Created a deep copier good for fake overrides.

Adapted it from inliner copier.
It is capable of producing type substituted members.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/inline/DeepCopyIrTreeWithDescriptors.kt

@@ -21,7 +21,6 @@ import org.jetbrains.kotlin.ir.visitors.acceptVoid
 import org.jetbrains.kotlin.name.Name
 
 internal class DeepCopyIrTreeWithSymbolsForInliner(
-    val context: CommonBackendContext,
     val typeArguments: Map<IrTypeParameterSymbol, IrType?>?,
     val parent: IrDeclarationParent?
 ) {

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/inline/FunctionInlining.kt

@@ -120,7 +120,7 @@ class FunctionInlining(
                 (0 until callSite.typeArgumentsCount).map {
                     typeParameters[it].symbol to callSite.getTypeArgument(it)
                 }.associate { it }
-            DeepCopyIrTreeWithSymbolsForInliner(context, typeArguments, parent)
+            DeepCopyIrTreeWithSymbolsForInliner(typeArguments, parent)
         }
 
         val substituteMap = mutableMapOf<IrValueParameter, IrExpression>()

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

@@ -62,22 +62,22 @@ open class DeepCopyIrTreeWithSymbols(
         }
     }
 
-    private fun mapDeclarationOrigin(origin: IrDeclarationOrigin) = origin
+    protected fun mapDeclarationOrigin(origin: IrDeclarationOrigin) = origin
-    private fun mapStatementOrigin(origin: IrStatementOrigin?) = origin
+    protected fun mapStatementOrigin(origin: IrStatementOrigin?) = origin
 
-    private inline fun <reified T : IrElement> T.transform() =
+    protected inline fun <reified T : IrElement> T.transform() =
         transform(this@DeepCopyIrTreeWithSymbols, null) as T
 
-    private inline fun <reified T : IrElement> List<T>.transform() =
+    protected inline fun <reified T : IrElement> List<T>.transform() =
         map { it.transform() }
 
-    private inline fun <reified T : IrElement> List<T>.transformTo(destination: MutableList<T>) =
+    protected inline fun <reified T : IrElement> List<T>.transformTo(destination: MutableList<T>) =
         mapTo(destination) { it.transform() }
 
-    private fun <T : IrDeclarationContainer> T.transformDeclarationsTo(destination: T) =
+    protected fun <T : IrDeclarationContainer> T.transformDeclarationsTo(destination: T) =
         declarations.transformTo(destination.declarations)
 
-    private fun IrType.remapType() = typeRemapper.remapType(this)
+    protected fun IrType.remapType() = typeRemapper.remapType(this)
 
     override fun visitElement(element: IrElement): IrElement =
         throw IllegalArgumentException("Unsupported element type: $element")
@@ -201,7 +201,7 @@ open class DeepCopyIrTreeWithSymbols(
             }
         }
 
-    private fun IrMutableAnnotationContainer.transformAnnotations(declaration: IrAnnotationContainer) {
+    protected fun IrMutableAnnotationContainer.transformAnnotations(declaration: IrAnnotationContainer) {
         annotations = declaration.annotations.transform()
     }
 
@@ -322,7 +322,7 @@ open class DeepCopyIrTreeWithSymbols(
             transformAnnotations(declaration)
         }
 
-    private fun IrTypeParametersContainer.copyTypeParametersFrom(other: IrTypeParametersContainer) {
+    protected fun IrTypeParametersContainer.copyTypeParametersFrom(other: IrTypeParametersContainer) {
         this.typeParameters = other.typeParameters.map {
             copyTypeParameter(it)
         }

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/DeepCopyIrTreeWithSymbolsForFakeOverrides.kt

