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/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/diagnostics/DefaultErrorMessagesJvm.java

@@ -45,6 +45,7 @@ public class DefaultErrorMessagesJvm implements DefaultErrorMessages.Extension {
     static {
         MAP.put(ErrorsJvm.CONFLICTING_JVM_DECLARATIONS, "Platform declaration clash: {0}", CONFLICTING_JVM_DECLARATIONS_DATA);
         MAP.put(ErrorsJvm.ACCIDENTAL_OVERRIDE, "Accidental override: {0}", CONFLICTING_JVM_DECLARATIONS_DATA);
+        MAP.put(ErrorsJvm.CONFLICTING_INHERITED_JVM_DECLARATIONS, "Inherited platform declarations clash: {0}", CONFLICTING_JVM_DECLARATIONS_DATA);
         MAP.put(ErrorsJvm.JVM_STATIC_NOT_IN_OBJECT, "Only functions in named objects and companion objects of classes can be annotated with 'JvmStatic'");
         MAP.put(ErrorsJvm.OVERRIDE_CANNOT_BE_STATIC, "Override member cannot be 'JvmStatic' in object");
         MAP.put(ErrorsJvm.OVERLOADS_WITHOUT_DEFAULT_ARGUMENTS, "''JvmOverloads'' annotation has no effect for methods without default arguments");

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/diagnostics/ErrorsJvm.java

@@ -39,6 +39,8 @@ public interface ErrorsJvm {
 
     DiagnosticFactory1<PsiElement, ConflictingJvmDeclarationsData> ACCIDENTAL_OVERRIDE =
             DiagnosticFactory1.create(ERROR, DECLARATION_SIGNATURE_OR_DEFAULT);
+    DiagnosticFactory1<PsiElement, ConflictingJvmDeclarationsData> CONFLICTING_INHERITED_JVM_DECLARATIONS =
+            DiagnosticFactory1.create(ERROR, DECLARATION_SIGNATURE_OR_DEFAULT);
 
     DiagnosticFactory0<JetDeclaration> OVERRIDE_CANNOT_BE_STATIC = DiagnosticFactory0.create(ERROR, DECLARATION_SIGNATURE);
     DiagnosticFactory0<JetDeclaration> JVM_STATIC_NOT_IN_OBJECT = DiagnosticFactory0.create(ERROR, DECLARATION_SIGNATURE);