KT-4198 On covariant projection, do not throw whole var's away

#KT-4198 Fixed

core/descriptors/src/org/jetbrains/jet/lang/descriptors/PropertyDescriptor.java

@@ -30,6 +30,16 @@ public interface PropertyDescriptor extends VariableDescriptor, CallableMemberDe
     @Nullable
     PropertySetterDescriptor getSetter();
 
+    /**
+     * In the following case, the setter is projected out:
+     *
+     *     trait Tr<T> { var v: T }
+     *     fun test(tr: Tr<out String>) {
+     *         tr.v = null!! // the assignment is illegal, although a read would be fine
+     *     }
+     */
+    boolean isSetterProjectedOut();
+
     @NotNull
     List<PropertyAccessorDescriptor> getAccessors();
 

core/descriptors/src/org/jetbrains/jet/lang/descriptors/impl/PropertyDescriptorImpl.java

@@ -30,6 +30,7 @@ import org.jetbrains.jet.lang.types.DescriptorSubstitutor;
 import org.jetbrains.jet.lang.types.JetType;
 import org.jetbrains.jet.lang.types.TypeSubstitutor;
 import org.jetbrains.jet.lang.types.Variance;
+import org.jetbrains.jet.lang.types.lang.KotlinBuiltIns;
 
 import java.util.Collections;
 import java.util.List;
@@ -49,6 +50,7 @@ public class PropertyDescriptorImpl extends VariableDescriptorImpl implements Pr
     private List<TypeParameterDescriptor> typeParameters;
     private PropertyGetterDescriptorImpl getter;
     private PropertySetterDescriptor setter;
+    private boolean setterProjectedOut;
 
     private PropertyDescriptorImpl(
             @Nullable PropertyDescriptor original,
@@ -125,6 +127,10 @@ public class PropertyDescriptorImpl extends VariableDescriptorImpl implements Pr
         this.setter = setter;
     }
 
+    public void setSetterProjectedOut(boolean setterProjectedOut) {
+        this.setterProjectedOut = setterProjectedOut;
+    }
+
     public void setVisibility(@NotNull Visibility visibility) {
         this.visibility = visibility;
     }
@@ -182,6 +188,11 @@ public class PropertyDescriptorImpl extends VariableDescriptorImpl implements Pr
         return setter;
     }
 
+    @Override
+    public boolean isSetterProjectedOut() {
+        return setterProjectedOut;
+    }
+
     @Override
     @NotNull
     public List<PropertyAccessorDescriptor> getAccessors() {
@@ -253,7 +264,15 @@ public class PropertyDescriptorImpl extends VariableDescriptorImpl implements Pr
         if (newSetter != null) {
             List<ValueParameterDescriptor> substitutedValueParameters = FunctionDescriptorImpl.getSubstitutedValueParameters(newSetter, setter, substitutor);
             if (substitutedValueParameters == null) {
-                return null;
+                // The setter is projected out, e.g. in this case:
+                //     trait Tr<T> { var v: T }
+                //     fun test(tr: Tr<out Any?>) { ... }
+                // we want to tell the user that although the property is declared as a var,
+                // it can not be assigned to because of the projection
+                substitutedDescriptor.setSetterProjectedOut(true);
+                substitutedValueParameters = Collections.<ValueParameterDescriptor>singletonList(
+                        PropertySetterDescriptorImpl.createSetterParameter(newSetter, KotlinBuiltIns.getInstance().getNothingType())
+                );
             }
             if (substitutedValueParameters.size() != 1) {
                 throw new IllegalStateException();

core/descriptors/src/org/jetbrains/jet/lang/descriptors/impl/PropertySetterDescriptorImpl.java

@@ -67,7 +67,16 @@ public class PropertySetterDescriptorImpl extends PropertyAccessorDescriptorImpl
 
     public void initializeDefault() {
         assert parameter == null;
-        parameter = new ValueParameterDescriptorImpl(this, 0, Collections.<AnnotationDescriptor>emptyList(), Name.special("<set-?>"), getCorrespondingProperty().getReturnType(), false, null);
+        parameter = createSetterParameter(this, getCorrespondingProperty().getReturnType());
+    }
+
+    public static ValueParameterDescriptorImpl createSetterParameter(
+            @NotNull PropertySetterDescriptor setterDescriptor,
+            @NotNull JetType type
+    ) {
+        return new ValueParameterDescriptorImpl(
+                setterDescriptor, 0, Collections.<AnnotationDescriptor>emptyList(), Name.special("<set-?>"), type, false, null
+        );
     }
 
     @NotNull