why is this causing so much trouble? (protocols and typealiases on their associated types)

Question

I\'m trying to do something along the lines of this:

protocol Protocol {
    associatedtype T
    associatedtype ArrayT = Array
}

struct Struct

        

Answer 1

The line associatedtype ArrayT = Array only tells the compiler that the default value of ArrayT is Array. An adaption of the protocol can still change ArrayT like:

struct U: Protocol {
    typealias T = UInt32
    typealias ArrayT = UInt64   // <-- valid!
}

If you want a fixed type, you should use a typealias...

// does not work yet.
protocol Protocol {
    associatedtype T
    typealias ArrayT = Array
}

But the compiler complains that the type is too complex