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>

compiler/testData/codegen/box/bridges/fakeGenericContravariantOverride1.kt

@@ -1,30 +0,0 @@
-interface A<T> {
-    fun foo(t: T): String
-}
-
-interface B {
-    fun foo(t: Int) = "B"
-}
-
-class Z1 : A<Int>, B
-
-class Z2 : B, A<Int>
-
-fun box(): String {
-    val z1 = Z1()
-    val z2 = Z2()
-    val z1a: A<Int> = z1
-    val z1b: B = z1
-    val z2a: A<Int> = z2
-    val z2b: B = z2
-
-    return when {
-        z1.foo( 0)  != "B" -> "Fail #1"
-        z1a.foo( 0) != "B" -> "Fail #2"
-        z1b.foo( 0) != "B" -> "Fail #3"
-        z2.foo( 0)  != "B" -> "Fail #4"
-        z2a.foo( 0) != "B" -> "Fail #5"
-        z2b.foo( 0) != "B" -> "Fail #6"
-        else -> "OK"
-    }
-}

compiler/testData/codegen/box/bridges/fakeGenericContravariantOverride2.kt

@@ -1,30 +0,0 @@
-interface A<T> {
-    fun foo(t: T): String
-}
-
-interface B<T : Number> {
-    fun foo(a: T) = "B"
-}
-
-class Z1 : A<Int>, B<Int>
-
-class Z2 : B<Int>, A<Int>
-
-fun box(): String {
-    val z1 = Z1()
-    val z2 = Z2()
-    val z1a: A<Int> = z1
-    val z1b: B<Int> = z1
-    val z2a: A<Int> = z2
-    val z2b: B<Int> = z2
-
-    return when {
-        z1.foo( 0)  != "B" -> "Fail #1"
-        z1a.foo( 0) != "B" -> "Fail #2"
-        z1b.foo( 0) != "B" -> "Fail #3"
-        z2.foo( 0)  != "B" -> "Fail #4"
-        z2a.foo( 0) != "B" -> "Fail #5"
-        z2b.foo( 0) != "B" -> "Fail #6"
-        else -> "OK"
-    }
-}

compiler/testData/codegen/box/bridges/fakeOverrideOfPropertySetterInTraitImpl.kt

@@ -1,25 +0,0 @@
-var result = ""
-
-interface A {
-    var foo: String
-        get() = result
-        set(value) {
-            result += value
-        }
-}
-
-abstract class B {
-    abstract var foo: Any
-}
-
-class C : A, B()
-
-fun box(): String {
-    val c = C()
-    c.foo = "1"
-    val b: B = c
-    val a: A = c
-    b.foo = "2"
-    a.foo = "3"
-    return if (result == "123") "OK" else "Fail: $result"
-}

compiler/testData/codegen/box/bridges/fakeOverrideWithImplementationInTrait.kt

@@ -1,20 +0,0 @@
-interface A {
-    fun foo(): String = "A"
-}
-
-interface B {
-    fun foo(): Any
-}
-
-class C : A, B
-
-fun box(): String {
-    val c = C()
-    var result = ""
-    val b: B = c
-    val a: A = c
-    result += c.foo()
-    result += b.foo()
-    result += a.foo()
-    return if (result == "AAA") "OK" else "Fail: $result"
-}

compiler/testData/codegen/box/builtinsProperties/bridges.kt

@@ -9,6 +9,8 @@ class B0 : Collection<String>, A0 {
     override fun contains(o: String) = throw UnsupportedOperationException()
     override fun iterator() = throw UnsupportedOperationException()
     override fun containsAll(c: Collection<String>) = throw UnsupportedOperationException()
+    override val size: Int
+        get() = super.size
 }
 
 open class A1 {
@@ -45,6 +47,8 @@ class B4 : Collection<String>, I4<Int> {
     override fun contains(o: String) = throw UnsupportedOperationException()
     override fun iterator() = throw UnsupportedOperationException()
     override fun containsAll(c: Collection<String>) = throw UnsupportedOperationException()
+    override val size: Int
+        get() = super.size
 }
 
 interface I5 : Collection<String> {

compiler/testData/codegen/box/classes/kt2477.kt

@@ -10,8 +10,8 @@ interface B {
         get() = "FAIL"
 }
 
-abstract class C {
-    abstract protected val c: String
+open class C {
+    private val c: String = "FAIL"
 }
 
 open class D: C(), A, B {