diff --git a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt
index 3381bf4f8e3..defaf337dbc 100644
--- a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt
+++ b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt
@@ -560,7 +560,7 @@ class Fir2IrConverter(
             val firModuleDescriptor = irModuleFragment.descriptor as? FirModuleDescriptor
             val targetPlatform = firModuleDescriptor?.platform
             val languageVersionSettings = firModuleDescriptor?.session?.languageVersionSettings ?: return
-            val intrinsicConstEvaluation = languageVersionSettings.supportsFeature(LanguageFeature.IntrinsicConstEvaluation) == true
+            val intrinsicConstEvaluation = languageVersionSettings.supportsFeature(LanguageFeature.IntrinsicConstEvaluation)
 
             val configuration = IrInterpreterConfiguration(
                 platform = targetPlatform,
@@ -572,7 +572,9 @@ class Fir2IrConverter(
 
             components.session.javaElementFinder?.propertyEvaluator = { it.evaluate(components, interpreter, mode) }
 
-            val ktDiagnosticReporter = KtDiagnosticReporterWithImplicitIrBasedContext(fir2IrConfiguration.diagnosticReporter, languageVersionSettings)
+            val ktDiagnosticReporter = KtDiagnosticReporterWithImplicitIrBasedContext(
+                fir2IrConfiguration.diagnosticReporter, languageVersionSettings
+            )
             irModuleFragment.files.forEach {
                 it.transformConst(
                     it,
@@ -618,7 +620,7 @@ class Fir2IrConverter(
         }
 
         // TODO: drop this function in favor of using [IrModuleDescriptor::shouldSeeInternalsOf] in FakeOverrideBuilder KT-61384
-        fun friendModulesMap(session: FirSession) = mapOf(
+        private fun friendModulesMap(session: FirSession) = mapOf(
             session.moduleData.name.asStringStripSpecialMarkers() to session.moduleData.friendDependencies.map {
                 it.name.asStringStripSpecialMarkers()
             }
@@ -669,7 +671,8 @@ class Fir2IrConverter(
             )
 
             if (fir2IrConfiguration.useIrFakeOverrideBuilder) {
-                FakeOverrideRebuilder(commonMemberStorage.symbolTable, components.fakeOverrideBuilder).rebuildFakeOverrides(irModuleFragment)
+                val rebuilder = FakeOverrideRebuilder(commonMemberStorage.symbolTable, components.fakeOverrideBuilder)
+                rebuilder.rebuildFakeOverrides(irModuleFragment)
             }
 
             return Fir2IrResult(irModuleFragment, components, moduleDescriptor)
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 900a50f0e08..d45380ea045 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
@@ -187,7 +187,7 @@ class Fir2IrDeclarationStorage(
         firOrigin: FirDeclarationOrigin,
     ): IrExternalPackageFragment {
         val isBuiltIn = fqName in BUILT_INS_PACKAGE_FQ_NAMES
-        return when(isBuiltIn) {
+        return when (isBuiltIn) {
             true -> getIrBuiltInsPackageFragment(fqName)
             false -> getIrExternalPackageFragment(fqName, moduleData, firOrigin)
         }
@@ -233,12 +233,18 @@ class Fir2IrDeclarationStorage(
 
     // ------------------------------------ functions ------------------------------------
 
-    fun getCachedIrFunctionSymbol(function: FirFunction, fakeOverrideOwnerLookupTag: ConeClassLikeLookupTag? = null): IrSimpleFunctionSymbol? {
+    fun getCachedIrFunctionSymbol(
+        function: FirFunction,
+        fakeOverrideOwnerLookupTag: ConeClassLikeLookupTag? = null,
+    ): IrSimpleFunctionSymbol? {
         return if (function is FirSimpleFunction) getCachedIrFunctionSymbol(function, fakeOverrideOwnerLookupTag)
         else localStorage.getLocalFunctionSymbol(function)
     }
 
