Sent from my iPhone
> On Dec 18, 2017, at 6:27 PM, Dave Abrahams <dabrah...@apple.com> wrote:
>
>
>
>> On Dec 18, 2017, at 4:52 PM, 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
>>
>> Thoughts?
>
> I'm confused; I thought that overlapping conformances could result in cases
> where it's ambiguous which extension to use to satisfy a requirement, but I
> don't see anything in your proposal for handling that. Does that become a
> dynamic failure?
My proposal doesn’t handle that. I guess I was unclear: the first code snippet,
which is well-formed in Swift 4, will get an “overlapping conformance” error in
Swift 4.1. It’s a source compatibility break, but one we feel is necessary.
The fixed code only has a single (conditional) conformance, so there’s nothing
to overlap.
- Doug
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