std::experimental::ranges::Readable
来自cppreference.com
                    
                                        
                    < cpp | experimental | ranges
                    
                                                            
                    |   template < class In > concept bool Readable =  | 
(范围 TS) | |
概念 Readable 为通过应用 operator* 可读的类型,例如指针、智能指针和迭代器所满足。