KT-13355

An overridden abstract member with a more specific "return type" than an overridden concrete member
should be explicitly overridden even if the class in question can have abstract members.

core/util.runtime/src/org/jetbrains/kotlin/utils/addToStdlib.kt

@@ -80,6 +80,7 @@ fun <T: Any> T.check(predicate: (T) -> Boolean): T? = if (predicate(this)) this
 
 inline fun <reified T : Any> Any?.safeAs(): T? = this as? T
 inline fun <reified T : Any> Any?.cast(): T = this as T
+inline fun <reified T : Any> Any?.assertedCast(message: () -> String): T = this as? T ?: throw AssertionError(message())
 
 fun <T : Any> constant(calculator: () -> T): T {
     val cached = constantMap[calculator]