As far as I'm aware, eliminating retroactive conformances is a non-starter.<br><br><br><div class="gmail_quote"><div dir="ltr">On Mon, Apr 10, 2017 at 21:44 Howard Lovatt <<a href="mailto:howard.lovatt@gmail.com">howard.lovatt@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto" class="gmail_msg"><div class="gmail_msg">@Xiaodi,</div><div id="m_3934634403776160645AppleMailSignature" class="gmail_msg"><br class="gmail_msg"></div><div id="m_3934634403776160645AppleMailSignature" class="gmail_msg">You make two drugs. </div><div id="m_3934634403776160645AppleMailSignature" class="gmail_msg"><br class="gmail_msg"></div><div id="m_3934634403776160645AppleMailSignature" class="gmail_msg">1. Deliberately making retroactive conformance outside of the file in which the type is declared illegal because of the problems it causes. See all the questions on Swift Users and watch people learning Swift get caught out. </div><div id="m_3934634403776160645AppleMailSignature" class="gmail_msg"><br class="gmail_msg"></div><div id="m_3934634403776160645AppleMailSignature" class="gmail_msg">2. Outside of the file in which the type is declared the static final extension is restricted to internal or fileprivate so that multiple modules can add static final extensions without clashes. </div></div><div dir="auto" class="gmail_msg"><div id="m_3934634403776160645AppleMailSignature" class="gmail_msg"><br class="gmail_msg"><br class="gmail_msg">-- Howard. </div></div><div dir="auto" class="gmail_msg"><div class="gmail_msg"><br class="gmail_msg">On 11 Apr 2017, at 8:51 am, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="gmail_msg" target="_blank">xiaodi.wu@gmail.com</a>> wrote:<br class="gmail_msg"><br class="gmail_msg"></div><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div dir="ltr" class="gmail_msg">On Mon, Apr 10, 2017 at 5:35 PM, Howard Lovatt via swift-evolution <span dir="ltr" class="gmail_msg"><<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br class="gmail_msg"><div class="gmail_extra gmail_msg"><div class="gmail_quote gmail_msg"><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto" class="gmail_msg"><div class="gmail_msg">In response to Jordan Rose's comment I suggest the following change:</div><div id="m_3934634403776160645m_-5335225969984223547AppleMailSignature" class="gmail_msg"><br class="gmail_msg"></div><div id="m_3934634403776160645m_-5335225969984223547AppleMailSignature" class="gmail_msg"><p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">Proposal: Split extension usage up into implementing methods and adding static functions</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">Currently extension methods are confusing because they have different dispatch rules for the same syntax. EG:</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> protocol P {</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> func m()</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> extension P {</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> func m() { print("P.m") }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> struct S: P {</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> func m() { print("S.m") }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> val p: P = S() // Note typed as P</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> p.m() // Surprisingly prints P.m even though S implements its own m</span></p></div></div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">This is incorrect. This prints "S.m", not "P.m".</div><div class="gmail_msg"> </div><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto" class="gmail_msg"><div id="m_3934634403776160645m_-5335225969984223547AppleMailSignature" class="gmail_msg">
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> val s = S() // Note typed as S</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> s.m() // Prints S.m as expected </span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">This proposal cures the above problem by separating extension methods into two seperate use cases: implementations for methods and adding static functions. </span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">First implementing methods.</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">If the extension is in the same file as the protocol/struct/class declaration then it implements the methods and is dispatched using a Vtable. EG:</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">File P.swift</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> protocol/struct/class P {</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> func m()</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> extension P {</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> func m() { print("P.m") }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">Same or other file</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> struct S: P {</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> override func m() { print("S.m") } // Note override required because m already has an implementation from the extension</span></p></div></div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">Requiring `override` breaks retroactive conformance of types to protocols. This idea has been brought up over half a dozen times. Each time it fails in not being able to accommodate retroactive conformance.</div><div class="gmail_msg"> </div><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto" class="gmail_msg"><div id="m_3934634403776160645m_-5335225969984223547AppleMailSignature" class="gmail_msg">
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> val p: P = S() // Note typed as P</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> p.m() // Now prints S.m as expected </span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">Extensions in the same file as the declaration can have any access, can be final, and can have where clauses and provide inheritable implementations. </span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">The implementation needed to achieve this is that a value instance typed as a protocol is copied onto the heap, a pointer to its Vtable added, and it is passed as a pointer. IE it becomes a class instance. No change needed for a class instance typed as a protocol. </span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">The second use case is adding static functions.</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">A new type of extension is proposed, a static final extension, which can be either in or outside the file in which the protocol/struct/class declaration is in. EG:</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> static final extension P { // Note extension marked static final</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> func m() { print("P.m") }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">Which is called as any other static function would be called:</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> val s = S()</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> P.m(s) // Prints P.m as expected</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">The new static final extension is shorthand, particularly in the case of multiple functions, for:</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> extension P {</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> static final func m(_ this: P) { print("P.m") }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"> }</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">If the static final extension is outside of the file in which the protocol/struct/class declaration is in then the extension and the methods can only have fileprivate and internal access.