The following code works as expected:

class FooCollection {
    private var items = [Foo]()

    func removeAllItems() -> [Foo] {

        defer { items.removeAll() }
        return items
    }
}

But do you know what “expected” means in this case?

As a reader, you assume the author had an intention. You look for the mens auctoris and are an overall benevolent reader, I hope. Presupposing said intention, you may assume that it does something special if you put the call to items.removeAll() in a defer block.

The removeAllItems method returns an array of items. If the internal collection was empty when the return statement is reached, that’d be pointless, wouldn’t it? Since your benevolent, you assume that the author isn’t stupid and that it does indeed return a non-empty collection in some cases.

Say the author had added a documentation line:

/// - returns: Array of items that were removed.
func removeAllItems() -> [Foo] { // ... }

Now that should tip the scale! So the internal collection of items is returned and afterwards emptied. Aha! How clever!

As a critical reader, you should be able to solve the puzzle and call the author names. Because, why make it so non-obvious to the reader? Why the guessing that either requires manual (or unit-) testing or the Swift doc (and trust in its truth) to verify the assumptions?

I was curious about the outcome of this approach so I just tried it before writing this up. Then I deleted the """clever""" code and replaced it with what I had before:

class FooCollection {
    private var items = [Foo]()

    func removeAllItems() -> [Foo] {
        let removedItems = items
        items.removeAll()
        return removedItems
    }
}

I prefer this any day. I hope you do, too.