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

idea/testData/quickfix/changeSignature/notEditable.kt

@@ -1,11 +1,11 @@
 // "class org.jetbrains.kotlin.idea.quickfix.RemoveFunctionParametersFix" "false"
 //ERROR: No value passed for parameter other
 
-interface StringComparable {
+abstract class StringComparable {
     public fun compareTo(other: String): Int = 0
 }
 
-class X: Comparable<String>, StringComparable
+class X: Comparable<String>, StringComparable()
 
 fun main(args: Array<String>) {
     X().compareTo(<caret>)

idea/tests/org/jetbrains/kotlin/idea/highlighter/DiagnosticMessageTestGenerated.java

@@ -31,6 +31,12 @@ import java.util.regex.Pattern;
 @TestDataPath("$PROJECT_ROOT")
 @RunWith(JUnit3RunnerWithInners.class)
 public class DiagnosticMessageTestGenerated extends AbstractDiagnosticMessageTest {
+    @TestMetadata("abstractBaseClassMemberNotImplemented.kt")
+    public void testAbstractBaseClassMemberNotImplemented() throws Exception {
+        String fileName = JetTestUtils.navigationMetadata("idea/testData/diagnosticMessage/abstractBaseClassMemberNotImplemented.kt");
+        doTest(fileName);
+    }
+
     public void testAllFilesPresentInDiagnosticMessage() throws Exception {
         JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("idea/testData/diagnosticMessage"), Pattern.compile("^(.+)\\.kt$"), false);
     }
@@ -202,10 +208,4 @@ public class DiagnosticMessageTestGenerated extends AbstractDiagnosticMessageTes
         String fileName = JetTestUtils.navigationMetadata("idea/testData/diagnosticMessage/upperBoundViolated.kt");
         doTest(fileName);
     }
-
-    @TestMetadata("wrongReturnTypeInImplementation.kt")
-    public void testWrongReturnTypeInImplementation() throws Exception {
-        String fileName = JetTestUtils.navigationMetadata("idea/testData/diagnosticMessage/wrongReturnTypeInImplementation.kt");
-        doTest(fileName);
-    }
 }