This is nice. Thanks for taking the time to write it up. I do have some concerns/questions:<br><br>Do the rules you spell out align with those for Codable? I think it is very important that these are paralleled as closely as possible, and that any deviations are explicitly called out in the text with reasoning as to why it must deviate. Knowing when something is synthesized is difficult enough with one set of rules--two is certainly one too many.<br><br>For example, is it permitted to extend a type in the same module in order to obtain synthesized Codable conformance? How about for a type in a different module? The same rules should then apply for Equatable and Hashable synthesis.<br><br>Furthermore, does Codable ignore computed properties? If not, then neither should Equatable and Hashable. There are also some complicated rules with generics, if I recall, that may force something to be a computed property. It would be worth exploring if such rules make ignoring computed properties counterintuitive. For instance, if a user has to redesign the type, changing a stored property to a computed property just to satisfy certain rules of the language, and all of a sudden the definition of equality has silently changed as a consequence, then that could end up being very hard to debug. If we find that this is a plausible issue, then it might be worth considering refusing to synthesize Equatable conformance for a type with any computed properties--obviously limiting, but better limiting than surprising. To be clear, I'm not suggesting that we do make this limitation, just that I don't know that the consequences have been adequately explored for not including computed properties.<br><br><br><div class="gmail_quote"><div dir="ltr">On Mon, May 15, 2017 at 17:21 Tony Allevato via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Yes—the PR of the proposal is here: <a href="https://github.com/apple/swift-evolution/pull/706" target="_blank">https://github.com/apple/swift-evolution/pull/706</a><div><br></div><div>It needs to be updated slightly—I'll remove the references to the "multiplicative hash function" recommendation because I ended up using the existing _mixInt and xor, which is how the standard library implements its Collection hashValues. (The proposal probably really doesn't need to state anything about the hash function used, and its entirely an implementation detail.)</div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr">On Mon, May 15, 2017 at 3:18 PM Andrew Bennett <<a href="mailto:cacoyi@gmail.com" target="_blank">cacoyi@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><div>Nice work Tony! Is this proposal up for PR on swift-evolution as well?</div></div><div><div><br><div class="gmail_quote"><div>On Tue, 16 May 2017 at 7:30 am, Tony Allevato <<a href="mailto:tony.allevato@gmail.com" target="_blank">tony.allevato@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>Just to update everyone on the thread—it took a little longer than I'd hoped to get the kinks out, but I finally have the implementation up as a PR: <a href="https://github.com/apple/swift/pull/9619" target="_blank">https://github.com/apple/swift/pull/9619</a><div><br></div><div>Hopefully there's still enough time to get the proposal reviewed, make any changes needed, and get this into Swift 4!</div><div><br></div></div><br><div class="gmail_quote"><div>On Tue, May 9, 2017 at 10:27 PM Brent Royal-Gordon <<a href="mailto:brent@architechies.com" target="_blank">brent@architechies.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><blockquote type="cite"><div>On May 9, 2017, at 3:53 PM, Tony Allevato via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br class="m_-7634931506526190142m_5760772590623056947m_-3114140352773685947m_5619269197906473925m_-7922117415000113549Apple-interchange-newline"><div><span 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;float:none;display:inline!important">Likewise, proposing a new public addition to the standard library would inspire far more design discussion than I believe we have time for if we want this to make Swift 4. :)</span></div></blockquote></div><div><br></div></div><div style="word-wrap:break-word"><div>Agreed. What I would do here is add an `_combineHashes` function (or `Hashable` extension method, or whatever is most convenient) to the standard library in Swift 4, have your compiler magic feature use it, and defer the name-and-interface discussion until Swift 5.</div></div><div style="word-wrap:break-word"><br><div>
<span class="m_-7634931506526190142m_5760772590623056947m_-3114140352773685947m_5619269197906473925m_-7922117415000113549Apple-style-span" style="border-collapse:separate;color:rgb(0,0,0);font-family:Helvetica;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:auto;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><div><div style="font-size:12px">-- </div><div style="font-size:12px">Brent Royal-Gordon</div><div style="font-size:12px">Architechies</div></div></span>
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