Kotlin契约:两个属性的链接不为空

假设我有一门课是这样的:

data class URLAndPath(
   val baseUrl: URL,
   val path: String?
) {
    val url get(): URL? =
        try { path?.let { URL(baseUrl, it) } }
        catch(_: Exception) { null }

    init { require(path == null || url != null) { "Invalid URL $baseUrl$path" } } 
}

这个类确保如果 path != null 当且仅当 url != null

Kotlin contracts 似乎是告诉编译器这类关系的方法。上述不变量是否可以用Kotlin契约建模?

我的最终结果是让如下代码编译:

val x = URLAndPath(URL("http://example.org/"), "index.html")
if(x.path != null) {
    // currently: Error: Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type URL?
    println(x.url.toURI())
}