protocol ElementProtocol {
    associatedtype BaseElement: BaseElementProtocol = Self
}
protocol BaseElementProtocol: ElementProtocol where BaseElement == Self {}
protocol ArrayProtocol {
    associatedtype Element: ElementProtocol
}
protocol NestedArrayProtocol: ArrayProtocol where Element: ArrayProtocol, Element.Element.BaseElement == Element.BaseElement {
    associatedtype BaseElement = Element.BaseElement
}
extension Array: ArrayProtocol where Element: ElementProtocol {}
extension Array: NestedArrayProtocol where Element: ElementProtocol, Element: ArrayProtocol, Element.Element.BaseElement == Element.BaseElement {
    // with the typealias uncommented you do not get a crash.
    // typealias BaseElement = Element.BaseElement
}