Optional binding bug on Swift 2.2?

Question

if let mathematicalSymbol = sender.currentTitle {
    brain.performOperation(mathematicalSymbol)
}

The code above introduces the error below;

Value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?

As can be seen in this screen shot;

sender.currentTitle is an optional.

Here is an excerpt from Apple's "The Swift Programming Language (Swift 2.2)" with its example code just below it;

If the optional value is nil, the conditional is false and the code in braces is skipped. Otherwise, the optional value is unwrapped and assigned to the constant after let, which makes the unwrapped value available inside the block of code.

Here is the sample code for that excerpt;

var optionalName: String? = "John Appleseed"
var greeting = "Hello!"
if let name = optionalName {
    greeting = "Hello, \(name)"
}

So for these reasons, I'm thinking that either I'm missing something or that I'm hitting a bug.

I've also tried something similar on a Playground, and didn't get a similar error;

Here is my Swift version;

Apple Swift version 2.2 (swiftlang-703.0.18.8 clang-703.0.31)
Target: x86_64-apple-macosx10.9

Answer 1

If you look at currentTitle, you'll see it is likely inferred to be String??. For example, go to currentTitle in Xcode and hit the esc key to see code completion options, and you'll see what type it thinks it is:

I suspect you have this in a method defining sender as AnyObject, such as:

@IBAction func didTapButton(sender: AnyObject) {
    if let mathematicalSymbol = sender.currentTitle {
        brain.performOperation(mathematicalSymbol)
    }
}

But if you explicitly tell it what type sender is, you can avoid this error, namely either:

@IBAction func didTapButton(sender: UIButton) {
    if let mathematicalSymbol = sender.currentTitle {
        brain.performOperation(mathematicalSymbol)
    }
}

Or

@IBAction func didTapButton(sender: AnyObject) {
    if let button = sender as? UIButton, let mathematicalSymbol = button.currentTitle {
        brain.performOperation(mathematicalSymbol)
    }
}