-    fun getCachedIrFunctionSymbol(function: FirSimpleFunction, fakeOverrideOwnerLookupTag: ConeClassLikeLookupTag? = null): IrSimpleFunctionSymbol? {
+    fun getCachedIrFunctionSymbol(
+        function: FirSimpleFunction,
+        fakeOverrideOwnerLookupTag: ConeClassLikeLookupTag? = null,
+    ): IrSimpleFunctionSymbol? {
         return getCachedIrFunctionSymbol(function, fakeOverrideOwnerLookupTag) {
             signatureComposer.composeSignature(function, fakeOverrideOwnerLookupTag)
         }
@@ -377,7 +383,7 @@ class Fir2IrDeclarationStorage(
         return getOrCreateIrConstructor(constructor, { irParent }, predefinedOrigin, isLocal)
     }
 
-    fun getOrCreateIrConstructor(
+    private fun getOrCreateIrConstructor(
         constructor: FirConstructor,
         irParent: () -> IrClass,
         predefinedOrigin: IrDeclarationOrigin? = null,
@@ -395,10 +401,7 @@ class Fir2IrDeclarationStorage(
 
     fun getIrConstructorSymbol(firConstructorSymbol: FirConstructorSymbol): IrConstructorSymbol {
         val fir = firConstructorSymbol.fir
-        return getOrCreateIrConstructor(
-            fir,
-            { findIrParent(fir, fakeOverrideOwnerLookupTag = null) as IrClass }
-        ).symbol
+        return getOrCreateIrConstructor(fir, { findIrParent(fir, fakeOverrideOwnerLookupTag = null) as IrClass }).symbol
     }
 
 
@@ -451,7 +454,7 @@ class Fir2IrDeclarationStorage(
         return getOrCreateIrProperty(property, { irParent }, predefinedOrigin, isLocal, fakeOverrideOwnerLookupTag)
     }
 
-    fun getOrCreateIrProperty(
+    private fun getOrCreateIrProperty(
         property: FirProperty,
         irParent: () -> IrDeclarationParent?,
         predefinedOrigin: IrDeclarationOrigin? = null,
@@ -526,6 +529,7 @@ class Fir2IrDeclarationStorage(
         }
     }
 
+    @Suppress("DuplicatedCode")
     private fun FirField.toStubProperty(): FirProperty {
         val field = this
         return buildProperty {
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 a2117abee52..37246c26392 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
@@ -90,21 +90,18 @@ class Fir2IrVisitor(
     }
 
     override fun visitElement(element: FirElement, data: Any?): IrElement {
-        TODO("Should not be here: ${element::class} ${element.render()}")
+        error("Should not be here: ${element::class} ${element.render()}")
     }
 
     override fun visitField(field: FirField, data: Any?): IrField = whileAnalysing(session, field) {
-        if (field.isSynthetic) {
-            @OptIn(UnsafeDuringIrConstructionAPI::class)
-            return declarationStorage.getCachedIrDelegateOrBackingFieldSymbol(field)!!.owner.apply {
-                // If this is a property backing field, then it has no separate initializer,
-                // so we shouldn't convert it
-                if (correspondingPropertySymbol == null) {
-                    memberGenerator.convertFieldContent(this, field)
-                }
+        require(field.isSynthetic) { "Non-synthetic field found during traversal of FIR tree: ${field.render()}" }
+        @OptIn(UnsafeDuringIrConstructionAPI::class)
+        return declarationStorage.getCachedIrDelegateOrBackingFieldSymbol(field)!!.owner.apply {
+            // If this is a property backing field, then it has no separate initializer,
+            // so we shouldn't convert it
+            if (correspondingPropertySymbol == null) {
+                memberGenerator.convertFieldContent(this, field)
             }
-        } else {
-            throw AssertionError("Unexpected field: ${field.render()}")
         }
     }
 
@@ -266,7 +263,7 @@ class Fir2IrVisitor(
                             statement is FirProperty && statement.origin == FirDeclarationOrigin.ScriptCustomization.ResultProperty -> {
                                 // Generating the result property only for expressions with a meaningful result type
                                 // otherwise skip the property and convert the expression into the statement
-                                if (statement.returnTypeRef.let { (it.isUnit || it.isNothing || it.isNullableNothing) } == true) {
+                                if (statement.returnTypeRef.let { (it.isUnit || it.isNothing || it.isNullableNothing) }) {
                                     statement.initializer!!.toIrStatement()
                                 } else {
                                     (statement.accept(this@Fir2IrVisitor, null) as? IrDeclaration)?.also {