std::nostopstate, std::nostopstate_t
From cppreference.com
                    
                                        
                    < cpp | thread | stop source
                    
                                                            
                    |   Defined in header  <stop_token>
  | 
||
|   struct nostopstate_t { explicit nostopstate_t() = default; };  | 
(1) | (since C++20) | 
|   inline constexpr std::nostopstate_t nostopstate {};  | 
(2) | (since C++20) | 
1) Empty tag type intended for use as a placeholder in std::stop_source non-default constructor, that makes the constructed std::stop_source empty with no associated stop-state.
2) The corresponding constant object instance of 
std::nostopstate_t for use in constructing an empty std::stop_source, as a placeholder value in the non-default constructor.See also
|    (C++20)  | 
  class representing a request to stop one or more std::jthreads  (class)  |