例えばCollectionだと、現在は非表示になってる_Indexableを使わずに済む様になり、associatedtype Elementやvar startIndex: Indexの定義がCollectionへ移り、それらがCollection準拠に必要なものだということが判りやすくなる。 https://github.com/apple/swift/pull/11923/files#diff-1e8313acf545a6c1b6fbb71978690ff6R348

[WIP] [SE-0157] Standard library uses of Recursive Protocol Constraints by DougGregor · Pull Request #11923 · apple/swift

This pull request implements the standard library part of SE-0157: Recursive Protocol Constraints, which includes: Make the SubSequence associated type have the same capabilities as its enclosing ...