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

[Douglas Gregor]

Sent from my iPhone

> On Dec 18, 2017, at 6:27 PM, Dave Abrahams <dabrahams at apple.com> wrote:
> 
> 
> 
>> On Dec 18, 2017, at 4:52 PM, 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
>> 
>> 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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