Issue Description:

The following extension on Array works as expected:

extension Array where Element: CustomStringConvertible {
    func f(_ x: Element) -> String {
        return x.description
    }
}

But when I use Iterator.Element instead, I get an error message (error: value of type 'Element' has no member 'description'):

extension Array where Iterator.Element: CustomStringConvertible {
    func f(_ x: Iterator.Element) -> String {
        return x.description
    }
}

However, this should work since Array is a Sequence and since Iterator.Element and Element refer to the same type, namely the type of the array elements.