Recursive Protocol Constraintsが入ると、Standard Libraryのプロトコル階層がシンプルになる。 https://github.com/apple/swift/pull/11923/files (edited)

[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 ...