffset: Int,
             endOffset: Int,
             descriptor: PropertyDescriptor,
+            receiver: IrExpression?,
             value: IrExpression,
-            operator: IrOperator? = null
-    ) : this(startOffset, endOffset, descriptor, operator) {
+            operator: IrOperator? = null,
+            superQualifier: ClassDescriptor? = null
+    ) : this(startOffset, endOffset, descriptor, operator, superQualifier) {
+        this.receiver = receiver
         this.value = value
     }
 
@@ -92,12 +133,13 @@ class IrSetBackingFieldImpl(
     override fun getChild(slot: Int): IrElement? =
             when (slot) {
                 CHILD_EXPRESSION_SLOT -> value
-                else -> null
+                else -> super.getChild(slot)
             }
 
     override fun replaceChild(slot: Int, newChild: IrElement) {
         when (slot) {
             CHILD_EXPRESSION_SLOT -> value = newChild.assertCast()
+            else -> super.replaceChild(slot, newChild)
         }
     }
 
@@ -106,6 +148,7 @@ class IrSetBackingFieldImpl(
     }
 
     override fun <D> acceptChildren(visitor: IrElementVisitor<Unit, D>, data: D) {
+        receiver?.accept(visitor, data)
         value.accept(visitor, data)
     }
 }

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DumpIrTree.kt

@@ -94,6 +94,19 @@ class DumpIrTreeVisitor(out: Appendable): IrElementVisitor<Unit, String> {
         }
     }
 
+    override fun visitGetBackingField(expression: IrGetBackingField, data: String) {
+        expression.dumpLabeledElementWith(data) {
+            expression.receiver?.accept(this, "receiver")
+        }
+    }
+
+    override fun visitSetBackingField(expression: IrSetBackingField, data: String) {
+        expression.dumpLabeledElementWith(data) {
+            expression.receiver?.accept(this, "receiver")
+            expression.value.accept(this, "value")
+        }
+    }
+
     override fun visitWhen(expression: IrWhen, data: String) {
         expression.dumpLabeledElementWith(data) {
             for (i in 0 .. expression.branchesCount - 1) {

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/visitors/IrElementVisitor.kt

@@ -62,8 +62,9 @@ interface IrElementVisitor<out R, in D> {
     fun visitGetEnumValue(expression: IrGetEnumValue, data: D) = visitSingletonReference(expression, data)
     fun visitGetVariable(expression: IrGetVariable, data: D) = visitDeclarationReference(expression, data)
     fun visitSetVariable(expression: IrSetVariable, data: D) = visitDeclarationReference(expression, data)
-    fun visitGetBackingField(expression: IrGetBackingField, data: D) = visitDeclarationReference(expression, data)
-    fun visitSetBackingField(expression: IrSetBackingField, data: D) = visitDeclarationReference(expression, data)
+    fun visitBackingFieldReference(expression: IrBackingFieldExpression, data: D) = visitDeclarationReference(expression, data)
+    fun visitGetBackingField(expression: IrGetBackingField, data: D) = visitBackingFieldReference(expression, data)
+    fun visitSetBackingField(expression: IrSetBackingField, data: D) = visitBackingFieldReference(expression, data)
     fun visitGetExtensionReceiver(expression: IrGetExtensionReceiver, data: D) = visitDeclarationReference(expression, data)
     fun visitGeneralCall(expression: IrGeneralCall, data: D) = visitDeclarationReference(expression, data)
     fun visitCall(expression: IrCall, data: D) = visitGeneralCall(expression, data)

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/visitors/IrElementVisitorVoid.kt

@@ -129,10 +129,13 @@ interface IrElementVisitorVoid : IrElementVisitor<Unit, Nothing?> {
     fun visitSetVariable(expression: IrSetVariable) = visitDeclarationReference(expression)
     override fun visitSetVariable(expression: IrSetVariable, data: Nothing?) = visitSetVariable(expression)
 
-    fun visitGetBackingField(expression: IrGetBackingField) = visitDeclarationReference(expression)
+    fun visitBackingFieldReference(expression: IrBackingFieldExpression) = visitDeclarationReference(expression)
+    override fun visitBackingFieldReference(expression: IrBackingFieldExpression, data: Nothing?) = visitBackingFieldReference(expression)
+
+    fun visitGetBackingField(expression: IrGetBackingField) = visitBackingFieldReference(expression)
     override fun visitGetBackingField(expression: IrGetBackingField, data: Nothing?) = visitGetBackingField(expression)
 
-    fun visitSetBackingField(expression: IrSetBackingField) = visitDeclarationReference(expression)
+    fun visitSetBackingField(expression: IrSetBackingField) = visitBackingFieldReference(expression)
     override fun visitSetBackingField(expression: IrSetBackingField, data: Nothing?) = visitSetBackingField(expression)
 
     fun visitGetExtensionReceiver(expression: IrGetExtensionReceiver) = visitDeclarationReference(expression)