protocol Hoge { associatedtype Fuga func piyo(_ index: Int) -> Fuga } extension Array: Hoge where Element == String { typealias Fuga = String func piyo(_ index: Int) -> String { return self[index] + "でーす" } } extension Array: Hoge where Element == Int { typealias Fuga = Int func piyo(_ index: Int) -> Int { return self[index] + 10 } }