Swift Array.firstObject() and Array.lastObject() Extensions

gistfile1.swift

extension Array {
    func firstObject() -> T! {
        var firstItem: T!
        if !self.isEmpty {
            firstItem = self[0]
        }
        return firstItem
    }
    
    func lastObject() -> T! {
        var lastItem: T!
        if !self.isEmpty {
            lastItem = self[self.count-1]
        }
        
        return lastItem
    }
}

var foo = [1,2,3]

var firstObject = foo.firstObject() // should return 1
var lastObject = foo.lastObject() // should return 3

firstObject == foo[0] // verify they equal

var bar: Int[] = [] // empty array

bar.firstObject() // should be nil
bar.lastObject() // should be nil

What made you choose not to have the methods return T! instead of T?