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.

idea/testData/diagnosticMessage/abstractBaseClassMemberNotImplemented.kt

@@ -1,5 +1,5 @@
 // !DIAGNOSTICS_NUMBER: 1
-// !DIAGNOSTICS: ABSTRACT_MEMBER_NOT_IMPLEMENTED
+// !DIAGNOSTICS: ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED
 // !MESSAGE_TYPE: TEXT
 
 interface T1

idea/testData/diagnosticMessage/abstractBaseClassMemberNotImplemented1.txt

@@ -0,0 +1,2 @@
+<!-- abstractBaseClassMemberNotImplemented1 -->
+Class 'E' must be declared abstract or implement abstract base class member public abstract fun f(): T4 defined in D

idea/testData/diagnosticMessage/wrongReturnTypeInImplementation1.txt

@@ -1,2 +0,0 @@
-<!-- wrongReturnTypeInImplementation1 -->
-Class 'E' must be declared abstract or implement abstract member public abstract fun f(): T3 defined in C