kotlin - 为什么 inc() 不能定义为与接收者类型不同的类中的扩展函数？

Question

这是我在做 Koan N28 时遇到的：

class DateRange(override val start: MyDate, override val endInclusive: MyDate) : ClosedRange<MyDate>, Iterable<MyDate> {

    private operator fun MyDate.inc() : MyDate {
        return this.nextDay()
    }

    override fun iterator(): Iterator<MyDate> {
        return object : Iterator<MyDate> {
            private var point = start

            override fun hasNext(): Boolean {
                return point <= endInclusive
            }

            override fun next(): MyDate {
                return if (hasNext()) point++ else throw NoSuchElementException()
            }


        }
    }
}

但是，编译器在该operator位置大喊大叫，说'operator' modifier is inapplicable on this function: receiver must be a supertype of the return type. 为了让代码编译，我必须将扩展函数移到顶层。但是，如果我MyDate.unaryPlus()在同一个地方重载，代码会编译。