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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Hi Slava,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Thank you very much for the quick response. As you requested, I filed the bug under the following link:
<a href="https://bugs.swift.org/browse/SR-6561">https://bugs.swift.org/browse/SR-6561</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Best Regards<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Mario<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black"><spestov@apple.com> on behalf of Slava Pestov <spestov@apple.com><br>
<b>Date: </b>Thursday, 7 December 2017 at 22:27<br>
<b>To: </b>Mario Meili <mario.meili@live.com><br>
<b>Cc: </b>"swift-users@swift.org" <swift-users@swift.org><br>
<b>Subject: </b>Re: [swift-users] Multiple Class Types in Protocol Composition<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hi Mario, <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">You’re right that this part of the proposal was not fully implemented. Computing the common ancestor of two superclasses is a little tricky here because it would force us to resolve the superclass type of a class when realizing a subclass
existential. This would introduce circular dependencies. Either I missed something obvious or it would require some re-architecting of the type checker to make it work.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Either way, yes it’s a known limitation, but not one that is tracked by a JIRA presently, so please do file a bug!<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Slava<o:p></o:p></p>
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<p class="MsoNormal">On Dec 7, 2017, at 7:17 AM, Mario Meili via swift-users <<a href="mailto:swift-users@swift.org">swift-users@swift.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="background:white">Hi everyone,<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white">I noticed strange behaviour of the Swift 4.0.3 compiler regarding class and subtype existentials.<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white">According to the proposal SE-0156 (<a href="https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fapple%2Fswift-evolution%2Fblob%2Fmaster%2Fproposals%2F0156-subclass-existentials.md&data=02%7C01%7C%7Cf12145f448334d2a644c08d53db941d1%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636482788230444913&sdata=bjKRAVds%2BubJz5Mpi27bQmpbBigAfekf%2FiXopHekabE%3D&reserved=0"><span style="color:#954F72">https://github.com/apple/swift-evolution/blob/master/proposals/0156-subclass-existentials.md</span></a>),
multiple class types in protocol compositions should be allowed, if:<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<li class="MsoNormal" style="margin-left:0cm;mso-list:l0 level1 lfo1;background:white">
The class types are the same<span style="font-size:12.0pt"><o:p></o:p></span></li><li class="MsoNormal" style="margin-left:0cm;mso-list:l0 level1 lfo1;background:white">
Or, one class type must be a subtype of the other.<span style="font-size:12.0pt"><o:p></o:p></span></li></ul>
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<p class="MsoNormal" style="background:white"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white">However, when copying the exact code from the proposal into Xcode 9.2, the result is the following:<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"><image001.png><span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white">My conclusion here is that the compiler does not allow multiple class types in one protocol composition.<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white">My question is:<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white">Is this behaviour intended or should I open a bug report?<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white">Thank you very much<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white">BR<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:white">Mario<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;background:white">_______________________________________________</span><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif"><br>
<span style="background:white">swift-users mailing list</span><br>
</span><a href="mailto:swift-users@swift.org"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;color:#954F72;background:white">swift-users@swift.org</span></a><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif"><br>
</span><a href="https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.swift.org%2Fmailman%2Flistinfo%2Fswift-users&data=02%7C01%7C%7Cf12145f448334d2a644c08d53db941d1%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636482788230444913&sdata=pY7NkpjW9jmvnhjXYa3axI%2F631OaDOdJ0F4KsIb%2FXHI%3D&reserved=0"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif;color:#954F72;background:white">https://lists.swift.org/mailman/listinfo/swift-users</span></a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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