Best way to check non-optional values for nil in Swift

Question

In Swift, it's rare but possible to end up with a value of a non-optional type that has a nil value. As explained in answers to this question, this can be caused by bad Objective-C code bridged to Swift:

- (NSObject * _Nonnull)someObject {
    return nil;
}

Or by bad Swift code:

class C {}
let x: C? = nil
let y: C = unsafeBitCast(x, to: C.self)

In practice, I've run into this with the MFMailComposeViewController API in MessageUI. The following creates a non-optional MFMailComposeViewController, but if the user has not set up an email account in Mail, the following code crashes with EXC_BAD_ACCESS:

let mailComposeViewController = MFMailComposeViewController()
print("\(mailComposeViewController)")

The debugger shows the value of mailComposeViewController like this:

mailComposeViewController = (MFMailComposeViewController) 0x0000000000000000

I have a couple of questions here:

1. I note that the documentation for unsafeBitCast(_:to:) says it "breaks the guarantees of the Swift type system," but is there a place in Swift documentation that explains that these guarantees can be broken, and how/when?
2. What's the best, most idiomatic way to check for this case? The compiler won't let me check whether mailComposeViewController == nil since it's not optional.

Answer 1

Even Apple's APIs sometimes return nil for a type that is not marked in the API as Optional. The solution is to assign to an Optional.

For example, for a while traitCollectionDidChange returned a UITraitCollection even though it could in fact be nil. You couldn't check it for nil because Swift won't let you check a non-Optional for nil.

The workaround was to assign the returned value immediately to a UITraitCollection? and check that for nil. That sort of thing should work for whatever your use case is as well (though your mail example is not a use case, because you're doing it wrong from the get-go).