diff --git a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/UnambiguousFqName.kt b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/UnambiguousFqName.kt
deleted file mode 100644
index b6325b346cb..00000000000
--- a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/UnambiguousFqName.kt
+++ /dev/null
@@ -1,22 +0,0 @@
-
-/*
- * Copyright 2010-2018 JetBrains s.r.o. 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.fir
-
-import org.jetbrains.kotlin.name.FqName
-import org.jetbrains.kotlin.name.FqNameUnsafe
-
-/*
- * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
- * that can be found in the license/LICENSE.txt file.
- */
-
-data class UnambiguousFqName(val packageFqName: FqNameUnsafe, val classFqName: FqName) {
-
-    override fun toString(): String {
-        return packageFqName.asString() + "/" + classFqName.asString()
-    }
-}
\ No newline at end of file
diff --git a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/descriptors/ConeDescriptors.kt b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/descriptors/ConeDescriptors.kt
index f890ba35fff..af34b9d0687 100644
--- a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/descriptors/ConeDescriptors.kt
+++ b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/descriptors/ConeDescriptors.kt
@@ -5,8 +5,8 @@
 
 package org.jetbrains.kotlin.fir.descriptors
 
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
+import org.jetbrains.kotlin.name.ClassId
 
 
 interface ConeDescriptor
@@ -16,7 +16,7 @@ interface ConeClassifierDescriptor : ConeDescriptor
 interface ConeClassifierDescriptorWithTypeParameters : ConeClassifierDescriptor {
     val typeParameters: List<ConeTypeParameterDescriptor>
 
-    val fqName: UnambiguousFqName
+    val fqName: ClassId
 }
 
 interface ConeTypeParameterDescriptor : ConeClassifierDescriptor
diff --git a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeKotlinType.kt b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeKotlinType.kt
index a85a03a5958..b9659631ccf 100644
--- a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeKotlinType.kt
+++ b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeKotlinType.kt
@@ -5,7 +5,7 @@
 
 package org.jetbrains.kotlin.fir.types
 
-import org.jetbrains.kotlin.fir.UnambiguousFqName
+import org.jetbrains.kotlin.name.ClassId
 
 sealed class ConeKotlinTypeProjection(val kind: ProjectionKind)
 
@@ -29,5 +29,5 @@ abstract class ConeKotlinType : ConeKotlinTypeProjection(ProjectionKind.INVARIAN
 }
 
 abstract class ConeClassType : ConeKotlinType() {
-    abstract val fqName: UnambiguousFqName
+    abstract val fqName: ClassId
 }
\ No newline at end of file
diff --git a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/impl/Impl.kt b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/impl/Impl.kt
index 391aadc3c35..22d5714d71b 100644
--- a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/impl/Impl.kt
+++ b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/impl/Impl.kt
@@ -5,11 +5,11 @@
 
 package org.jetbrains.kotlin.fir.types.impl
 
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.types.*
+import org.jetbrains.kotlin.name.ClassId
 
 class ConeClassTypeImpl(
-    override val fqName: UnambiguousFqName,
+    override val fqName: ClassId,
     override val typeArguments: List<ConeKotlinTypeProjection>
 ) : ConeClassType()
 
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/ConeClassDescriptorImpl.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/ConeClassDescriptorImpl.kt
index b434c8aa098..13e9dc5d97c 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/ConeClassDescriptorImpl.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/ConeClassDescriptorImpl.kt
@@ -5,16 +5,16 @@
 
 package org.jetbrains.kotlin.descriptors
 
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.declarations.FirResolvedClass
 import org.jetbrains.kotlin.fir.descriptors.ConeClassDescriptor
 import org.jetbrains.kotlin.fir.descriptors.ConeClassifierDescriptor
 import org.jetbrains.kotlin.fir.descriptors.ConeTypeParameterDescriptor
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
+import org.jetbrains.kotlin.name.ClassId
 
