std::experimental::nonesuch
From cppreference.com
                    
                                        
                    < cpp | experimental
                    
                                                            
                    | Defined in header  <experimental/type_traits> | ||
| struct nonesuch {     nonesuch() = delete; | (library fundamentals TS v2) | |
std::experimental::nonesuch is a class type used by std::experimental::detected_t to indicate detection failure. 
[edit] Notes
nonesuch is not constructible, destructible, or copyable.


