关于toMutableList()与数据类型转化的问题

Question

在学习kotlin泛型型变时,写了以下代码

val list2 = mutableListOf<Int>(4,4,5)
val list : MutableList<out Number> = list2

然后尝试了一下错误的写法

val list3 : MutableList<Number> = list2//error

发现以下代码不会报错

val list3 : MutableList<Number> = list2.toMutableList()

通过源码可以看到数据类型没有变化

/**
 * Constructs a list containing the elements of the specified
 * collection, in the order they are returned by the collection's
 * iterator.
 *
 * @param c the collection whose elements are to be placed into this list
 * @throws NullPointerException if the specified collection is null
 */
public ArrayList(Collection<? extends E> c) {
    Object[] a = c.toArray();
    if ((size = a.length) != 0) {
        if (c.getClass() == ArrayList.class) {
            elementData = a;
        } else {
            elementData = Arrays.copyOf(a, size, Object[].class);
        }
    } else {
        // replace with empty array.
        elementData = EMPTY_ELEMENTDATA;
    }
}

所以是如何跳过类型转换的？