Why do nested function not work within closures?

gistfile1.txt

func functionWithClosure(closure: () -> ()) {
    closure()
}

func topFunction() {
    let button = UIButton()
    let image = UIImage()
    
    func nestedFunction() {
        button.setImage(image, forState: .Normal)
    }
    
    nestedFunction() // works
    
    functionWithClosure {
        nestedFunction() // doesn't work
    }
}

Though it's a little dense, the error you get here is descriptive: "Cannot reference a local function with capture from another local function". See this Stack Overflow answer.

The key is that nested functions and closures capture data from their surroundings a little bit differently. Closures are always work on the assumption that they can be called from another context; functions don't.

There might be actual limitations I'm not aware of, but you can think of it as artificial: you get into weird situations with memory when closures capture functions, since functions are always meant to exist within their parent scope.

(FWIW, it works if you pass nestedFunction directly to functionWithClosure, like functionWithClosure(nestedFunction).)