diff --git a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrDeclarationStorage.kt b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrDeclarationStorage.kt
index 6fb6acbfd65..7deeff03991 100644
--- a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrDeclarationStorage.kt
+++ b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrDeclarationStorage.kt
@@ -140,16 +140,18 @@ class Fir2IrDeclarationStorage(
         leaveScope(descriptor)
     }
 
+    private fun IrTypeParametersContainer.setTypeParameters(owner: FirTypeParametersOwner) {
+        typeParameters = owner.typeParameters.mapIndexed { index, typeParameter ->
+            getIrTypeParameter(typeParameter, index).apply { parent = this@setTypeParameters }
+        }
+    }
+
     private fun IrClass.declareSupertypesAndTypeParameters(klass: FirClass<*>): IrClass {
-        for (superTypeRef in klass.superTypeRefs) {
-            superTypes += superTypeRef.toIrType(session, this@Fir2IrDeclarationStorage)
+        superTypes = klass.superTypeRefs.map { superTypeRef ->
+            superTypeRef.toIrType(session, this@Fir2IrDeclarationStorage)
         }
         if (klass is FirRegularClass) {
-            for ((index, typeParameter) in klass.typeParameters.withIndex()) {
-                typeParameters += getIrTypeParameter(typeParameter, index).apply {
-                    parent = this@declareSupertypesAndTypeParameters
-                }
-            }
+            setTypeParameters(klass)
         }
         return this
     }
@@ -320,19 +322,21 @@ class Fir2IrDeclarationStorage(
     private fun <T : IrFunction> T.declareDefaultSetterParameter(type: IrType): T {
         val parent = this
         val descriptor = WrappedValueParameterDescriptor()
-        valueParameters += irSymbolTable.declareValueParameter(
-            startOffset, endOffset, origin, descriptor, type
-        ) { symbol ->
-            IrValueParameterImpl(
-                startOffset, endOffset, IrDeclarationOrigin.DEFINED, symbol,
-                Name.special("<set-?>"), 0, type,
-                varargElementType = null,
-                isCrossinline = false, isNoinline = false
-            ).apply {
-                this.parent = parent
-                descriptor.bind(this)
+        valueParameters = listOf(
+            irSymbolTable.declareValueParameter(
+                startOffset, endOffset, origin, descriptor, type
+            ) { symbol ->
+                IrValueParameterImpl(
+                    startOffset, endOffset, IrDeclarationOrigin.DEFINED, symbol,
+                    Name.special("<set-?>"), 0, type,
+                    varargElementType = null,
+                    isCrossinline = false, isNoinline = false
+                ).apply {
+                    this.parent = parent
+                    descriptor.bind(this)
+                }
             }
-        }
+        )
         return this
     }
 
@@ -342,16 +346,14 @@ class Fir2IrDeclarationStorage(
             val type = function.valueParameters.first().returnTypeRef.toIrType(session, this@Fir2IrDeclarationStorage)
             declareDefaultSetterParameter(type)
         } else if (function != null) {
-            for ((index, valueParameter) in function.valueParameters.withIndex()) {
-                valueParameters += createAndSaveIrParameter(valueParameter, index).apply { this.parent = parent }
+            valueParameters = function.valueParameters.mapIndexed { index, valueParameter ->
+                createAndSaveIrParameter(valueParameter, index).apply { this.parent = parent }
             }
         }
         if (function !is FirConstructor) {
             val thisOrigin = IrDeclarationOrigin.DEFINED
             if (function is FirSimpleFunction) {
-                for ((index, typeParameter) in function.typeParameters.withIndex()) {
-                    typeParameters += getIrTypeParameter(typeParameter, index).apply { this.parent = parent }
-                }
+                setTypeParameters(function)
             }
             val receiverTypeRef = function?.receiverTypeRef
             if (receiverTypeRef != null) {
diff --git a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt
index 086bb443bfc..557c8806dbe 100644
--- a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt
+++ b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt
@@ -163,9 +163,8 @@ class Fir2IrVisitor(
                 declarations += irDeclaration
             }
 
-            file.annotations.forEach {
-                val irCall = it.accept(this@Fir2IrVisitor, data) as? IrConstructorCall ?: return@forEach
-                annotations += irCall
+            annotations = file.annotations.mapNotNull {
+                it.accept(this@Fir2IrVisitor, data) as? IrConstructorCall
             }
         }
     }
@@ -305,9 +304,8 @@ class Fir2IrVisitor(
                 }
             }
             addFakeOverrides(klass, processedCallableNames)
-            klass.annotations.forEach {
-                val irCall = it.accept(this@Fir2IrVisitor, null) as? IrConstructorCall ?: return@forEach
-                annotations += irCall
+            annotations = klass.annotations.mapNotNull {
+                it.accept(this@Fir2IrVisitor, null) as? IrConstructorCall
             }
         }
         if (irPrimaryConstructor != null) {
@@ -377,7 +375,7 @@ class Fir2IrVisitor(
             if (firOverriddenSymbol != null && this is IrSimpleFunction && firFunctionSymbol != null) {
                 val overriddenSymbol = declarationStorage.getIrFunctionSymbol(firOverriddenSymbol)
                 if (overriddenSymbol is IrSimpleFunctionSymbol) {
-                    overriddenSymbols += overriddenSymbol
+                    overriddenSymbols = listOf(overriddenSymbol)
                 }
             }
             var body = firFunction?.body?.convertToIrBlockBody()
@@ -581,18 +579,18 @@ class Fir2IrVisitor(
             property.getter, this, propertyType, property.getter is FirDefaultPropertyGetter, property.getter == null
         )
         getter?.apply {
-            overriddenProperty?.owner?.getter?.symbol?.let { overriddenSymbols += it }
+            overriddenProperty?.owner?.getter?.symbol?.let { overriddenSymbols = listOf(it) }
         }
         if (property.isVar) {
             setter?.setPropertyAccessorContent(
                 property.setter, this, propertyType, property.setter is FirDefaultPropertySetter, property.setter == null
             )
             setter?.apply {
-                overriddenProperty?.owner?.setter?.symbol?.let { overriddenSymbols += it }
+                overriddenProperty?.owner?.setter?.symbol?.let { overriddenSymbols = listOf(it) }
             }
         }
-        property.annotations.forEach {
-            annotations += it.accept(this@Fir2IrVisitor, null) as IrConstructorCall
+        annotations = property.annotations.mapNotNull {
+            it.accept(this@Fir2IrVisitor, null) as? IrConstructorCall
         }
         declarationStorage.leaveScope(descriptor)
         return this