protocol and associated type test

ProtocolTest.swift

//: Playground - noun: a place where people can play

import UIKit

protocol A
{
    associatedtype SomeType
    var a : SomeType { get }
}

protocol B : A
{
    typealias SomeType = String
    var a : String { get }
}

class C
{
// if uncommented, then it works. But we have a heavy dependency on `class AA`
//     var ivar : AA? = nil
    
    var ivar : B? = nil
}


class AA : A
{
    var a: String = "hello"
}

var aa = AA()
var cc = C()

cc.ivar = aa


if let cIvar = cc.ivar
{
    print(cIvar.a)
}

class AA : B<String>

AA must be a sub-class of A since they belong to a third-party library in my case. And I can't change the hierarchy, I'm afraid.
P.S. They are Store class and Store protocol of the ReSwift library