@swift-4.2.4
enum Result<Value, Error: Swift.Error> { case success(Value) case failure(Error) func get() throws -> Value { switch self { case .success(let value): return value case .failure(let error): throw error } } } extension Result where Error == Never { func get() -> Value { switch self { case .success(let value): return value } } } let a: Result<Int, Never> = .success(42) print(a.get())