Code that doesn't work because existentials don't conform to their own protocols.

Nodes.swift

protocol MyNode {
    var total: Int { get }
}

protocol MyParentNode: MyNode {
    // Each MyParentNode-conforming type can only have certain types of child nodes
    // (basically, type A can have B, C, or D; B can have C or D; C can have D; and D is not a MyParentNode).
    // I want to model these limitations in the types of children permitted.
    associatedtype Child: MyNode
    var children: [Child] { get set }
}
extension MyParentNode {
    var total: Int {
        return children.map { $0.total }.reduce(0, +)
    }
}

protocol SpecificNodeChild: MyNode {}
struct SpecificNode: MyParentNode {
    typealias Child = SpecificNodeChild
    var children: [Child] = []
    // Error: Type 'SpecificNode' does not conform to protocol 'MyParentNode'
    //      Protocol requires nested type 'Child'; do you want to add it?
    //      Possibly intended match 'SpecificNode.Child' (aka 'SpecificNodeChild') does not conform to 'SpecificNode'
}