(Int) -> Int // #1: Normal function (Int) throws -> Int // #2: Throwing function (Int) async -> Int // #3: Asynchronous function (Int) async throws -> Int // #4: Asynchronous function, can also throw. (Int) -> Int // #1: Normal function (Int) throws -> Int // #2: Throwing function (Int) async -> Int // #3: Asynchronous function (Int) async throws -> Int // #4: Asynchronous function, can also throw.reasync がなさそうだから swift-evolution に書いておくか。await! もほしいな。! を使うのが良いのか自信がない。