 class ConeClassDescriptorImpl(
     override val typeParameters: List<ConeTypeParameterDescriptor>,
-    override val fqName: UnambiguousFqName,
+    override val fqName: ClassId,
     override val superTypes: List<ConeKotlinType>,
     override val nestedClassifiers: List<ConeClassifierDescriptor>
 ) : ConeClassDescriptor, AbstractFirBasedDescriptor<FirResolvedClass>() {
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirClassifierResolveTransformer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirClassifierResolveTransformer.kt
index 87fd5f1e578..c7a6a7c9145 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirClassifierResolveTransformer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirClassifierResolveTransformer.kt
@@ -7,34 +7,29 @@ package org.jetbrains.kotlin.descriptors
 
 import org.jetbrains.kotlin.fir.FirDescriptorOwner
 import org.jetbrains.kotlin.fir.FirElement
-import org.jetbrains.kotlin.fir.UnambiguousFqName
-import org.jetbrains.kotlin.fir.declarations.*
+import org.jetbrains.kotlin.fir.declarations.FirClass
+import org.jetbrains.kotlin.fir.declarations.FirDeclaration
+import org.jetbrains.kotlin.fir.declarations.FirFile
+import org.jetbrains.kotlin.fir.declarations.FirResolvedClass
 import org.jetbrains.kotlin.fir.declarations.impl.FirResolvedClassImpl
 import org.jetbrains.kotlin.fir.descriptors.ConeClassifierDescriptor
 import org.jetbrains.kotlin.fir.descriptors.ConeTypeParameterDescriptor
-import org.jetbrains.kotlin.fir.resolve.FirTypeResolver
-import org.jetbrains.kotlin.fir.scopes.FirImportingScope
-import org.jetbrains.kotlin.fir.scopes.impl.FirCompositeImportingScope
-import org.jetbrains.kotlin.fir.scopes.impl.FirExplicitImportingScope
-import org.jetbrains.kotlin.fir.scopes.impl.FirSelfImportingScope
 import org.jetbrains.kotlin.fir.types.FirResolvedType
-import org.jetbrains.kotlin.fir.types.FirType
-import org.jetbrains.kotlin.fir.types.impl.FirResolvedTypeImpl
 import org.jetbrains.kotlin.fir.visitors.CompositeTransformResult
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 import org.jetbrains.kotlin.fir.visitors.compose
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
-import org.jetbrains.kotlin.name.FqNameUnsafe
 
 class FirClassifierResolveTransformer : FirTransformer<Nothing?>() {
     override fun <E : FirElement> transformElement(element: E, data: Nothing?): CompositeTransformResult<E> {
         return element.compose()
     }
 
-    lateinit var packageFqName: FqNameUnsafe
+    lateinit var packageFqName: FqName
 
     override fun transformFile(file: FirFile, data: Nothing?): CompositeTransformResult<FirFile> {
-        packageFqName = file.packageFqName.toUnsafe()
+        packageFqName = file.packageFqName
         return file.also { it.transformChildren(this, null) }.compose()
     }
 
@@ -60,7 +55,7 @@ class FirClassifierResolveTransformer : FirTransformer<Nothing?>() {
 
         val descriptor = ConeClassDescriptorImpl(
             typeParameters,
-            UnambiguousFqName(packageFqName, actualClassName),
+            ClassId(packageFqName, actualClassName, false),
             superTypes,
             nestedClassifiers
         )
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirImportResolveTransformer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirImportResolveTransformer.kt
index 00f90d87030..4be353ff548 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirImportResolveTransformer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirImportResolveTransformer.kt
@@ -6,7 +6,6 @@
 package org.jetbrains.kotlin.descriptors
 
 import org.jetbrains.kotlin.fir.FirElement
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.declarations.FirFile
 import org.jetbrains.kotlin.fir.declarations.FirImport
 import org.jetbrains.kotlin.fir.declarations.impl.FirResolvedImportImpl
@@ -14,6 +13,7 @@ import org.jetbrains.kotlin.fir.resolve.FirProvider
 import org.jetbrains.kotlin.fir.visitors.CompositeTransformResult
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 import org.jetbrains.kotlin.fir.visitors.compose
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 
 class FirImportResolveTransformer : FirTransformer<Nothing?>() {
@@ -37,7 +37,7 @@ class FirImportResolveTransformer : FirTransformer<Nothing?>() {
                 lastPart.add(0, firstPart.shortName().asString())
                 firstPart = firstPart.parent()
 
