fun box(): String {
    val nullStr = JavaClass.nullString()
    val nonnullStr = JavaClass.nonnullString()

    if (nullStr.foo() != true) return "fail 1"
    if (nonnullStr.foo() != true) return "fail 2"

    if (nullStr.fooN() != true) return "fail 3"
    if (nonnullStr.fooN() != true) return "fail 4"

    return "OK"
}

inline fun <reified T> T.foo(): Boolean = this is T

inline fun <reified T> T.fooN(): Boolean = this is T?