class StrList : List<String?> {
    override val size: Int
        get() = throw UnsupportedOperationException()

    override fun isEmpty(): Boolean {
        throw UnsupportedOperationException()
    }

    override fun contains(o: String?) = o == null || o == "abc"

    override fun iterator(): Iterator<String> {
        throw UnsupportedOperationException()
    }

    override fun containsAll(c: Collection<String?>) = false
    override fun get(index: Int): String {
        throw UnsupportedOperationException()
    }

    override fun indexOf(o: String?): Int {
        throw UnsupportedOperationException()
    }

    override fun lastIndexOf(o: String?): Int {
        throw UnsupportedOperationException()
    }

    override fun listIterator(): ListIterator<String?> {
        throw UnsupportedOperationException()
    }

    override fun listIterator(index: Int): ListIterator<String?> {
        throw UnsupportedOperationException()
    }

    override fun subList(fromIndex: Int, toIndex: Int): List<String?> {
        throw UnsupportedOperationException()
    }
}

fun <E> Collection<E>.forceContains(x: Any?): Boolean = contains(x as E)

fun box(): String {
    val strList = StrList()

    if (strList.forceContains(1)) return "fail 1"
    if (!strList.forceContains(null)) return "fail 2"
    if (strList.forceContains("cde")) return "fail 3"
    if (!strList.forceContains("abc")) return "fail 4"

    return "OK"
}