@swift-4.1.3
import Foundation struct Empty: Codable { } struct Foo: Codable { var empty: Empty } struct Bar: Codable { var empty: [Empty] } struct Piyo<E: Codable>: Codable { var empty: E } let encoder = JSONEncoder() let decoder = JSONDecoder() do { print(try decoder.decode(Bar.self, from: encoder.encode(Foo.init(empty: .init())))) } catch { print(error) } do { print(try decoder.decode(Foo.self, from: encoder.encode(Bar.init(empty: .init())))) } catch { print(error) } do { print(try decoder.decode(Piyo<Empty>.self, from: encoder.encode(Piyo<[Empty]>(empty: .init())))) } catch { print(error) } do { print(try decoder.decode(Piyo<[Empty]>.self, from: encoder.encode(Piyo<Empty>(empty: .init())))) } catch { print(error) }