[Commonizer] Remove unnecessary nullability at CirKnownClassifiers.commonDependeeLibraries

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/core/CommonizationVisitor.kt

@@ -128,7 +128,7 @@ internal class CommonizationVisitor(
         val supertypesMap: MutableMap<CirType, CommonizedGroup<CirType>> = linkedMapOf() // preserve supertype order
         for ((index, typeAlias) in targetDeclarations.withIndex()) {
             val expandedClassId = typeAlias!!.expandedType.classifierId
-            if (classifiers.commonDependeeLibraries?.hasClassifier(expandedClassId) == true)
+            if (classifiers.commonDependeeLibraries.hasClassifier(expandedClassId))
                 return null // this case is not supported yet
 
             val expandedClassNode = classifiers.commonized.classNode(expandedClassId) ?: return null

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/core/TypeAliasCommonizer.kt

@@ -76,7 +76,7 @@ private class TypeAliasShortCircuitingCommonizer(
     private tailrec fun computeCommonizedUnderlyingType(underlyingType: CirClassOrTypeAliasType): CirClassOrTypeAliasType {
         return when (underlyingType) {
             is CirClassType -> underlyingType
-            is CirTypeAliasType -> if (classifiers.commonDependeeLibraries?.hasClassifier(underlyingType.classifierId) == true)
+            is CirTypeAliasType -> if (classifiers.commonDependeeLibraries.hasClassifier(underlyingType.classifierId))
                 underlyingType
             else
                 computeCommonizedUnderlyingType(underlyingType.underlyingType)

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/core/TypeCommonizer.kt

@@ -220,7 +220,7 @@ private class TypeArgumentListCommonizer(classifiers: CirKnownClassifiers) : Abs
 )
 
 private fun commonizeClass(classId: ClassId, classifiers: CirKnownClassifiers): Boolean {
-    if (classifiers.commonDependeeLibraries?.hasClassifier(classId) == true) {
+    if (classifiers.commonDependeeLibraries.hasClassifier(classId)) {
         // The class is from common fragment of dependee library (ex: stdlib). Already commonized.
         return true
     } else if (classId.packageFqName.isUnderKotlinNativeSyntheticPackages) {
@@ -247,7 +247,7 @@ private fun commonizeClass(classId: ClassId, classifiers: CirKnownClassifiers):
 }
 
 private fun commonizeTypeAlias(typeAliasId: ClassId, classifiers: CirKnownClassifiers): CommonizedTypeAliasAnswer {
-    if (classifiers.commonDependeeLibraries?.hasClassifier(typeAliasId) == true) {
+    if (classifiers.commonDependeeLibraries.hasClassifier(typeAliasId)) {
         // The type alias is from common fragment of dependee library (ex: stdlib). Already commonized.
         return SUCCESS_FROM_DEPENDEE_LIBRARY
     }

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/mergedtree/CirClassifiersCache.kt

@@ -25,7 +25,8 @@ class CirKnownClassifiers(
     val dependeeLibraries: Map<Target, CirProvidedClassifiers>
 ) {
     // a shortcut for fast access
-    val commonDependeeLibraries: CirProvidedClassifiers? = dependeeLibraries.filterKeys { it is SharedTarget }.values.singleOrNull()
+    val commonDependeeLibraries: CirProvidedClassifiers =
+        dependeeLibraries.filterKeys { it is SharedTarget }.values.singleOrNull() ?: CirProvidedClassifiers.EMPTY
 }
 
 interface CirCommonizedClassifiers {
@@ -86,6 +87,10 @@ interface CirProvidedClassifiers {
     //fun classifier(classifierId: ClassId): Any?
 
     companion object {
+        internal val EMPTY = object : CirProvidedClassifiers {
+            override fun hasClassifier(classifierId: ClassId) = false
+        }
+
         // N.B. This is suboptimal implementation. It will be replaced by another implementation that will
         // retrieve classifier information directly from the metadata.
         fun fromModules(storageManager: StorageManager, modules: () -> Collection<ModuleDescriptor>) = object : CirProvidedClassifiers {