<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><span></span></div><div><div></div><div>Looks good. Minor comments below:</div><div><br></div><div><pre class="" style="box-sizing: border-box; margin-top: 0px; margin-bottom: 0px; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal;"><blockquote type="cite"><font face="UICTFontTextStyleBody"><span style="white-space: normal; background-color: rgba(255, 255, 255, 0);"><span class="pl-k" style="box-sizing: border-box;">let</span> t<span class="pl-k" style="box-sizing: border-box;">:</span> P <span class="pl-k" style="box-sizing: border-box;">&</span> C <span class="pl-c" style="box-sizing: border-box;"><span class="pl-c" style="box-sizing: border-box;">//</span> Compiler error: subclass contraint must be in first position</span></span></font></blockquote></pre><pre class="" style="box-sizing: border-box; margin-top: 0px; margin-bottom: 0px; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal;"><font face="UICTFontTextStyleBody"><span style="white-space: normal; background-color: rgba(255, 255, 255, 0);"><span class="pl-c" style="box-sizing: border-box;">Typo: contraint should be constraint</span></span></font></pre></div><blockquote type="cite"><div class=""><h3 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; font-size: 1.25em; line-height: 1.25; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; background-color: rgb(255, 255, 255);" class=""><a id="user-content-3-when-a-protocol-composition-type-contains-a-typealias-the-validity-of-the-type-is-determined-using-the-following-steps" class="anchor" href="https://github.com/hartbit/swift-evolution/blob/e6411d8a9e7924bbd8a48fc292bf08d58a8d1199/proposals/XXXX-subclass-existentials.md#3-when-a-protocol-composition-type-contains-a-typealias-the-validity-of-the-type-is-determined-using-the-following-steps" aria-hidden="true" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a></h3><div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);"><pre style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; margin-top: 0px; margin-bottom: 0px; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; background-color: rgb(247, 247, 247); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal;" class="">...
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">typealias</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">TA4</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> D <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P2
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">typealias</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">TA5</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> E <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P2
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">typealias</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">TA5</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> TA1 <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> TA2
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">typealias</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">TA5</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> class <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P1 <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> class <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P2 <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">//</span> Expansion</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">typealias</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">TA5</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> class <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P1 <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P2 <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">//</span> Normalization</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">//</span> TA5 is valid</span>
</pre></div></div></blockquote><div><br></div>The typealias 'T5' is repeated as both an initial composition, and as a demonstration of combining typealiases. <div><br></div><div><br><blockquote type="cite"><div class=""><div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);"><pre style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; margin-top: 0px; margin-bottom: 0px; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; background-color: rgb(247, 247, 247); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal;" class=""><span style="font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);">This proposal merges the concepts of </span><code class="" style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">class</code><span style="font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);"> and </span><code class="" style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">AnyObject</code><span style="font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);">, which now have the same meaning: they represent an existential for classes. They are four solutions to this dilemna:</span></pre></div><ol style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""><li style="box-sizing: border-box;" class="">Do nothing.</li><li style="box-sizing: border-box; margin-top: 0.25em;" class="">Replace all uses of <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">AnyObject</code> by <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">class</code>, breaking source compatibility.</li><li style="box-sizing: border-box; margin-top: 0.25em;" class="">Replace all uses of <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">class</code> by <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">AnyObject</code>, breaking source compatibility.</li><li style="box-sizing: border-box; margin-top: 0.25em;" class="">Redefine <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">AnyObject</code> as <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">typealias AnyObject = class</code>.</li></ol></div></blockquote><div><br></div>I agree with other comments on recommending 4 here, and covering the others as alternatives. <div><blockquote type="cite"><div class=""><h2 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; line-height: 1.25; padding-bottom: 0.3em; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(238, 238, 238); color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; background-color: rgb(255, 255, 255);" class=""><a id="user-content-source-compatibility" class="anchor" href="https://github.com/hartbit/swift-evolution/blob/e6411d8a9e7924bbd8a48fc292bf08d58a8d1199/proposals/XXXX-subclass-existentials.md#source-compatibility" aria-hidden="true" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a></h2></div></blockquote></div></div></div></body></html>