How to iterate over Dictionary.Values plus one additional element?

Using Swift
3

Perhaps you may move the loop body out into a function / closure:

func doSomething(_ y: Y) {
    ...
}

doSomething(extra)
dictionary.values.forEach(doSomething)

Below would probably also achieve what you want, but looks like overkill:

struct ArrayListIterator<T>: IteratorProtocol {
    private var iterators: [Array<T>.Iterator]
    
    init(_ arrays: [[T]]) {
        iterators = arrays.map { $0.makeIterator() }
    }
    
    mutating func next() -> T? {
        while !iterators.isEmpty {
            if let element = iterators[0].next() {
                return element
            }
            iterators.removeFirst()
        }
        return nil
    }
}

struct ArrayList<T>: Sequence {
    let arrays: [[T]]
    
    func makeIterator() -> ArrayListIterator<T> {
        ArrayListIterator(arrays)
    }
}

for v in ArrayList(arrays: [["a", "b", "c"], ["d"], ["e", "f"]]) {
    print(v, terminator: "")
}
print() // abcdef