A FIFO request queue with type erasure (any / some)

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Task

You have a protocol with associated types. Write a queue that runs requests strictly one at a time, in insertion order. add must accept any Request.

protocol Request {
    associatedtype Response
    associatedtype Failure: Error

    typealias Handler = (Result<Response, Failure>) -> Void

    var handler: Handler { get }
    func perform(then handler: @escaping Handler)
}

final class RequestQueue {
    func add(_ request: Request) { } // doesn't compile — why?
}