[swift-dev] Conditional conformance: Removing the opt-in flag

[Ted Kremenek]

> On Dec 19, 2017, at 8:57 PM, Douglas Gregor <dgregor at apple.com> wrote:
> 
> 
> 
> Sent from my iPhone
> 
>> On Dec 19, 2017, at 8:31 PM, Ted Kremenek <kremenek at apple.com> wrote:
>> 
>> 
>> 
>>> On Dec 19, 2017, at 3:59 PM, Douglas Gregor <dgregor at apple.com> wrote:
>>> 
>>> 
>>> 
>>>> On Dec 19, 2017, at 2:26 PM, Ted Kremenek via swift-dev <swift-dev at swift.org> wrote:
>>>> 
>>>> 
>>>>> On Dec 18, 2017, 4:53 PM -0800, Douglas Gregor via swift-dev <swift-dev at 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
>> 
>> 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.
>> 
>> 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. 
> 
> I’d rather not make it a runtime error, because code that’s doing dynamic casting to a protocol is generally already handling the “nil” case (“as?” syntax), so aborting the program feels far too strong. 
> 
>   - Doug

For me I think the part I’m struggling with is that making it a warning conflates two things together: expected failure in the dynamic cast because the value you are casting doesn’t have that type or — in this case — failure because the cast can never succeed because it is not supported yet.  I feel like we would be silently swallowing an unsupported condition.  If that didn’t matter, why bother issuing a warning?  Clearly were trying to send some kind of message here about this not being supported.
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