<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div></div><div><br></div><div><br>On Dec 19, 2017, at 3:59 PM, Douglas Gregor <<a href="mailto:dgregor@apple.com">dgregor@apple.com</a>> wrote:<br><br></div><blockquote type="cite"><div><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><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></blockquote><br><div>Hmm. I’m concerned that a warning runtime would be to settle. Many people would possibly not even notice it. It’s essentially an edge case in a feature that isn’t fully implemented and thus that part of the feature should not be used yet.</div><div><br></div><div>What do you think about making this a hard runtime error instead, similar to how we are approaching runtime issues for exclusivity checking? That would be impossible to miss and would convey the optics that this runtime aspect of the feature is not yet supported and thus should not be used. </div></body></html>