within: in: にどのViewいれても同じ結果になると思います。
public func samePosition( in utf8: String.UTF8View ) -> String.UTF8View.Index? { return String.UTF8View.Index(self, within: utf8) } public func samePosition( in utf16: String.UTF16View ) -> String.UTF16View.Index? { return String.UTF16View.Index(self, within: utf16) }