Is it possible to add type constraints to a Swift protocol conformance extension?

The good news is that what you are asking for Conditional Conformance is coming in Swift 4.1:

https://swift.org/blog/conditional-conformance/

EDIT: As noted in the updated question, this is now possible since Swift 4.1

---

This is not currently possible in Swift (as of Xcode 7.1). As the error indicates, you can't restrict protocol conformance ("inheritance clause") to a type-constrained extension. Maybe someday. I don't believe there's any deep reason for this to be impossible, but it's currently not implemented.

The closest you can get is to create a wrapper type such as:

struct FriendlyArray<Element: Friendly>: Friendly {
    let array: [Element]
    init(_ array: [Element]) {
        self.array = array
    }
    func sayHi() {
        for elem in array {
            elem.sayHi()
        }
    }
}

let friendly: Friendly = FriendlyArray(["Foo", "Bar"])

(You would likely want to extend FriendlyArray to be a CollectionType.)

For a tale of my own descent into the madness of trying to make this work, and my crawl back from the edge, see NSData, My Old Friend.