std::experimental::ranges::Mergeable
From cppreference.com
                    
                                        
                    < cpp | experimental | ranges
                    
                                                            
                    | template < class I1, class I2, class Out, class R = ranges::less<>,            class P1 = ranges::identity, class P2 = ranges::identity > | (ranges TS) | |
The Mergeable concept specifies the common requirements of algorithms that merge sorted sequences into an output sequence by copying the elements (for example, ranges::merge).