Protocol doesn't conform to itself?

Question

Why doesn\'t this Swift code compile?

protocol P { }
struct S: P { }

let arr:[P] = [ S() ]

extension Array where Element : P {
    func test() -&g         


        

Answer 1

If you extend CollectionType protocol instead of Array and constraint by protocol as a concrete type, you can rewrite the previous code as follows.

protocol P { }
struct S: P { }

let arr:[P] = [ S() ]

extension CollectionType where Generator.Element == P {
    func test() -> [T] {
        return []
    }
}

let result : [S] = arr.test()