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.