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

[Douglas Gregor]

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?

	- Doug

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