Swift Toggleable Enum protocol extension

Toggleable.swift

import Foundation

protocol Toggleable: CaseIterable, Equatable {
    mutating func toggle()
}

extension Toggleable {
    mutating func toggle() {
        guard Self.allCases.count == 2, let firstCase = Self.allCases.first else {
            fatalError("Enum must have only two elements")
        }
        if self == firstCase {
            self = Self.allCases[1 as! Self.AllCases.Index]
        } else {
            self = firstCase
        }
    }
}

enum TestEnum: Toggleable {
    case expanded, collapsed
}

var test = TestEnum.expanded
test.toggle()