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.send { color:#77bb77; }
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.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="bloop_markdown"><p>Why can’t we completely ban this?</p>
<pre><code class="swift">protocol A {}
protocol B {}
typealias AB = A & B
protocol C : AB {} // Allowed, but could be also banned
protocol D : A & B {} // Error
</code></pre>
<p></p></div><div class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div> <br> <div id="bloop_sign_1487064559862729984" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 14. Februar 2017 um 10:25:43, Slava Pestov via swift-evolution (<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div></div><div>
<title></title>
<br class="">
<div>
<blockquote type="cite" class="">
<div class="">On Feb 12, 2017, at 12:32 PM, David Hart 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 style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi Matthew,
<div class=""><br class=""></div>
<div class="">Your arguments made sense to me. I modified the
proposal to choose strategy number 3: deprecating and removing
<b class="">class</b> over several versions to favour <b class="">AnyObject</b>. Mind having another proof read?</div>
<div class=""><br class=""></div>
<div class=""><a href="https://github.com/hartbit/swift-evolution/blob/subclass-existentials/proposals/XXXX-subclass-existentials.md" class="">https://github.com/hartbit/swift-evolution/blob/subclass-existentials/proposals/XXXX-subclass-existentials.md</a></div>
<div class=""><br class=""></div>
<div class="">Anybody has counter arguments?</div>
<div class=""><br class=""></div>
<div class="">
<h1 style="box-sizing: border-box; margin-right: 0px; margin-bottom: 16px; margin-left: 0px; 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); margin-top: 0px !important;" class="">Class and Subtype existentials</h1>
<ul 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="">
Proposal: <a href="https://github.com/hartbit/swift-evolution/blob/subclass-existentials/proposals/XXXX-subclass-existentials.md" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none;" class="">SE-XXXX</a></li>
<li style="box-sizing: border-box; margin-top: 0.25em;" class="">
Authors: <a href="http://github.com/hartbit/" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none;" class="">David Hart</a>, <a href="http://github.com/austinzheng" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none;" class="">Austin Zheng</a></li>
<li style="box-sizing: border-box; margin-top: 0.25em;" class="">
Review Manager: TBD</li>
<li style="box-sizing: border-box; margin-top: 0.25em;" class="">
Status: TBD</li>
</ul>
<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-introduction" class="anchor" href="https://github.com/hartbit/swift-evolution/tree/subclass-existentials#introduction" 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>Introduction</h2>
<p style="box-sizing: border-box; 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="">This proposal brings more expressive power to the type
system by allowing Swift to represent existentials of classes and
subtypes which conform to protocols.</p>
<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-motivation" class="anchor" href="https://github.com/hartbit/swift-evolution/tree/subclass-existentials#motivation" 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>Motivation</h2>
<p style="box-sizing: border-box; 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="">Currently, the only existentials which can be represented
in Swift are conformances to a set of protocols, using
the <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="">&</code>protocol composition syntax:</p>
<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="">Protocol1 <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> Protocol2</pre></div>
<p style="box-sizing: border-box; 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="">On the other hand, Objective-C is capable of expressing
existentials of classes and subclasses conforming to protocols with
the following syntax:</p>
<div class="highlight highlight-source-objc" 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);">id</span><Protocol1, Protocol2>
Base<Protocol>*</pre></div>
<p style="box-sizing: border-box; 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="">We propose to provide similar expressive power to Swift,
which will also improve the bridging of those types from
Objective-C.</p>
<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-proposed-solution" class="anchor" href="https://github.com/hartbit/swift-evolution/tree/subclass-existentials#proposed-solution" 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>Proposed solution</h2>
<p style="box-sizing: border-box; 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="">The proposal keeps the existing <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="">&</code> syntax but allows the first element, and
only the first, to be either the <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>keyword or of class type. The equivalent
to the above Objective-C types would look like this:</p>
<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-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">AnyObject</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> Protocol1 <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> Protocol2
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Base</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> Protocol</pre></div>
<p style="box-sizing: border-box; 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="">As in Objective-C, the first line is an existential of
classes which conform to <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="">Protocol1</code> and <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="">Protocol2</code>, and the second line is an existential of
subtypes 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="">Base</code> which conform to <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="">Protocol</code>.</p>
