<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Dec 19, 2017, at 2:26 PM, Ted Kremenek via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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On Dec 18, 2017, 4:53 PM -0800, Douglas Gregor via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>>, wrote:<br class="">
<blockquote type="cite" style="margin: 5px 5px; padding-left: 10px; border-left: thin solid #1abc9c;" class="">Hi all,
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<div class="">A little while back, I added an error to the Swift 4.1 compiler that complains if one tries to use conditional conformances, along with a flag “-enable-experimental-conditional-conformances” to enable the feature. We did this because we haven’t implemented the complete proposal yet; specifically, we don’t yet handle dynamic casting that involves conditional conformances, and won’t in Swift 4.1.</div>
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<div class="">I’d like to take away the "-enable-experimental-conditional-conformances” flag and always allow conditional conformances in Swift 4.1, because the changes in the standard library that make use of conditional conformances can force users to change their code *to themselves use conditional conformances*. Specifically, if they had code like this:</div>
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<div class=""><font face="Menlo" class="">extension MutableSlice : P { }</font></div>
<div class=""><font face="Menlo" class="">extension MutableBidirectionalSlice : P { }</font></div>
<div class=""><font face="Menlo" class="">// …</font></div>
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<div class="">they’ll get an error about overlapping conformances, and need to do something like the following to fix the issue:</div>
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<div class=""><font face="Menlo" class="">extension Slice: P where Base: MutableCollection { }</font></div>
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<div class="">which is way more elegant, but would require passing "-enable-experimental-conditional-conformances”. That seems… unfortunate… given that we’re forcing them to use this feature.</div>
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<div class="">My proposal is, specifically:</div>
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<li class="">Allow conditional conformances to be used in Swift 4.1 (no flag required)</li>
<li class="">Drop the -enable-experimental-conditional-conformances flag entirely</li>
<li class="">Add a runtime warning when an attempt to dynamic cast fails due to a conditional conformance, so at least users know what’s going on</li>
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<div class="">The last bullet doesn’t feel right to me. It sounds like we would ship a feature that we know only partially works, but issue a runtime warning in the case we know isn’t fully implemented? I’m I interpretting that point correctly?</div>
</div></div></div></blockquote><br class=""></div><div>Yes, that’s correct. We will fail to match the conformance (i.e., return “nil” from an “as?” cast), which might be correct and might be wrong.</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Doug</div><div><br class=""></div><br class=""></body></html>