Downcast from Any to a protocol

Question

I have the following code. How can I resolve the error in the last line?

protocol Animal {
    func walk()
}

struct Cat: Animal {
    func walk() {}

    init() { }
}

var obj: Any = Cat()
var cat = obj as Animal // ERROR: cannot downcast from Any to unrelated type Animal

Answer 1

Update: This has been fixed in Swift 1.2+ (Xcode 6.3+). The Xcode 6.3 beta release notes say:

Dynamic casts (“as!", “as?" and “is”) now work with Swift protocol types, so long as they have no associated types.

---

You can only check for protocol conformance (which includes is, as, and as?) with an @objc protocol. Animal is not @objc.

See the Checking for Protocol Conformance section of the Swift book.

NOTE

You can check for protocol conformance only if your protocol is marked with the @objc attribute

Answer 2

You can workaround it by doing

 var cat = obj as Cat as Animal

but this workaround is almost useless... because you need to know the type of obj first

---

Edit:

As @newacct point out, it is not bug, see his answer for more information

xcrun swift
Welcome to Swift!  Type :help for assistance.
  1>
  2> @objc protocol Animal {
  3.         func walk()
  4. }

@objc class Cat: Animal {
    func walk() {}

    init() { }
}

var obj: AnyObject = Cat()

var cat = obj as Animal
  5>
  6> @objc class Cat: Animal {
  7.         func walk() {}
  8.
  9.         init() { }
 10. }
 11>
 12> var obj: AnyObject = Cat()
obj: Cat = {}
 13>
 14> var cat = obj as Animal
cat: Cat = {}
 15>

Animal protocol need @objc attribute and Cat need to be @objc class.