SE-0143はSwift 4.2をもってImplementedとなっていて、ランタイムチェックも4.2で実装されてます https://github.com/apple/swift-evolution/blob/master/proposals/0143-conditional-conformances.md

swift-evolution - This maintains proposals for changes and user-visible enhancements to the Swift Programming Language.

2:09 PM

実装されたのは https://github.com/apple/swift/pull/14368 これで2月ですね

[Runtime] SE-0143: Evaluate conditional conformances at runtime. b...

When evaluating whether a given type conforms to a protocol, evaluate the conditional requirements and pass the results to the witness table accessor function. This provides the ability to query co...

2:10 PM

https://github.com/apple/swift/commit/87f7b4e5fb58cc32fd33144e5e4207d53d2e5745 このマージコミットを見ればどのタグから入っているか分かりますね

Merge pull request #14368 from DougGregor/se-0143-runtime-conditio...

…-conformances

2:11 PM

^ @Nonchalant