enum AorB { case a(A) case b(B) } protocol AorBProtocol { var _value: AorB { get } } extension A: AorBProtocol { var _value: AorB { return .a(self) } extension B: AorBProtocol { var _value: AorB { return .b(self) } extension Array: Foo where Element: AorBProtocol { func fooFunc() { forEach { switch $0._value { case .a(let a): // write a case here case .b(let b): // write b case here } } } } (edited)