FIR: Avoid creating empty nested classes scopes

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirAbstractTreeTransformerWithSuperTypes.kt

@@ -48,11 +48,11 @@ abstract class FirAbstractTreeTransformerWithSuperTypes(
                 firClass.addTypeParametersScope()
                 val companionObject = firClass.companionObject
                 if (companionObject != null) {
-                    towerScope.addScope(nestedClassifierScope(companionObject))
+                    nestedClassifierScope(companionObject)?.let(towerScope::addScope)
                 }
             }
 
-            towerScope.addScope(nestedClassifierScope(firClass))
+            nestedClassifierScope(firClass)?.let(towerScope::addScope)
 
             transformElement(firClass, data)
         }

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirSupertypesResolution.kt

@@ -133,9 +133,9 @@ private fun createScopesForNestedClasses(
         addIfNotNull(klass.typeParametersScope())
         val companionObjects = klass.declarations.filterIsInstance<FirRegularClass>().filter { it.isCompanion }
         for (companionObject in companionObjects) {
-            add(nestedClassifierScope(companionObject))
+            addIfNotNull(nestedClassifierScope(companionObject))
         }
-        add(nestedClassifierScope(klass))
+        addIfNotNull(nestedClassifierScope(klass))
     }
 
 fun FirRegularClass.resolveSupertypesInTheAir(session: FirSession): List<FirTypeRef> {

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirClassDeclaredMemberScope.kt

@@ -72,5 +72,7 @@ class FirClassDeclaredMemberScope(
     override fun processClassifiersByNameWithSubstitution(
         name: Name,
         processor: (FirClassifierSymbol<*>, ConeSubstitutor) -> Unit
-    ) = nestedClassifierScope.processClassifiersByNameWithSubstitution(name, processor)
+    ) {
+        nestedClassifierScope?.processClassifiersByNameWithSubstitution(name, processor)
+    }
 }

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirDeclaredMemberScopeProvider.kt

@@ -13,11 +13,12 @@ import org.jetbrains.kotlin.fir.resolve.declaredMemberScopeProvider
 import org.jetbrains.kotlin.fir.scopes.FirScope
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.utils.getOrPutNullable
 
 class FirDeclaredMemberScopeProvider : FirSessionComponent {
 
     private val declaredMemberCache = mutableMapOf<FirClass<*>, FirScope>()
-    private val nestedClassifierCache = mutableMapOf<FirClass<*>, FirNestedClassifierScope>()
+    private val nestedClassifierCache = mutableMapOf<FirClass<*>, FirNestedClassifierScope?>()
 
     fun getClassByClassId(classId: ClassId): FirClass<*>? {
         for ((clazz, _) in declaredMemberCache) {
@@ -48,9 +49,9 @@ class FirDeclaredMemberScopeProvider : FirSessionComponent {
         }
     }
 
-    fun nestedClassifierScope(klass: FirClass<*>): FirNestedClassifierScope {
-        return nestedClassifierCache.getOrPut(klass) {
-            FirNestedClassifierScope(klass)
+    fun nestedClassifierScope(klass: FirClass<*>): FirNestedClassifierScope? {
+        return nestedClassifierCache.getOrPutNullable(klass) {
+            FirNestedClassifierScope(klass).takeUnless { it.isEmpty() }
         }
     }
 }
@@ -73,7 +74,7 @@ fun declaredMemberScopeWithLazyNestedScope(
         .declaredMemberScope(klass, useLazyNestedClassifierScope = true, existingNames = existingNames, symbolProvider = symbolProvider)
 }
 
-fun nestedClassifierScope(klass: FirClass<*>): FirNestedClassifierScope {
+fun nestedClassifierScope(klass: FirClass<*>): FirNestedClassifierScope? {
     return klass
         .session
         .declaredMemberScopeProvider
@@ -84,7 +85,8 @@ fun lazyNestedClassifierScope(
     classId: ClassId,
     existingNames: List<Name>,
     symbolProvider: FirSymbolProvider
-): FirLazyNestedClassifierScope {
+): FirLazyNestedClassifierScope? {
+    if (existingNames.isEmpty()) return null
     return FirLazyNestedClassifierScope(classId, existingNames, symbolProvider)
 }
 

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirNestedClassifierScope.kt

@@ -25,6 +25,8 @@ class FirNestedClassifierScope(val klass: FirClass<*>) : FirScope() {
         result
     }
 
+    fun isEmpty() = classIndex.isEmpty()
+
     override fun processClassifiersByNameWithSubstitution(
         name: Name,
         processor: (FirClassifierSymbol<*>, ConeSubstitutor) -> Unit