Java 8 rules for method overrides:

- base class method wins against a (default) interface method,
so an abstract base class method should always be implemented
in a derived class;

- interface methods clash regardless of abstract/default
with possibly undefined behavior at run-time,
so a class or interface should always define its own method
for methods inherited from multiple interfaces and not from base class;

- meaningful diagnostics for class inheriting conflicting JVM signatures.
Since no override will happen under Java 8 rules,
ACCIDENTAL_OVERRIDE is misleading for this case;

- update testData.

compiler/backend/src/org/jetbrains/kotlin/codegen/state/BuilderFactoryForDuplicateSignatureDiagnostics.kt

@@ -118,7 +118,12 @@ class BuilderFactoryForDuplicateSignatureDiagnostics(
             if (elementToReportOn == null) return // TODO: it'd be better to report this error without any element at all
 
             val data = ConflictingJvmDeclarationsData(classInternalName, classOrigin, rawSignature, origins)
-            diagnostics.report(ErrorsJvm.ACCIDENTAL_OVERRIDE.on(elementToReportOn, data))
+            if (memberElement != null) {
+                diagnostics.report(ErrorsJvm.ACCIDENTAL_OVERRIDE.on(elementToReportOn, data))
+            }
+            else {
+                diagnostics.report(ErrorsJvm.CONFLICTING_INHERITED_JVM_DECLARATIONS.on(elementToReportOn, data))
+            }
         }
     }