<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 Feb 22, 2017, at 10:20, David Sweeris 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=""><span 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; float: none; display: inline !important;" class="">What if we extended that to any type that’s ExpressibleByNilLiteral & Equatable?</span></div></blockquote></div><br class=""><div class="">Note that this is not how the Unsafe*Pointer optimization works: in that case, the type Unsafe*Pointer is not ExpressibleByNilLiteral, it just has some spare/guaranteed-to-be-unused sentinel values in its representation, that the compiler knows about. This also works for types like `enum X { case A, B }`: an optional like X? (and X??, all the way up to 254 ?’s) is also represented as a single byte, because there’s spare values that the compiler knows won't be used by valid instances of X.</div><div class=""><br class=""></div><div class="">Converting valid instances to .none would break round-tripping: Optional(x)! would sometimes fail, if x was the sentinel value. This seems likely to cause generic code to stop working.</div><div class=""><br class=""></div><div class="">Huon</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">Example program for the enum optimization:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">enum</span><span style="font-variant-ligatures: no-common-ligatures" class=""> X {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> A, B</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O2<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">??</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O4<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O2</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O2</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O8<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O4</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O4</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O16<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O8</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O8</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O32<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O16</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O16</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O64<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O32</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O32</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O128<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O64</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O64</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O256<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O128</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O128</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O254<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O128</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O64</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O32</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O16</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O8</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O4</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O2</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>>>>>>></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">typealias</span><span style="font-variant-ligatures: no-common-ligatures" class=""> O255<T> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">O254</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>?</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">func</span><span style="font-variant-ligatures: no-common-ligatures" class=""> size<T>(</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">_</span><span style="font-variant-ligatures: no-common-ligatures" class="">: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">.Type) -> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> MemoryLayout<</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">>.size</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">print(size(X.</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">), </span><span style="font-variant-ligatures: no-common-ligatures; color: #008400" class="">// 1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> size((X?).</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">), </span><span style="font-variant-ligatures: no-common-ligatures; color: #008400" class="">// 1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> size(O254<</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">X</span><span style="font-variant-ligatures: no-common-ligatures" class="">>.</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">), </span><span style="font-variant-ligatures: no-common-ligatures; color: #008400" class="">// 1</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> size(O255<</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">X</span><span style="font-variant-ligatures: no-common-ligatures" class="">>.</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">), </span><span style="font-variant-ligatures: no-common-ligatures; color: #008400" class="">// 2</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> size(O256<</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">X</span><span style="font-variant-ligatures: no-common-ligatures" class="">>.</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">)) </span><span style="font-variant-ligatures: no-common-ligatures; color: #008400" class="">// 3</span></div></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><div style="font-family: Helvetica; font-size: 12px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px;" class="">(The last is 3 because the compiler currently essentially treats O255<X> like an opaque/all-values-valid 2-byte type, like Int16, and so adds an extra byte for the extra ?. It could theoretically be 2 if the unused values in the extra byte in O255<X> are reused.)</div></div></body></html>