-                val resolvedFqName = UnambiguousFqName(firstPart.toUnsafe(), FqName.fromSegments(lastPart))
+                val resolvedFqName = ClassId(firstPart, FqName.fromSegments(lastPart), false)
                 val foundClassifier = firProvider.getFirClassifierByFqName(resolvedFqName)
 
                 if (foundClassifier != null) {
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirTypeResolveTransformer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirTypeResolveTransformer.kt
index e965501e17b..9682464c2bd 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirTypeResolveTransformer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/descriptors/FirTypeResolveTransformer.kt
@@ -29,7 +29,7 @@ class FirTypeResolveTransformer : FirTransformer<Nothing?>() {
     override fun transformFile(file: FirFile, data: Nothing?): CompositeTransformResult<FirFile> {
         importingScope = FirCompositeImportingScope(
             FirExplicitImportingScope(file.imports),
-            FirSelfImportingScope(file.packageFqName.toUnsafe(), file.session)
+            FirSelfImportingScope(file.packageFqName, file.session)
         )
         return file.also { it.transformChildren(this, null) }.compose()
     }
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/FirProvider.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/FirProvider.kt
index 310862ceef6..4a0687d3536 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/FirProvider.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/FirProvider.kt
@@ -6,23 +6,23 @@
 package org.jetbrains.kotlin.fir.resolve
 
 import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.declarations.FirFile
 import org.jetbrains.kotlin.fir.declarations.FirMemberDeclaration
 import org.jetbrains.kotlin.fir.declarations.FirTypeParameter
 import org.jetbrains.kotlin.fir.service
-import org.jetbrains.kotlin.name.FqNameUnsafe
+import org.jetbrains.kotlin.name.ClassId
+import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 
 interface FirProvider {
-    fun getFirFilesByPackage(fqName: FqNameUnsafe): List<FirFile>
+    fun getFirClassifierByFqName(fqName: ClassId): FirMemberDeclaration?
 
-    fun getFirClassifierByFqName(fqName: UnambiguousFqName): FirMemberDeclaration?
-
-    fun getFirTypeParameterByFqName(fqName: UnambiguousFqName, parameterName: Name): FirTypeParameter?
+    fun getFirTypeParameterByFqName(fqName: ClassId, parameterName: Name): FirTypeParameter?
 
 
     companion object {
         fun getInstance(session: FirSession): FirProvider = session.service()
     }
+
+    fun getFirFilesByPackage(fqName: FqName): List<FirFile>
 }
\ No newline at end of file
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/FirQualifierResolver.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/FirQualifierResolver.kt
index 96d6c6bc3cd..d4dd3708d29 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/FirQualifierResolver.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/FirQualifierResolver.kt
@@ -6,14 +6,14 @@
 package org.jetbrains.kotlin.fir.resolve
 
 import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.service
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.FirQualifierPart
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 
 interface FirQualifierResolver {
-    fun resolveTypeWithPrefix(parts: List<FirQualifierPart>, prefix: UnambiguousFqName): ConeKotlinType?
+    fun resolveTypeWithPrefix(parts: List<FirQualifierPart>, prefix: ClassId): ConeKotlinType?
 
     fun resolveType(parts: List<FirQualifierPart>): ConeKotlinType?
 
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirProviderImpl.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirProviderImpl.kt
index 6fc2abab042..53d00386a28 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirProviderImpl.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirProviderImpl.kt
@@ -7,18 +7,17 @@ package org.jetbrains.kotlin.fir.resolve.impl
 
 import org.jetbrains.kotlin.fir.FirElement
 import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.resolve.FirProvider
 import org.jetbrains.kotlin.fir.visitors.FirVisitorVoid
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
-import org.jetbrains.kotlin.name.FqNameUnsafe
 import org.jetbrains.kotlin.name.Name
 
 class FirProviderImpl(val session: FirSession) : FirProvider {
 
     fun recordFile(file: FirFile) {
-        val packageName = file.packageFqName.toUnsafe()
+        val packageName = file.packageFqName
         fileMap.merge(packageName, listOf(file)) { a, b -> a + b }
 
