diff --git a/compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/phaser/performByIrFile.kt b/compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/phaser/performByIrFile.kt
index 1d803aea687..9c270533b62 100644
--- a/compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/phaser/performByIrFile.kt
+++ b/compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/phaser/performByIrFile.kt
@@ -18,7 +18,7 @@ import org.jetbrains.kotlin.ir.expressions.impl.IrCallImpl
 import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
 import org.jetbrains.kotlin.ir.symbols.IrFileSymbol
 import org.jetbrains.kotlin.ir.symbols.IrSimpleFunctionSymbol
-import org.jetbrains.kotlin.ir.util.DeepCopySymbolRemapper
+import org.jetbrains.kotlin.ir.util.DeepCopySymbolRemapperPreservingSignatures
 import org.jetbrains.kotlin.ir.util.copyTypeAndValueArgumentsFrom
 import org.jetbrains.kotlin.ir.util.deepCopySavingMetadata
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
@@ -149,7 +149,7 @@ fun IrFile.copySavingMappings(
     return newIrFile
 }
 
-private class DeepCopySymbolRemapperSavingFunctions : DeepCopySymbolRemapper() {
+private class DeepCopySymbolRemapperSavingFunctions : DeepCopySymbolRemapperPreservingSignatures() {
     val declaredFunctions = mutableSetOf<IrSimpleFunctionSymbol>()
     val declaredClasses = mutableSetOf<IrClassSymbol>()
 
diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DeepCopySymbolRemapper.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DeepCopySymbolRemapper.kt
index 2bca5a2e8fa..1abe0783069 100644
--- a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DeepCopySymbolRemapper.kt
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DeepCopySymbolRemapper.kt
@@ -32,21 +32,21 @@ open class DeepCopySymbolRemapper(
     private val descriptorsRemapper: DescriptorsRemapper = NullDescriptorsRemapper
 ) : IrElementVisitorVoid, SymbolRemapper {
 
-    private val classes = hashMapOf<IrClassSymbol, IrClassSymbol>()
-    private val scripts = hashMapOf<IrScriptSymbol, IrScriptSymbol>()
-    private val constructors = hashMapOf<IrConstructorSymbol, IrConstructorSymbol>()
-    private val enumEntries = hashMapOf<IrEnumEntrySymbol, IrEnumEntrySymbol>()
-    private val externalPackageFragments = hashMapOf<IrExternalPackageFragmentSymbol, IrExternalPackageFragmentSymbol>()
-    private val fields = hashMapOf<IrFieldSymbol, IrFieldSymbol>()
-    private val files = hashMapOf<IrFileSymbol, IrFileSymbol>()
-    private val functions = hashMapOf<IrSimpleFunctionSymbol, IrSimpleFunctionSymbol>()
-    private val properties = hashMapOf<IrPropertySymbol, IrPropertySymbol>()
-    private val returnableBlocks = hashMapOf<IrReturnableBlockSymbol, IrReturnableBlockSymbol>()
-    private val typeParameters = hashMapOf<IrTypeParameterSymbol, IrTypeParameterSymbol>()
-    private val valueParameters = hashMapOf<IrValueParameterSymbol, IrValueParameterSymbol>()
-    private val variables = hashMapOf<IrVariableSymbol, IrVariableSymbol>()
-    private val localDelegatedProperties = hashMapOf<IrLocalDelegatedPropertySymbol, IrLocalDelegatedPropertySymbol>()
-    private val typeAliases = hashMapOf<IrTypeAliasSymbol, IrTypeAliasSymbol>()
+    protected val classes = hashMapOf<IrClassSymbol, IrClassSymbol>()
+    protected val scripts = hashMapOf<IrScriptSymbol, IrScriptSymbol>()
+    protected val constructors = hashMapOf<IrConstructorSymbol, IrConstructorSymbol>()
+    protected val enumEntries = hashMapOf<IrEnumEntrySymbol, IrEnumEntrySymbol>()
+    protected val externalPackageFragments = hashMapOf<IrExternalPackageFragmentSymbol, IrExternalPackageFragmentSymbol>()
+    protected val fields = hashMapOf<IrFieldSymbol, IrFieldSymbol>()
+    protected val files = hashMapOf<IrFileSymbol, IrFileSymbol>()
+    protected val functions = hashMapOf<IrSimpleFunctionSymbol, IrSimpleFunctionSymbol>()
+    protected val properties = hashMapOf<IrPropertySymbol, IrPropertySymbol>()
+    protected val returnableBlocks = hashMapOf<IrReturnableBlockSymbol, IrReturnableBlockSymbol>()
+    protected val typeParameters = hashMapOf<IrTypeParameterSymbol, IrTypeParameterSymbol>()
+    protected val valueParameters = hashMapOf<IrValueParameterSymbol, IrValueParameterSymbol>()
+    protected val variables = hashMapOf<IrVariableSymbol, IrVariableSymbol>()
+    protected val localDelegatedProperties = hashMapOf<IrLocalDelegatedPropertySymbol, IrLocalDelegatedPropertySymbol>()
+    protected val typeAliases = hashMapOf<IrTypeAliasSymbol, IrTypeAliasSymbol>()
 
