NewOptionalAssociatedType.swift

protocol Injectable {
    associatedtype Dependency = Void
    
    func inject(_ dependency: Dependency)
}

extension Injectable where Dependency == Void {
    func inject(_ depenency: Dependency) {
        
    }
}

class VC: Injectable {
    
}

class VC2: Injectable {
    typealias Dependency = Int
    
    func inject(_ dependency: Int) {
        print(dependency)
    }
}