为什么受保护的var是KProperty，而公共/私有var是KMutableProperty？

Question

在以下每种情况下，我都有一些 mutable var属性。根据Javadocs，可变属性由KMutableProperty表示，但是在这些示例中，protected属性由KProperty类表示。为什么会这样？

class FooA {
    var publicProp: String? = null
    protected var protectedProp: String? = null
    private var privateProp: String? = null

    fun foo() {
        val a = ::publicProp
        val b = ::protectedProp
        val c = ::privateProp
    }
}

open class FooB {
    var publicProp: String? = null
    protected var protectedProp: String? = null
    private var privateProp: String? = null

    fun foo() {
        val a = ::publicProp
        val b = ::protectedProp
        val c = ::privateProp
    }
}

class Bar : FooB() {
    fun bar() {
        val a = ::publicProp
        val b = ::protectedProp
    }
}

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