         file.acceptChildren(object : FirVisitorVoid() {
@@ -28,7 +27,7 @@ class FirProviderImpl(val session: FirSession) : FirProvider {
 
             override fun visitClass(klass: FirClass) {
                 val fqName = containerFqName.child(klass.name)
-                classifierMap[UnambiguousFqName(packageName, fqName)] = klass
+                classifierMap[ClassId(packageName, fqName, false)] = klass
 
                 containerFqName = fqName
                 klass.acceptChildren(this)
@@ -37,25 +36,23 @@ class FirProviderImpl(val session: FirSession) : FirProvider {
 
             override fun visitTypeAlias(typeAlias: FirTypeAlias) {
                 val fqName = containerFqName.child(typeAlias.name)
-                classifierMap[UnambiguousFqName(packageName, fqName)] = typeAlias
+                classifierMap[ClassId(packageName, fqName, false)] = typeAlias
             }
         })
     }
 
-    private val fileMap = mutableMapOf<FqNameUnsafe, List<FirFile>>()
-    private val classifierMap = mutableMapOf<UnambiguousFqName, FirMemberDeclaration>()
-
-    override fun getFirFilesByPackage(fqName: FqNameUnsafe): List<FirFile> {
-
+    private val fileMap = mutableMapOf<FqName, List<FirFile>>()
+    private val classifierMap = mutableMapOf<ClassId, FirMemberDeclaration>()
 
+    override fun getFirFilesByPackage(fqName: FqName): List<FirFile> {
         return fileMap[fqName].orEmpty()
     }
 
-    override fun getFirClassifierByFqName(fqName: UnambiguousFqName): FirMemberDeclaration? {
+    override fun getFirClassifierByFqName(fqName: ClassId): FirMemberDeclaration? {
         return classifierMap[fqName]
     }
 
-    override fun getFirTypeParameterByFqName(fqName: UnambiguousFqName, parameterName: Name): FirTypeParameter? {
+    override fun getFirTypeParameterByFqName(fqName: ClassId, parameterName: Name): FirTypeParameter? {
         val typeParameterContainer = (getFirClassifierByFqName(fqName) as? FirTypeParameterContainer) ?: return null
         // TODO: Optimize search here
         return typeParameterContainer.typeParameters.find { it.name == parameterName }
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirQualifierResolverImpl.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirQualifierResolverImpl.kt
index 282b40eec0b..0ab2e3bcda1 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirQualifierResolverImpl.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirQualifierResolverImpl.kt
@@ -6,7 +6,6 @@
 package org.jetbrains.kotlin.fir.resolve.impl
 
 import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.declarations.FirMemberDeclaration
 import org.jetbrains.kotlin.fir.resolve.FirProvider
 import org.jetbrains.kotlin.fir.resolve.FirQualifierResolver
@@ -14,12 +13,13 @@ import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.fir.types.impl.ConeClassTypeImpl
 import org.jetbrains.kotlin.fir.types.impl.ConeKotlinTypeProjectionInImpl
 import org.jetbrains.kotlin.fir.types.impl.ConeKotlinTypeProjectionOutImpl
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.types.Variance
 
 class FirQualifierResolverImpl(val session: FirSession) : FirQualifierResolver {
 
-    private fun FirMemberDeclaration.toConeKotlinType(fqName: UnambiguousFqName, parts: List<FirQualifierPart>): ConeKotlinType? {
+    private fun FirMemberDeclaration.toConeKotlinType(fqName: ClassId, parts: List<FirQualifierPart>): ConeKotlinType? {
         return ConeClassTypeImpl(fqName, parts.flatMap {
             it.typeArguments.map {
                 when (it) {
@@ -38,10 +38,14 @@ class FirQualifierResolverImpl(val session: FirSession) : FirQualifierResolver {
         })
     }
 
-    override fun resolveTypeWithPrefix(parts: List<FirQualifierPart>, prefix: UnambiguousFqName): ConeKotlinType? {
+    override fun resolveTypeWithPrefix(parts: List<FirQualifierPart>, prefix: ClassId): ConeKotlinType? {
         val firProvider = FirProvider.getInstance(session)
 
