var a: [Int] = [] var b: [String]? = a as? [String] // Cast from '[Int]' to unrelated type '[String]' always fails この警告だけ (edited)func foo(_ x: Any) { switch type(of: x) { case is [String].Type: print("strings \(x as! [String])") case is [Int].Type: print("integers \(x as! [Int])") default: print("unknwon") } } var a: [Int] = [] foo(a) // integers [] (edited)