In Swift can I use a tuple as the key in a dictionary?

As mentioned in the answer above, it is not possible. But you can wrap tuple into generic structure with Hashable protocol as a workaround:

struct Two<T:Hashable,U:Hashable> : Hashable {
  let values : (T, U)

  var hashValue : Int {
      get {
          let (a,b) = values
          return a.hashValue &* 31 &+ b.hashValue
      }
  }
}

// comparison function for conforming to Equatable protocol
func ==<T:Hashable,U:Hashable>(lhs: Two<T,U>, rhs: Two<T,U>) -> Bool {
  return lhs.values == rhs.values
}

// usage:
let pair = Two(values:("C","D"))
var pairMap = Dictionary<Two<String,String>,String>()
pairMap[pair] = "A"

The definition for Dictionary is struct Dictionary<KeyType : Hashable, ValueType> : ..., i.e. the type of the key must conform to the protocol Hashable. But the language guide tells us that protocols can be adopted by classes, structs and enums, i.e. not by tuples. Therefore, tuples cannot be used as Dictionary keys.

A workaround would be defining a hashable struct type containing two Ints (or whatever you want to put in your tuple).