这是文档的锅，主要是这里有一个递归的声明 SubSequence.SubSequence == SubSequence，所以文档生成器才被绕进去了，实际声明的代码其实很简单：

// https://github.com/apple/swift/blob/master/stdlib/public/core/Collection.swift
protocol Collection: Sequence {
  override associatedtype Element
  associatedtype Index: Comparable
  associatedtype SubSequence: Collection where SubSequence.Index == Index,
        Element == SubSequence.Element,
        SubSequence.SubSequence == SubSequence
  ...
}

继续保留侧滑返回就好了