std::variant_npos
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header  <variant> | ||
| inline constexpr std::size_t variant_npos = -1; | (since C++17) | |
This is a special value equal to the largest value representable by the type std::size_t, used as the return type of index() when valueless_by_exception() is true
[edit] See also
| returns the zero-based index of the alternative held by the variant (public member function) | |
| checks if the variant is in the invalid state (public member function) | 