-        val fqName = UnambiguousFqName(prefix.packageFqName, parts.fold(prefix.classFqName) { prefix, suffix -> prefix.child(suffix.name) })
+        val fqName = ClassId(
+            prefix.packageFqName,
+            parts.fold(prefix.relativeClassName) { prefix, suffix -> prefix.child(suffix.name) },
+            false
+        )
         val foundClassifier = firProvider.getFirClassifierByFqName(fqName)
 
         return foundClassifier?.toConeKotlinType(fqName, parts)
@@ -58,7 +62,7 @@ class FirQualifierResolverImpl(val session: FirSession) : FirQualifierResolver {
                 lastPart.add(0, firstPart.last())
                 firstPart.removeAt(firstPart.lastIndex)
 
-                val fqName = UnambiguousFqName(firstPart.toFqNameUnsafe(), lastPart.toFqName())
+                val fqName = ClassId(firstPart.toFqName(), lastPart.toFqName(), false)
                 val foundClassifier = firProvider.getFirClassifierByFqName(fqName)
 
                 if (foundClassifier != null) {
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/FirImportingScope.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/FirImportingScope.kt
index 989a669ff1e..5637c23ddc6 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/FirImportingScope.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/FirImportingScope.kt
@@ -5,9 +5,9 @@
 
 package org.jetbrains.kotlin.fir.scopes
 
-import org.jetbrains.kotlin.fir.UnambiguousFqName
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
 
 interface FirImportingScope {
-    fun processClassifiersByName(name: Name, processor: (UnambiguousFqName) -> Boolean): Boolean
+    fun processClassifiersByName(name: Name, processor: (ClassId) -> Boolean): Boolean
 }
\ No newline at end of file
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirCompositeImportingScope.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirCompositeImportingScope.kt
index c2f979c7f42..7d119fc67ee 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirCompositeImportingScope.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirCompositeImportingScope.kt
@@ -5,12 +5,12 @@
 
 package org.jetbrains.kotlin.fir.scopes.impl
 
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.scopes.FirImportingScope
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
 
 class FirCompositeImportingScope(vararg val scopes: FirImportingScope) : FirImportingScope {
-    override fun processClassifiersByName(name: Name, processor: (UnambiguousFqName) -> Boolean): Boolean {
+    override fun processClassifiersByName(name: Name, processor: (ClassId) -> Boolean): Boolean {
         for (scope in scopes) {
             if (!scope.processClassifiersByName(name, processor)) {
                 return false
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirExplicitImportingScope.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirExplicitImportingScope.kt
index bceae96c5a6..5bb22149bc9 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirExplicitImportingScope.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirExplicitImportingScope.kt
@@ -5,11 +5,10 @@
 
 package org.jetbrains.kotlin.fir.scopes.impl
 
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.declarations.FirImport
 import org.jetbrains.kotlin.fir.declarations.FirResolvedImport
-import org.jetbrains.kotlin.fir.resolve.FirQualifierResolver
 import org.jetbrains.kotlin.fir.scopes.FirImportingScope
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
 
 class FirExplicitImportingScope(imports: List<FirImport>) : FirImportingScope {
@@ -18,9 +17,9 @@ class FirExplicitImportingScope(imports: List<FirImport>) : FirImportingScope {
     private val simpleImports =
         imports.filterIsInstance<FirResolvedImport>()
             .filter { !it.isAllUnder }
-            .groupBy { it.aliasName ?: it.resolvedFqName.classFqName.shortName() }
+            .groupBy { it.aliasName ?: it.resolvedFqName.shortClassName }
 
-    override fun processClassifiersByName(name: Name, processor: (UnambiguousFqName) -> Boolean): Boolean {
+    override fun processClassifiersByName(name: Name, processor: (ClassId) -> Boolean): Boolean {
         val imports = simpleImports[name] ?: return true
         for (import in imports) {
             if (!processor(import.resolvedFqName)) {
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirSelfImportingScope.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirSelfImportingScope.kt
index f5e2b94a835..2896b675c4b 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirSelfImportingScope.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirSelfImportingScope.kt
@@ -6,20 +6,18 @@
 package org.jetbrains.kotlin.fir.scopes.impl
 
 import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.resolve.FirProvider
-import org.jetbrains.kotlin.fir.resolve.FirQualifierResolver
 import org.jetbrains.kotlin.fir.scopes.FirImportingScope
 import org.jetbrains.kotlin.fir.service
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
-import org.jetbrains.kotlin.name.FqNameUnsafe
 import org.jetbrains.kotlin.name.Name
 