@@ -0,0 +1,104 @@
+package org.jetbrains.kotlin.backend.common.serialization
+
+import org.jetbrains.kotlin.backend.common.DescriptorsToIrRemapper
+import org.jetbrains.kotlin.backend.common.WrappedDescriptorPatcher
+import org.jetbrains.kotlin.ir.IrElement
+import org.jetbrains.kotlin.ir.declarations.IrClass
+import org.jetbrains.kotlin.ir.declarations.IrTypeParametersContainer
+import org.jetbrains.kotlin.ir.expressions.IrConstructorCall
+import org.jetbrains.kotlin.ir.symbols.IrClassifierSymbol
+import org.jetbrains.kotlin.ir.symbols.IrTypeParameterSymbol
+import org.jetbrains.kotlin.ir.types.*
+import org.jetbrains.kotlin.ir.types.impl.buildSimpleType
+import org.jetbrains.kotlin.ir.types.impl.makeTypeProjection
+import org.jetbrains.kotlin.ir.util.*
+import org.jetbrains.kotlin.ir.visitors.acceptVoid
+
+// This is basicly modelled after the inliner copier.
+class DeepCopyIrTreeWithSymbolsForFakeOverrides(
+    val typeArguments: Map<IrTypeParameterSymbol, IrType>,
+    val superType: IrType,
+    val parent: IrClass
+) {
+
+    fun copy(irElement: IrElement): IrElement {
+        // Create new symbols.
+        irElement.acceptVoid(symbolRemapper)
+
+        // Make symbol remapper aware of the callsite's type arguments.
+        symbolRemapper.typeArguments = typeArguments
+
+        // Copy IR.
+        val result = irElement.transform(copier, data = null)
+
+        // Bind newly created IR with wrapped descriptors.
+        result.acceptVoid(WrappedDescriptorPatcher)
+
+        result.patchDeclarationParents(parent)
+        return result
+    }
+
+    private inner class FakeOverrideTypeRemapper(
+        val symbolRemapper: SymbolRemapper,
+        val typeArguments: Map<IrTypeParameterSymbol, IrType>
+    ) : TypeRemapper {
+
+        override fun enterScope(irTypeParametersContainer: IrTypeParametersContainer) {}
+
+        override fun leaveScope() {}
+
+        private fun remapTypeArguments(arguments: List<IrTypeArgument>) =
+            arguments.map { argument ->
+                (argument as? IrTypeProjection)?.let { makeTypeProjection(remapType(it.type), it.variance) }
+                    ?: argument
+            }
+
+        override fun remapType(type: IrType): IrType {
+            if (type !is IrSimpleType) return type
+
+            val substitutedType = typeArguments[type.classifier]
+
+            if (substitutedType is IrDynamicType) return substitutedType
+
+            if (substitutedType is IrSimpleType) {
+                return substitutedType.buildSimpleType {
+                    kotlinType = null
+                    hasQuestionMark = type.hasQuestionMark or substitutedType.isMarkedNullable()
+                }
+            }
+
+            return type.buildSimpleType {
+                kotlinType = null
+                classifier = symbolRemapper.getReferencedClassifier(type.classifier)
+                arguments = remapTypeArguments(type.arguments)
+                annotations = type.annotations.map { it.transform(copier, null) as IrConstructorCall }
+            }
+        }
+    }
+
+    private class FakeOverrideSymbolRemapperImpl(descriptorsRemapper: DescriptorsRemapper) :
+        DeepCopySymbolRemapper(descriptorsRemapper) {
+
+        var typeArguments = mapOf<IrTypeParameterSymbol, IrType>()
+            set(value) {
+                field = value.asSequence().associate {
+                    (getReferencedClassifier(it.key) as IrTypeParameterSymbol) to it.value
+                }
+            }
+
+        override fun getReferencedClassifier(symbol: IrClassifierSymbol): IrClassifierSymbol {
+            val result = super.getReferencedClassifier(symbol)
+            if (result !is IrTypeParameterSymbol)
+                return result
+            return typeArguments[result]?.classifierOrNull ?: result
+        }
+    }
+
+    private val symbolRemapper = FakeOverrideSymbolRemapperImpl(DescriptorsToIrRemapper)
+    private val copier =
+        FakeOverrideCopier(
+            symbolRemapper,
+            FakeOverrideTypeRemapper(symbolRemapper, typeArguments),
+            SymbolRenamer.DEFAULT
+        )
+}

