たぶんこんな感じになるんだけど
struct Foo : Decodable { let obj: Base public func encode(to encoder: Encoder) throws { let objType: ObjType switch self.obj { case is Concrete: objType = .concrete //... default: objType = .base } container.encode(objType, forKey: .objType) container.encode(obj, forKey: .obj) } public init(from decoder: Decoder) throws { objType = container.decode(ObjType.self, forKey: .objType) let ObjT : Base.Type switch { case .concrete: ObjT = Concrete.self // ... case .base: ObjT = Base.self } self.obj = container.decode(ObjT, forKey: .obj) } }
(edited)