diff --git a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinCodeBlockModificationListener.kt b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinCodeBlockModificationListener.kt
index 86f9b55d97f..41cb8ad942a 100644
--- a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinCodeBlockModificationListener.kt
+++ b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinCodeBlockModificationListener.kt
@@ -97,7 +97,7 @@ public class KotlinCodeBlockModificationListener(modificationTracker: PsiModific
             var child = element.getFirstChild()
             while (child != null) {
                 if (containsClassesInside(child)) return true
-                child = child!!.getNextSibling()
+                child = child.getNextSibling()
             }
 
             return false
diff --git a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightClass.kt b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightClass.kt
index dfecd5736c8..ffbe21456cd 100644
--- a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightClass.kt
+++ b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightClass.kt
@@ -20,6 +20,6 @@ import com.intellij.psi.PsiClass;
 import org.jetbrains.kotlin.name.FqName;
 import org.jetbrains.kotlin.psi.JetClassOrObject
 
-public trait KotlinLightClass : PsiClass, KotlinLightElement<JetClassOrObject, PsiClass> {
+public interface KotlinLightClass : PsiClass, KotlinLightElement<JetClassOrObject, PsiClass> {
     public fun getFqName(): FqName
 }
diff --git a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightElement.kt b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightElement.kt
index 4f0963d9340..12fe3e0699b 100644
--- a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightElement.kt
+++ b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightElement.kt
@@ -19,7 +19,7 @@ package org.jetbrains.kotlin.asJava
 import org.jetbrains.kotlin.psi.JetDeclaration
 import com.intellij.psi.PsiElement
 
-public trait KotlinLightElement<T : JetDeclaration, D : PsiElement> {
+public interface KotlinLightElement<T : JetDeclaration, D : PsiElement> {
     public fun getOrigin(): T?
 
     public fun getDelegate(): D
diff --git a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightMethod.kt b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightMethod.kt
index eb4a67782a7..7d40c73d9b6 100644
--- a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightMethod.kt
+++ b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/KotlinLightMethod.kt
@@ -19,4 +19,4 @@ package org.jetbrains.kotlin.asJava
 import com.intellij.psi.PsiMethod
 import org.jetbrains.kotlin.psi.JetDeclaration
 
-public trait KotlinLightMethod: PsiMethod, KotlinLightElement<JetDeclaration, PsiMethod>
+public interface KotlinLightMethod: PsiMethod, KotlinLightElement<JetDeclaration, PsiMethod>
diff --git a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/LightClassStubWithData.kt b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/LightClassStubWithData.kt
index 7f4b18c85d7..b011a196ec8 100644
--- a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/LightClassStubWithData.kt
+++ b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/LightClassStubWithData.kt
@@ -22,14 +22,14 @@ import org.jetbrains.kotlin.psi.JetClassOrObject
 import org.jetbrains.kotlin.resolve.diagnostics.Diagnostics
 import org.jetbrains.kotlin.name.FqName
 
-trait LightClassData
+interface LightClassData
 
-trait WithFileStubAndExtraDiagnostics {
+interface WithFileStubAndExtraDiagnostics {
     val javaFileStub: PsiJavaFileStub
     val extraDiagnostics: Diagnostics
 }
 
-trait LightClassDataForKotlinClass: LightClassData {
+interface LightClassDataForKotlinClass: LightClassData {
     val classOrObject: JetClassOrObject
     val descriptor: ClassDescriptor?
     val jvmQualifiedName: FqName
diff --git a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/duplicateJvmSignatureUtil.kt b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/duplicateJvmSignatureUtil.kt
index 1b0dc3e0c8a..a90dade088b 100644
--- a/compiler/light-classes/src/org/jetbrains/kotlin/asJava/duplicateJvmSignatureUtil.kt
+++ b/compiler/light-classes/src/org/jetbrains/kotlin/asJava/duplicateJvmSignatureUtil.kt
@@ -57,10 +57,10 @@ public fun getJvmSignatureDiagnostics(element: PsiElement, otherDiagnostics: Dia
 
         when (parent) {
             is JetFile -> {
-                return getDiagnosticsForPackage(parent as JetFile)
+                return getDiagnosticsForPackage(parent)
             }
             is JetClassBody -> {
-                val parentsParent = parent?.getParent()
+                val parentsParent = parent.getParent()
 
                 if (parentsParent is JetClassOrObject) {
                     return getDiagnosticsForClass(parentsParent)