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/FakeOverrideCopier.kt

@@ -0,0 +1,86 @@
+package org.jetbrains.kotlin.backend.common.serialization
+
+import org.jetbrains.kotlin.ir.declarations.IrDeclarationOrigin
+import org.jetbrains.kotlin.ir.declarations.IrFunction
+import org.jetbrains.kotlin.ir.declarations.IrProperty
+import org.jetbrains.kotlin.ir.declarations.IrSimpleFunction
+import org.jetbrains.kotlin.ir.declarations.impl.IrFakeOverrideFunctionImpl
+import org.jetbrains.kotlin.ir.declarations.impl.IrFakeOverridePropertyImpl
+import org.jetbrains.kotlin.ir.util.*
+
+class FakeOverrideCopier(
+    symbolRemapper: SymbolRemapper,
+    private val typeRemapper: TypeRemapper,
+    private val symbolRenamer: SymbolRenamer
+) : DeepCopyIrTreeWithSymbols(symbolRemapper, typeRemapper, symbolRenamer) {
+
+    private fun <T : IrFunction> T.transformFunctionChildren(declaration: T): T =
+        apply {
+            transformAnnotations(declaration)
+            copyTypeParametersFrom(declaration)
+            typeRemapper.withinScope(this) {
+                // This is the more correct way to produce dispatch receiver for a fake override,
+                // but some lowerings still expect the below behavior as produced by the current psi2ir.
+                /*
+                val superDispatchReceiver = declaration.dispatchReceiverParameter!!
+                val dispatchReceiverSymbol = IrValueParameterSymbolImpl(WrappedReceiverParameterDescriptor())
+                val dispatchReceiverType = destinationClass.defaultType
+                dispatchReceiverParameter = IrValueParameterImpl(
+                    superDispatchReceiver.startOffset,
+                    superDispatchReceiver.endOffset,
+                    superDispatchReceiver.origin,
+                    dispatchReceiverSymbol,
+                    superDispatchReceiver.name,
+                    superDispatchReceiver.index,
+                    dispatchReceiverType,
+                    null,
+                    superDispatchReceiver.isCrossinline,
+                    superDispatchReceiver.isNoinline
+                )
+                */
+                // Should fake override's receiver be the current class is an open question.
+                dispatchReceiverParameter = declaration.dispatchReceiverParameter?.transform()
+                extensionReceiverParameter = declaration.extensionReceiverParameter?.transform()
+                returnType = typeRemapper.remapType(declaration.returnType)
+                this.valueParameters = declaration.valueParameters.transform()
+            }
+        }
+
+    override fun visitSimpleFunction(declaration: IrSimpleFunction): IrSimpleFunction =
+        IrFakeOverrideFunctionImpl(
+            declaration.startOffset, declaration.endOffset,
+            IrDeclarationOrigin.FAKE_OVERRIDE,
+            symbolRenamer.getFunctionName(declaration.symbol),
+            declaration.visibility,
+            declaration.modality,
+            declaration.returnType,
+            isInline = declaration.isInline,
+            isExternal = false,
+            isTailrec = declaration.isTailrec,
+            isSuspend = declaration.isSuspend,
+            isExpect = declaration.isExpect,
+            isOperator = declaration.isOperator
+        ).apply {
+            transformFunctionChildren(declaration)
+        }
+
+
+    override fun visitProperty(declaration: IrProperty): IrProperty =
+        IrFakeOverridePropertyImpl(
+            declaration.startOffset, declaration.endOffset,
+            IrDeclarationOrigin.FAKE_OVERRIDE,
+            declaration.name,
+            declaration.visibility,
+            declaration.modality,
+            isVar = declaration.isVar,
+            isConst = declaration.isConst,
+            isLateinit = declaration.isLateinit,
+            isDelegated = declaration.isDelegated,
+            isExpect = declaration.isExpect,
+            isExternal = false
+        ).apply {
+            transformAnnotations(declaration)
+            this.getter = declaration.getter?.transform()
+            this.setter = declaration.setter?.transform()
+        }
+}