     override fun visitElement(element: IrElement) {
         element.acceptChildrenVoid(this)
diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DeepCopySymbolRemapperPreservingSignatures.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DeepCopySymbolRemapperPreservingSignatures.kt
new file mode 100644
index 00000000000..ba8aa38f0b2
--- /dev/null
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DeepCopySymbolRemapperPreservingSignatures.kt
@@ -0,0 +1,54 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.ir.util
+
+import org.jetbrains.kotlin.ir.declarations.*
+import org.jetbrains.kotlin.ir.symbols.impl.*
+import org.jetbrains.kotlin.ir.visitors.acceptChildrenVoid
+
+open class DeepCopySymbolRemapperPreservingSignatures : DeepCopySymbolRemapper() {
+    override fun visitClass(declaration: IrClass) {
+        remapSymbol(classes, declaration) { symbol ->
+            symbol.signature?.let { sig -> IrClassPublicSymbolImpl(sig) } ?: IrClassSymbolImpl()
+        }
+        declaration.acceptChildrenVoid(this)
+    }
+
+    override fun visitConstructor(declaration: IrConstructor) {
+        remapSymbol(constructors, declaration) { symbol ->
+            symbol.signature?.let { sig -> IrConstructorPublicSymbolImpl(sig) } ?: IrConstructorSymbolImpl()
+        }
+        declaration.acceptChildrenVoid(this)
+    }
+
+    override fun visitEnumEntry(declaration: IrEnumEntry) {
+        remapSymbol(enumEntries, declaration) { symbol ->
+            symbol.signature?.let { sig -> IrEnumEntryPublicSymbolImpl(sig) } ?: IrEnumEntrySymbolImpl()
+        }
+        declaration.acceptChildrenVoid(this)
+    }
+
+    override fun visitSimpleFunction(declaration: IrSimpleFunction) {
+        remapSymbol(functions, declaration) { symbol ->
+            symbol.signature?.let { sig -> IrSimpleFunctionPublicSymbolImpl(sig) } ?: IrSimpleFunctionSymbolImpl()
+        }
+        declaration.acceptChildrenVoid(this)
+    }
+
+    override fun visitProperty(declaration: IrProperty) {
+        remapSymbol(properties, declaration) { symbol ->
+            symbol.signature?.let { sig -> IrPropertyPublicSymbolImpl(sig) } ?: IrPropertySymbolImpl()
+        }
+        declaration.acceptChildrenVoid(this)
+    }
+
+    override fun visitTypeAlias(declaration: IrTypeAlias) {
+        remapSymbol(typeAliases, declaration) { symbol ->
+            symbol.signature?.let { sig -> IrTypeAliasPublicSymbolImpl(sig) } ?: IrTypeAliasSymbolImpl()
+        }
+        declaration.acceptChildrenVoid(this)
+    }
+}
\ No newline at end of file
diff --git a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrFakeOverrideUtils.kt b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrFakeOverrideUtils.kt
index 02e4499893c..4e60955a217 100644
--- a/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrFakeOverrideUtils.kt
+++ b/compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrFakeOverrideUtils.kt
@@ -58,9 +58,9 @@ fun Collection<IrOverridableMember>.collectAndFilterRealOverrides(
     val realOverrides = mutableMapOf<Any, IrOverridableMember>()
 
     /*
-        Due to IR copying in performByIrFile, overrides should only be distinguished up to their fqNames.
+        Due to IR copying in performByIrFile, overrides should only be distinguished up to their signatures.
      */
-    fun IrOverridableMember.toKey(): Any = fqNameWhenAvailable ?: this
+    fun IrOverridableMember.toKey(): Any = symbol.signature ?: this
 
     fun overriddenSymbols(declaration: IrOverridableMember) = when (declaration) {
         is IrSimpleFunction -> declaration.overriddenSymbols