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/testData/diagnostics/tests/delegation/DelegationAndOverriding.kt

@@ -9,7 +9,7 @@ open <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class Br<!>(<!UNUSED_PARAMETER!>t<!> : T
 
 }
 
-<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class Br3<!>(t : T) : Br(t) {
+<!ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED!>class Br3<!>(t : T) : Br(t) {
 
 }
 
@@ -35,4 +35,4 @@ open class GC1(g : G<Int>) : G<Int> by g {
 
 open class GC2(g : G<Int>) : GC1(g) {
 
-}
+}

compiler/testData/diagnostics/tests/delegation/Delegation_MultipleDelegates.kt

@@ -14,6 +14,6 @@ interface TwoImpl : Two {
   public override fun foo() = 2
 }
 
-<!MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class Test1<!>() : TwoImpl, OneImpl {}
+<!MANY_INTERFACES_MEMBER_NOT_IMPLEMENTED!>class Test1<!>() : TwoImpl, OneImpl {}
 class Test2(a : One) : One by a, Two {}
 <!MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>class Test3<!>(a : One, b : Two) : Two by b, One by a {}