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

@@ -0,0 +1,27 @@
+// !DIAGNOSTICS_NUMBER: 1
+// !DIAGNOSTICS: ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED
+// !MESSAGE_TYPE: TEXT
+
+interface T1
+interface T2 : T1
+interface T3 : T2
+interface T4 : T3
+
+
+interface A {
+    fun f(): T1
+}
+
+interface B {
+    fun f(): T2 = null!!
+}
+
+interface C {
+    fun f(): T3
+}
+
+abstract class D {
+    abstract fun f(): T4
+}
+
+class E : A, B, C, D()