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/codegen/box/bridges/complexTraitImpl.kt

@@ -9,7 +9,9 @@ interface B {
     fun foo(): ArrayList<String> = ArrayList(Arrays.asList("B"))
 }
 
-open class C : A(), B
+open class C : A(), B {
+    override fun foo(): ArrayList<String> = super<B>.foo()
+}
 
 interface D {
     fun foo(): Collection<String>