a bit of nullability cleanup

compiler/backend/src/org/jetbrains/jet/codegen/CallableMethod.java

@@ -84,10 +84,12 @@ public class CallableMethod implements Callable {
         return signature.getValueParameterTypes();
     }
 
+    @Nullable
     public JvmClassName getThisType() {
         return thisClass;
     }
 
+    @Nullable
     public Type getReceiverClass() {
         return receiverParameterType;
     }

compiler/backend/src/org/jetbrains/jet/codegen/ExpressionCodegen.java

@@ -1516,6 +1516,7 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> {
                     }
                     if (enclosed != context.getThisDescriptor()) {
                         CodegenContext c = context;
+                        //noinspection ConstantConditions
                         while (!(c instanceof CodegenContexts.ClassContext) ||
                                !DescriptorUtils.isSubclass(c.getThisDescriptor(), enclosed)) {
                             c = c.getParentContext();
@@ -1720,6 +1721,7 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> {
         }
     }
 
+    @Nullable
     private static JetExpression getReceiverForSelector(PsiElement expression) {
         if (expression.getParent() instanceof JetDotQualifiedExpression && !isReceiver(expression)) {
             final JetDotQualifiedExpression parent = (JetDotQualifiedExpression) expression.getParent();
@@ -1793,9 +1795,11 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> {
 
 
             assert cur != null;
-            if (DescriptorUtils.isSubclass(cur.getThisDescriptor(), calleeContainingClass)) {
+            final ClassDescriptor thisDescriptor = cur.getThisDescriptor();
-                if (!isObject || (cur.getThisDescriptor() == calleeContainingClass)) {
+            assert thisDescriptor != null;
-                    return castToRequiredTypeOfInterfaceIfNeeded(result, cur.getThisDescriptor(), calleeContainingClass);
+            if (DescriptorUtils.isSubclass(thisDescriptor, calleeContainingClass)) {
+                if (!isObject || (thisDescriptor == calleeContainingClass)) {
+                    return castToRequiredTypeOfInterfaceIfNeeded(result, thisDescriptor, calleeContainingClass);
                 }
                 else {
                     v.getstatic(type.getInternalName(), "$instance", type.getDescriptor());
@@ -2447,6 +2451,7 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> {
         v.invokevirtual("java/lang/StringBuilder", "append", appendDescriptor.getDescriptor());
     }
 
+    @Nullable
     private static JetSimpleNameExpression targetLabel(JetExpression expression) {
         if (expression.getParent() instanceof JetPrefixExpression) {
             JetPrefixExpression parent = (JetPrefixExpression) expression.getParent();

compiler/backend/src/org/jetbrains/jet/codegen/ImplementationBodyCodegen.java

@@ -205,6 +205,7 @@ public class ImplementationBodyCodegen extends ClassBodyCodegen {
         }
     }
 
+    @Nullable
     private static ClassDescriptor getContainingClassDescriptor(ClassDescriptor decl) {
         DeclarationDescriptor container = decl.getContainingDeclaration();
         while (container != null && !(container instanceof NamespaceDescriptor)) {
@@ -768,6 +769,7 @@ public class ImplementationBodyCodegen extends ClassBodyCodegen {
                     typeMapper.getClosureAnnotator().getEclosingClassDescriptor(descriptor).getDefaultType(), MapTypeMode.VALUE));
         }
         Method superCallMethod = new Method("<init>", Type.VOID_TYPE, parameterTypes.toArray(new Type[parameterTypes.size()]));
+        //noinspection ConstantConditions
         iv.invokespecial(typeMapper.mapType(superClassDescriptor.getDefaultType(), MapTypeMode.VALUE).getInternalName(), "<init>",
                          superCallMethod.getDescriptor());
     }

compiler/backend/src/org/jetbrains/jet/codegen/JetTypeMapper.java

@@ -341,6 +341,7 @@ public class JetTypeMapper {
         return mapType(jetType, signatureVisitor, MapTypeMode.VALUE);
     }
 
+    @Nullable
     public static String getLocalNameForObject(JetObjectDeclaration object) {
         PsiElement parent = object.getParent();
         if (parent instanceof JetClassObject) {
@@ -596,11 +597,7 @@ public class JetTypeMapper {
         }
     }
 
-    public CallableMethod mapToCallableMethod(FunctionDescriptor functionDescriptor, boolean superCall, OwnerKind kind) {
+    public CallableMethod mapToCallableMethod(@NotNull FunctionDescriptor functionDescriptor, boolean superCall, OwnerKind kind) {
-        if (functionDescriptor == null) {
-            return null;
-        }
-
         final DeclarationDescriptor functionParent = functionDescriptor.getOriginal().getContainingDeclaration();
 
         while (functionDescriptor.getKind() == CallableMemberDescriptor.Kind.FAKE_OVERRIDE) {