为什么我不能改变一个隐式解包的可选变量？

以下是重现问题的简短示例:

使用Array

var list: [Int]! = [1]
list.append(10) // Error here

Immutable value of type '[Int]' only has mutating members named 'append'

用Int

var number: Int! = 1
number = 2
number = 2 + number
number += 2 // Error here

Could not find an overload for '+=' that accepts the supplied arguments