</span></p></div></div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">What is the use case for having this restriction? What is the problem you are trying to solve? </div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg"> </div><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto" class="gmail_msg"><div id="m_3934634403776160645m_-5335225969984223547AppleMailSignature" class="gmail_msg"><p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">As at present protocol/struct/class can have both a static and instance method of the same name, m in the case of the example, because the usage syntax is distinct. As at present, static final extensions, both the extension and the individual functions, can have where clauses.</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">In summary.</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">The proposal formalises the split use of extensions into their two uses: implementing methods and adding static functions. Syntax is added that clarifies both for declarations and usage which type of extension is provided/in use.</span></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69);min-height:20.3px" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg"></span><br class="gmail_msg"></p>
<p style="margin:0px;line-height:normal;font-family:'.SF UI Text';color:rgb(69,69,69)" class="gmail_msg"><span style="font-family:'.SFUIText';font-size:17pt" class="gmail_msg">Note the distinction between an extension in the same file and in a separate file is consistent with the proposed use of private in <a href="https://github.com/apple/swift-evolution/blob/master/proposals/0169-improve-interaction-between-private-declarations-and-extensions.md" class="gmail_msg" target="_blank">https://github.com/apple/swift-evolution/blob/master/proposals/0169-improve-interaction-between-private-declarations-and-extensions.md</a>.</span></p><div id="m_3934634403776160645m_-5335225969984223547AppleMailSignature" class="gmail_msg"><br class="gmail_msg"></div>Comments?</div><div id="m_3934634403776160645m_-5335225969984223547AppleMailSignature" class="gmail_msg"><br class="gmail_msg">-- Howard.</div><div class="gmail_msg"><br class="gmail_msg">On 7 Apr 2017, at 4:49 am, Jordan Rose <<a href="mailto:jordan_rose@apple.com" class="gmail_msg" target="_blank">jordan_rose@apple.com</a>> wrote:<br class="gmail_msg"><br class="gmail_msg"></div><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div class="gmail_msg">[Proposal: <a href="https://github.com/apple/swift-evolution/blob/master/proposals/0164-remove-final-support-in-protocol-extensions.md" class="gmail_msg" target="_blank">https://github.com/apple/swift-evolution/blob/master/proposals/0164-remove-final-support-in-protocol-extensions.md</a>]</div><br class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg">On Apr 5, 2017, at 16:15, Howard Lovatt via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br class="m_3934634403776160645m_-5335225969984223547Apple-interchange-newline gmail_msg"><div class="gmail_msg"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><span style="background-color:rgba(255,255,255,0)" class="gmail_msg">The review of SE-0164 "Remove final support in protocol extensions"</span></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><ul style="box-sizing:border-box;padding-left:2em;margin-top:0px;margin-bottom:16px" class="gmail_msg"><li style="box-sizing:border-box" class="gmail_msg"><font class="gmail_msg"><span style="background-color:rgba(255,255,255,0)" class="gmail_msg">What is your evaluation of the proposal?</span></font></li></ul></blockquote><div class="gmail_msg">The present situation isn't great. People get confused about which method will called with protocol extensions. Seems like every week there is a variation on this confusion on Swift Users mailing list. Therefore something needs to be done. </div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">However I am not keen on this proposal since it makes behaviour inconsistent between methods in protocol extensions, classes, and structs. </div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">I think a better solution would be one of the following alternatives:</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg"> 1. Must use final and final means it cannot be overridden; or</div><div class="gmail_msg"> 2. If not final dispatches using a table like a class and if marked final cannot be overridden and if marked dynamic uses obj-c dispatching; or</div><div class="gmail_msg"> 3. Must be marked dynamic and uses obj-c dispatching. </div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">My preference would be option 2 but I think any of the three is superior to the present situation or the proposal. </div></div></div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">People have suggested all of these before, but none of them are obviously correct. It's true that we have a difference between extension members that satisfy requirements and those that don't, and that that confuses people. However, an extension-only member of one protocol can be used to satisfy the requirements of another protocol today, which is a tool for code reuse.</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">(I <i class="gmail_msg">think</i> we managed to convince everyone that it's just a bug that a protocol extension method that satisfies a requirement cannot be overridden in a subclass, so at least that isn't an issue on top of the rest of this.)</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">Oh, and we can't retroactively add members of a protocol extension to existing adopters, which is why protocol extension members cannot be @objc. There are limited circumstances where that would be safe, but that would be a separate proposal.</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">Jordan</div></div></div></blockquote></div><br class="gmail_msg">_______________________________________________<br class="gmail_msg">
swift-evolution mailing list<br class="gmail_msg">
<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a><br class="gmail_msg">
<a href="https://lists.swift.org/mailman/listinfo/swift-evolution" rel="noreferrer" class="gmail_msg" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br class="gmail_msg">
<br class="gmail_msg"></blockquote></div><br class="gmail_msg"></div></div>
</div></blockquote></div></blockquote></div>