/tmp/test_package_ugk1IN/Sources/TestApp/main.swift:14:24: warning: result of call to 'doIt(thingsLeft:)' is unused
                return doIt(thingsLeft: thingsLeft)
                       ^   ~~~~~~~~~~~~~~~~~~~~~~~~

enum Time {
    case milliseconds(Int)
}
struct Thing {}
struct Scheduled<T> {
    let futureResult: EventLoopFuture<T>
}
struct EventLoop {
    func makeSucceededFuture<T>(_ v: T) -> EventLoopFuture<T> {
        fatalError()
    }
    func scheduleTask<T>(in t: Time, _ body: @escaping () -> T) -> Scheduled<T> {
        fatalError()
    }
}
struct EventLoopFuture<T> {
    func flatMap<V>(_ body: @escaping (T) -> EventLoopFuture<V>) -> EventLoopFuture<V> {
        fatalError()
    }
}


func bar(eventLoop: EventLoop, process: @escaping (Thing) -> EventLoopFuture<Void>) -> EventLoopFuture<Void> {
    func doIt(thingsLeft: ArraySlice<Thing>) -> EventLoopFuture<Void> {
        var thingsLeft = thingsLeft
        guard let first = thingsLeft.popFirst() else {
            return eventLoop.makeSucceededFuture(())
        }

        return process(first).flatMap { [thingsLeft] in
            return eventLoop.scheduleTask(in: .milliseconds(100)) {
                return doIt(thingsLeft: thingsLeft)
            }.futureResult
        }
    }
    return doIt(thingsLeft: [][...])
}