Don't treat values of type "Unit?" to be always equal to Unit.VALUE

Two changes here: StackValue.Constant does cast iff value is non-null (if null,
no cast between classes is really needed, as null can be an instance of
anything), and codegen for safe qualified expressions uses correct type for the
expression

 #KT-4265 Fixed

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

@@ -2622,13 +2622,13 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> implem
     public StackValue visitSafeQualifiedExpression(@NotNull JetSafeQualifiedExpression expression, StackValue unused) {
         JetExpression receiver = expression.getReceiverExpression();
         JetExpression selector = expression.getSelectorExpression();
+        Type type = boxType(expressionType(expression));
         Type receiverType = expressionType(receiver);
 
         gen(receiver, receiverType);
 
         if (isPrimitive(receiverType)) {
             StackValue propValue = genQualified(StackValue.onStack(receiverType), selector);
-            Type type = boxType(propValue.type);
             propValue.put(type, v);
 
             return StackValue.onStack(type);
@@ -2639,7 +2639,6 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> implem
         v.dup();
         v.ifnull(ifnull);
         StackValue propValue = genQualified(StackValue.onStack(receiverType), selector);
-        Type type = boxType(propValue.type);
         propValue.put(type, v);
         v.goTo(end);
 

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

@@ -430,7 +430,10 @@ public abstract class StackValue {
             else {
                 v.aconst(value);
             }
-            coerceTo(type, v);
+
+            if (value != null) {
+                coerceTo(type, v);
+            }
         }
 
         @Override