<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Dec 8, 2016, at 4:35 PM, Jordan Rose via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">Um, Sequence doesn’t have a subscript (or indexes). Sequences are single-pass. So if this is important, it needs to stay a Collection.</div><br class="Apple-interchange-newline"></div></blockquote></div><br class=""><div class="">Just because something fulfills one of the requirements of a Collection does not mean it should be one. It needs to tick all the boxes before its allowed to be elevated.</div><div class=""><br class=""></div><div class="">But it’s still allowed to have subscripts (UnsafePointer has subscripting but isn’t a collection) or be multi-pass (strides are multiples but are only sequences). That’s OK</div><div class=""><br class=""></div><div class="">In this case, yes it’s multi-pass, yes it has a subscript, but no it isn’t a collection because it doesn’t meet the requirements for slicing i.e. that indices of the slice be indices of the parent.</div><div class="">(relatedly… it appears this requirement is documented on the concrete Slice type rather than on Collection… which is a documentation bug we should fix).</div></body></html>