> On Dec 19, 2017, at 2:26 PM, Ted Kremenek via swift-dev <swift-dev@swift.org> > wrote: > > > On Dec 18, 2017, 4:53 PM -0800, Douglas Gregor via swift-dev > <swift-dev@swift.org>, wrote: >> Hi all, >> >> 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. >> >> 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: >> >> extension MutableSlice : P { } >> extension MutableBidirectionalSlice : P { } >> // … >> >> they’ll get an error about overlapping conformances, and need to do >> something like the following to fix the issue: >> >> extension Slice: P where Base: MutableCollection { } >> >> 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. >> >> My proposal is, specifically: >> >> Allow conditional conformances to be used in Swift 4.1 (no flag required) >> Drop the -enable-experimental-conditional-conformances flag entirely >> 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 >> > > 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?
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. - Doug
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