-class FirSelfImportingScope(val fqName: FqNameUnsafe, val session: FirSession) : FirImportingScope {
-    override fun processClassifiersByName(name: Name, processor: (UnambiguousFqName) -> Boolean): Boolean {
+class FirSelfImportingScope(val fqName: FqName, val session: FirSession) : FirImportingScope {
+    override fun processClassifiersByName(name: Name, processor: (ClassId) -> Boolean): Boolean {
 
 
-        val unambiguousFqName = UnambiguousFqName(fqName, FqName.topLevel(name))
+        val unambiguousFqName = ClassId(fqName, name)
 
         val firProvider = session.service<FirProvider>()
 
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt
index 18abb861364..5fad7d96f49 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt
@@ -384,7 +384,7 @@ class FirRenderer(builder: StringBuilder) : FirVisitorVoid() {
                     sb.append(packageFqName.asString().replace('.', '/'))
                 }
                 sb.append('.')
-                sb.append(fqName.classFqName.asString())
+                sb.append(fqName.relativeClassName.asString())
                 sb.append(typeArguments.joinToString { it ->
                     when (it) {
                         StarProjection -> "*"
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirResolvedImport.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirResolvedImport.kt
index 459de2a1209..1e8014455ca 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirResolvedImport.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirResolvedImport.kt
@@ -5,11 +5,11 @@
 
 package org.jetbrains.kotlin.fir.declarations
 
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.visitors.FirVisitor
+import org.jetbrains.kotlin.name.ClassId
 
 interface FirResolvedImport : FirImport {
-    val resolvedFqName: UnambiguousFqName
+    val resolvedFqName: ClassId
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R =
         visitor.visitResolvedImport(this, data)
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirResolvedImportImpl.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirResolvedImportImpl.kt
index 39c20e23d14..a9399743e0c 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirResolvedImportImpl.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirResolvedImportImpl.kt
@@ -5,14 +5,14 @@
 
 package org.jetbrains.kotlin.fir.declarations.impl
 
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.declarations.FirImport
 import org.jetbrains.kotlin.fir.declarations.FirResolvedImport
 import org.jetbrains.kotlin.fir.visitors.FirVisitor
+import org.jetbrains.kotlin.name.ClassId
 
 class FirResolvedImportImpl(
     val delegate: FirImport,
-    override val resolvedFqName: UnambiguousFqName
+    override val resolvedFqName: ClassId
 ) : FirResolvedImport, FirImport by delegate {
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R =
         visitor.visitResolvedImport(this, data)
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirBuiltinType.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirBuiltinType.kt
index 718949dce0c..434fb32f9e2 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirBuiltinType.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirBuiltinType.kt
@@ -7,12 +7,11 @@ package org.jetbrains.kotlin.fir.types.impl
 
 import com.intellij.psi.PsiElement
 import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.UnambiguousFqName
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.FirResolvedType
+import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
-import org.jetbrains.kotlin.name.FqNameUnsafe
 import org.jetbrains.kotlin.name.Name
 
 sealed class FirBuiltinType(
@@ -21,9 +20,9 @@ sealed class FirBuiltinType(
     val name: String
 ) : FirResolvedType {
     final override val type: ConeKotlinType = ConeClassTypeImpl(
-        UnambiguousFqName(
+        ClassId(
             KOTLIN_PACKAGE_FQ_NAME,
-            FqName.topLevel(Name.identifier(name))
+            Name.identifier(name)
         ), emptyList()
     )
 
@@ -33,7 +32,7 @@ sealed class FirBuiltinType(
         get() = emptyList()
 
     companion object {
-        private val KOTLIN_PACKAGE_FQ_NAME = FqNameUnsafe.topLevel(Name.identifier("kotlin"))
+        private val KOTLIN_PACKAGE_FQ_NAME = FqName.topLevel(Name.identifier("kotlin"))
     }
 }