A word on Self requirement

In the previous section, we've seen an example of associated type. Let's see what we mean when mentioning Self requirement.

Consider the following protocol:

protocol Thing {
    associatedtype AType
}

struct Node: Thing {
    typealias AType = Thing // Self associated type
}

Because Node conforms to Thing and sets its AType associatedtype to itself via typealias, you would probably imagine the following code would work:

let thing: Thing = Node()

Unfortunately, it doesn't. This is what we call a Self requirement and you will be hit by the same problem:

Protocol 'Thing' can only be used as a generic constraint because it has Self or associated type requirements