<p style="box-sizing: border-box; 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="">Here are the new proposed rules for what is valid in a
existential conjunction syntax:</p>
<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-1-the-first-element-in-the-protocol-composition-syntax-can-be-the-anyobject-keyword-to-enforce-a-class-constraint" class="anchor" href="https://github.com/hartbit/swift-evolution/tree/subclass-existentials#1-the-first-element-in-the-protocol-composition-syntax-can-be-the-anyobject-keyword-to-enforce-a-class-constraint" 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>1. The first element in the protocol composition syntax
can be the <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: inherit; 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> keyword to enforce a class
constraint:</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);">protocol</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">P</span> {}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">struct</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">S</span> : <span class="pl-e" style="box-sizing: border-box; color: rgb(121, 93, 163);">P </span>{}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">C</span> : <span class="pl-e" style="box-sizing: border-box; color: rgb(121, 93, 163);">P </span>{}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> t<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> P <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">AnyObject</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> Compiler error: AnyObject requirement must be in first position</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> u<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">AnyObject</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">S</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> Compiler error: S is not of class type</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> v<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">AnyObject</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">C</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> Compiles successfully</span></pre></div>
<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-2-the-first-element-in-the-protocol-composition-syntax-can-be-a-class-type-to-enforce-the-existential-to-be-a-subtype-of-the-class" class="anchor" href="https://github.com/hartbit/swift-evolution/tree/subclass-existentials#2-the-first-element-in-the-protocol-composition-syntax-can-be-a-class-type-to-enforce-the-existential-to-be-a-subtype-of-the-class" 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>2. The first element in the protocol composition syntax
can be a class type to enforce the existential to be a subtype of
the class:</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);">protocol</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">P</span> {}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">struct</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">S</span> {}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">C</span> {}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">D</span> : <span class="pl-e" style="box-sizing: border-box; color: rgb(121, 93, 163);">P </span>{}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">E</span> : <span class="pl-e" style="box-sizing: border-box; color: rgb(121, 93, 163);">C</span>, <span class="pl-e" style="box-sizing: border-box; color: rgb(121, 93, 163);">P </span>{}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> t<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> P <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> C <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">//</span> Compiler error: subclass constraint must be in first position</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> u<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> S <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">//</span> Compiler error: S is not of class type</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> v<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> C <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">D</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> Compiler error: D is not a subtype of C</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> w<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> C <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> P <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">E</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> Compiles successfully</span></pre></div>
<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/tree/subclass-existentials#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>3. When a protocol composition type contains a typealias,
the validity of the type is determined using the following
steps:</h3>
<ul 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="">Expand the
typealias</li>
<li style="box-sizing: border-box; margin-top: 0.25em;" class="">
Normalize the type by removing duplicate constraints and replacing
less specific constraints by more specific constraints
(a <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> constraint is less specific than a class
type constraint, which is less specific than a constraint of a
subclass of that class).</li>
<li style="box-sizing: border-box; margin-top: 0.25em;" class="">
Check that the type does not contain two class-type
constraints</li>
</ul>
</div>
</div>
</div>
</blockquote>
<div><br class=""></div>
You could generalize this and instead say that if the type contains
two class-type constraints, the resulting existential type is the
common base class of the two classes, or AnyObject if they do not
share a common base class.</div>
<div><br class=""></div>
<div>Also, I’d like to see some discussion about class-constrained
existentials appearing in the inheritance clause of a protocol.
