Xcode 8 :function types cannot have argument label breaking my build

The Swift designers decided to prohibit argument labels for function types.

The reasoning is explained here: https://github.com/apple/swift-evolution/blob/master/proposals/0111-remove-arg-label-type-significance.md

This is a frustrating and questionable choice, as prohibiting argument labels makes it much easier to incorrectly invoke closures, which seems more important than simplifying the language's type system.

Usability > ideology.

A workaround to consider. You can't do:

func doStuff(completion: (foo: Int, bar: String) -> Void) {
    ...
    completion(foo: 0, bar: "")
}

... but you can do:

func doStuff(completion: ((foo: Int, bar: String)) -> Void) {
    ...
    completion((foo: 0, bar: ""))
}

i.e. have a single unnamed argument to your closure which is a tuple, in this case (foo: Int, bar: String).

It's ugly in its own way, but at least you retain the argument labels.