gistfile1.kt

public final class Attrs<out T: HtmlTag>(tag: T): AbstractCommonAttribute<Attrs<T>>(tag)

open class A(containingTag: HtmlBodyTag): AllowText, HtmlBodyTag(containingTag, "a") {
//    public final class Attrs(htmlTag: HtmlTag): AbstractCommonAttribute<Attrs>(htmlTag)
    public open val attr: Attrs<A> = Attrs(this)
}
public var Attrs<A>.href: Link by Attributes.href

class A1: A("") {
    override val attr: Attrs<A1> = Attrs(this)
}

var Attrs<A1>.foo = 1

fun main(args: Array<String>) {
    A1().attr.foo
    A1().attr.href

}