Development Snapshotをちょっと使ってみたのだけど、
extension Sequence { fileprivate func grouped<U: Hashable>(by transform: (Iterator.Element) -> U) -> [U: [Iterator.Element]] { return reduce([:]) { dictionary, element in var dictionary = dictionary let key = transform(element) dictionary[key] = (dictionary[key] ?? []) + [element] return dictionary } } } で console.log error: redundant conformance constraint 'U': 'Hashable' fileprivate func grouped<U: Hashable>(by transform: (Iterator.Element) -> U) -> [U: [Iterator.Element]] { ^ note: conformance constraint 'U': 'Hashable' inferred from type here fileprivate func grouped<U: Hashable>(by transform: (Iterator.Element) -> U) -> [U: [Iterator.Element]] { ^ なんてエラーが出るようになったのね。