> 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?
-Dave
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