IMHO, we should ban this:</div>
<div><br class=""></div>
<div>typealias MyType = SomeClass & SomeProtocol</div>
<div><br class=""></div>
<div>protocol SomeOtherProtocol : MyType {}</div>
<div><br class=""></div>
<div>Slava</div>
<div><br class="">
<blockquote type="cite" class="">
<div class="">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">
<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 class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">C</span> {}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">D</span> : <span class="pl-e" style="box-sizing: border-box; color: rgb(121, 93, 163);">C </span>{}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">E</span> {}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">protocol</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">P1</span> {}
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">protocol</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">P2</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);">TA1</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">AnyObject</span> <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);">typealias</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">TA2</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">AnyObject</span> <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);">TA3</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> C <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);">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-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">//</span> Expansion: typealias TA5 = AnyObject & P1 & AnyObject & P2</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> Normalization: typealias TA5 = AnyObject & P1 & P2 </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>
<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);">TA6</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> TA3
<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: typealias TA6 = AnyObject & P1 & C & P2 </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> Normalization (AnyObject < C): typealias TA6 = C & P1 & P2 </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> TA6 is valid</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);">TA7</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> TA3 <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> TA4
<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: typealias TA7 = C & P2 & D & P2</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> Normalization (C < D): typealias TA7 = D & P2</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> TA7 is valid</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);">TA8</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> TA4 <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> TA5
<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: typealias TA8 = D & P2 & E & P2</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> Normalization: typealias TA8 = D & E & P2</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> TA8 is invalid because the D and E constraints are incompatible</span></pre></div>
<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-class-and-anyobject" class="anchor" href="https://github.com/hartbit/swift-evolution/tree/subclass-existentials#class-and-anyobject" 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><code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: inherit; 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> and <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: inherit; 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></h2>
<p style="box-sizing: border-box; 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="">This proposal merges the concepts 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> and <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>, which now have the same meaning: they
represent an existential for classes. To get rid of the
duplication, we suggest only keeping <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> around. To reduce source-breakage to
a minimum, <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> could be redefined 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 class = AnyObject</code> and give a
deprecation warning on <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> for the first version of Swift this
proposal is implemented in. Later, <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> could be removed in a subsequent version
of Swift.</p>
<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/tree/subclass-existentials#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>Source compatibility</h2>
<p style="box-sizing: border-box; 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="">This change will not break Swift 3 compability mode
because Objective-C types will continue to be imported as before.
But in Swift 4 mode, all types bridged from Objective-C which use
the equivalent Objective-C existential syntax could break code
which does not meet the new protocol requirements. For example, the
following Objective-C code:</p>
<div class="highlight highlight-source-objc" 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);">@interface</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">MyViewController</span>
- (<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">void</span>)<span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">setup</span><span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">:</span>(nonnull UIViewController<UITableViewDataSource,UITableViewDelegate>*)<span class="pl-smi" style="box-sizing: border-box;">tableViewController</span>;
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">@end</span></pre></div>
<p style="box-sizing: border-box; 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="">is imported into Swift-3 mode as:</p>
<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);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">MyViewController</span> {
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">setup</span>(<span class="pl-smi" style="box-sizing: border-box;"><span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">tableViewController</span></span>: UIViewController) {}
}</pre></div>
<p style="box-sizing: border-box; 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="">which allows calling the function with an invalid
parameter:</p>
<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);">let</span> myViewController<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">:</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">MyViewController</span>()
myViewController.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">setup</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">UIViewController</span>())</pre></div>
<p style="box-sizing: border-box; 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="">The previous code continues to compile but still crashs if
the Objective-C code calls a method 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="">UITableViewDataSource</code> or <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="">UITableViewDelegate</code>. But if this proposal is
accepted and implemented as-is, the Objective-C code will be
imported in Swift 4 mode as:</p>
<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);">class</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">MyViewController</span> {
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">setup</span>(<span class="pl-smi" style="box-sizing: border-box;"><span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">tableViewController</span></span>: UIViewController <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> UITableViewDataSource <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">&</span> UITableViewDelegate) {}
