Prohibit implicit overrides with lesser visibility

A non-abstract super-declaration is allowed to stand as 'an implementation' of
a fake override only if its visibility is not less than visibilities of all
other super-declarations

 #KT-2491 Fixed

compiler/testData/compileKotlinAgainstCustomBinaries/noWarningsOnJavaKotlinInheritance/noWarningsOnJavaKotlinInheritance.kt

@@ -8,7 +8,7 @@ class SimpleSubclass : JavaInterface {
 // Class extends both Java and Kotlin traits. Checks that there's no DIFFERENT_NAMES_FOR_THE_SAME_PARAMETER_IN_SUPERTYPES warning
 
 trait KotlinTrait {
-    fun foo(someOtherName: Int) {}
+    public fun foo(someOtherName: Int) {}
 }
 
 class BothTraitsSubclass : JavaInterface, KotlinTrait

compiler/testData/diagnostics/tests/override/kt2491.kt

@@ -0,0 +1,11 @@
+trait T {
+    public fun foo()
+}
+
+open class C {
+    protected fun foo() {}
+}
+
+class <!CANNOT_INFER_VISIBILITY!>D<!> : C(), T
+
+val obj: C = <!CANNOT_INFER_VISIBILITY!>object<!> : C(), T {}

compiler/testData/diagnostics/tests/override/parameterNames/kotlinInheritsBothJavaAndKotlin.kt

@@ -7,7 +7,7 @@ interface JavaInterface {
 // FILE: kotlin.kt
 
 trait KotlinTrait {
-    fun foo(someOtherName: Int) {}
+    public fun foo(someOtherName: Int) {}
 }
 
 class BothTraitsSubclass : JavaInterface, KotlinTrait

compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java

@@ -5342,6 +5342,11 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                 doTest("compiler/testData/diagnostics/tests/override/kt2052.kt");
             }
             
+            @TestMetadata("kt2491.kt")
+            public void testKt2491() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/override/kt2491.kt");
+            }
+            
             @TestMetadata("kt4763.kt")
             public void testKt4763() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/override/kt4763.kt");

core/descriptors/src/org/jetbrains/jet/lang/resolve/OverridingUtil.java

@@ -379,8 +379,18 @@ public class OverridingUtil {
             return;
         }
 
-        Visibility maxVisibility = computeVisibilityToInherit(memberDescriptor, cannotInferVisibility);
-        Visibility visibilityToInherit = maxVisibility == null ? Visibilities.PUBLIC : maxVisibility;
+        Visibility maxVisibility = computeVisibilityToInherit(memberDescriptor);
+        Visibility visibilityToInherit;
+        if (maxVisibility == null) {
+            if (cannotInferVisibility != null) {
+                cannotInferVisibility.invoke(memberDescriptor);
+            }
+            visibilityToInherit = Visibilities.PUBLIC;
+        }
+        else {
+            visibilityToInherit = maxVisibility;
+        }
+
         if (memberDescriptor instanceof PropertyDescriptorImpl) {
             ((PropertyDescriptorImpl) memberDescriptor).setVisibility(visibilityToInherit);
             for (PropertyAccessorDescriptor accessor : ((PropertyDescriptor) memberDescriptor).getAccessors()) {
@@ -398,18 +408,19 @@ public class OverridingUtil {
     }
 
     @Nullable
-    private static Visibility computeVisibilityToInherit(
-            @NotNull CallableMemberDescriptor memberDescriptor,
-            @Nullable Function1<CallableMemberDescriptor, Unit> cannotInferVisibility
-    ) {
-        Visibility maxVisibility = findMaxVisibility(memberDescriptor.getOverriddenDescriptors());
+    private static Visibility computeVisibilityToInherit(@NotNull CallableMemberDescriptor memberDescriptor) {
+        Set<? extends CallableMemberDescriptor> overriddenDescriptors = memberDescriptor.getOverriddenDescriptors();
+        Visibility maxVisibility = findMaxVisibility(overriddenDescriptors);
         if (maxVisibility == null) {
-            if (cannotInferVisibility != null) {
-                cannotInferVisibility.invoke(memberDescriptor);
-            }
             return null;
         }
         if (memberDescriptor.getKind() == CallableMemberDescriptor.Kind.FAKE_OVERRIDE) {
+            for (CallableMemberDescriptor overridden : overriddenDescriptors) {
+                // An implementation (a non-abstract overridden member) of a fake override should have the maximum possible visibility
+                if (overridden.getModality() != Modality.ABSTRACT && !overridden.getVisibility().equals(maxVisibility)) {
+                    return null;
+                }
+            }
             return maxVisibility;
         }
         return maxVisibility.normalize();
@@ -423,7 +434,7 @@ public class OverridingUtil {
         Visibility maxVisibility = null;
         for (CallableMemberDescriptor descriptor : descriptors) {
             Visibility visibility = descriptor.getVisibility();
-            assert visibility != Visibilities.INHERITED;
+            assert visibility != Visibilities.INHERITED : "Visibility should have been computed for " + descriptor;
             if (maxVisibility == null) {
                 maxVisibility = visibility;
                 continue;