}</pre></div>
<p style="box-sizing: border-box; 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="">That would then cause the Swift code run in version 4 mode
to fail to compile with an error which states
that <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="">UIViewController</code> does not conform to
the <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="">UITableViewDataSource</code> and <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="">UITableViewDelegate</code> protocols.</p>
<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-alternatives-considered" class="anchor" href="https://github.com/hartbit/swift-evolution/tree/subclass-existentials#alternatives-considered" 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>Alternatives considered</h2>
<p style="box-sizing: border-box; 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="">An alternative solution to the <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>/<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> duplication was to keep both,
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> and favor the
latter when used as a type name.</p>
<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-acknowledgements" class="anchor" href="https://github.com/hartbit/swift-evolution/tree/subclass-existentials#acknowledgements" 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>Acknowledgements</h2>
<div style="box-sizing: border-box; margin-top: 0px; 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); margin-bottom: 0px !important;" class="">Thanks to <a href="http://github.com/austinzheng" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none;" class="">Austin Zheng</a> and <a href="https://github.com/anandabits" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none;" class="">Matthew Johnson</a> who brought a lot of attention to
existentials in this mailing-list and from whom most of the ideas
in the proposal come from.</div>
</div>
<div class=""><br class="">
<div class="">
<blockquote type="cite" class="">
<div class="">On 9 Feb 2017, at 21:50, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
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<div class="">On Feb 9, 2017, at 2:44 PM, David Hart <<a href="mailto:david@hartbit.com" class="">david@hartbit.com</a>>
wrote:</div>
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On 9 Feb 2017, at 20:43, Matthew Johnson via swift-evolution
<<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class="">
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Sent from my iPad</div>
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On Feb 9, 2017, at 1:30 PM, Hooman Mehr via swift-evolution
<<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class="">
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<div class="">On Feb 9, 2017, at 10:47 AM, Joe Groff via
swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div>
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<blockquote type="cite" class="">
<div class="">On Feb 9, 2017, at 4:26 AM, Step Christopher via
swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div>
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<div class="">Looks good. Minor comments below:</div>
The typealias 'T5' is repeated as both an initial composition, and
as a demonstration of combining typealiases.
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<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 class="" 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;"><span class="" 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 class="" 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 class="" 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 class="" 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);">
<li class="" style="box-sizing: border-box;">Do nothing.</li>
<li class="" style="box-sizing: border-box; margin-top: 0.25em;">
Replace all uses of <code class="" 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;">AnyObject</code> by <code class="" 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</code>,
breaking source compatibility.</li>
<li class="" style="box-sizing: border-box; margin-top: 0.25em;">
Replace all uses of <code class="" 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</code> by <code class="" 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;">AnyObject</code>,
breaking source compatibility.</li>
<li class="" style="box-sizing: border-box; margin-top: 0.25em;">
Redefine <code class="" 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;">AnyObject</code> as <code class="" 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;">typealias
AnyObject = class</code>.</li>
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I agree with other comments on recommending 4 here, and covering
the others as alternatives
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<h2 class="" 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);">
<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" style="">
<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" style=""></path>
</svg></a></h2>
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<span 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; float: none; display: inline !important;">I
agree that we need the typealias for compatibility. I think it's
still worth discussing whether the `AnyObject` typealias should
*only* be there for compatibility; it could be deprecated or
obsoleted in Swift 4 or future language versions.</span></div>
</blockquote>
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<div class="">I think it might be worth keeping to provide a more
sensible capitalization alternative than lower case “class” when
used as a type name:</div>
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<div class="">var obj: class // this looks weird because of
capitalization.</div>
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<div class="">var obj: AnyObject // this looks better.</div>
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<div class="">I agree that it looks better and would choose
AnyObject if source compatibility weren't an issue. One
option that wasn't listed was to drop 'class' but use a
multi-release deprecation strategy and a fix-it to facilitate a
smooth transition. If the community is willing to adopt this
approach it would be my first choice.</div>
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<div 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;" class=""><br class=""></div>
<div 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;" class="">You mean option 3?</div>
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<div class="">Pretty much, but option 3 does not make it clear that
it won’t break source immediately in Swift 4. I think it
becomes much more reasonable if Swift 3.1 code still compiles in
Swift 4 mode, but with a deprecation warning.</div>
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<div class="">The reason I prefer `AnyObject` to `class` is because
I think it’s ugly to have `class` as the name of an existential
type. Type names are uppercase in Swift. It is also
used to compose with protocols which also use uppercase names in
Swift. Because it appears in contexts which use an uppercase
convention it makes sense for this to have an uppercase name.
`AnyObject` seems like the obvious choice if we’re going to
go in that direction.</div>
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-Joe</div>
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