std::atomic_flag::operator=
From cppreference.com
                    
                                        
                    < cpp | atomic | atomic flag
                    
                                                            
                    |   Defined in header  <atomic>
  | 
||
|   atomic_flag& operator=( const atomic_flag& ) = delete;  | 
(1) | (since C++11) | 
|   atomic_flag& operator=( const atomic_flag& ) volatile = delete;  | 
(2) | (since C++11) | 
std::atomic_flag is not assignable, its assignment operators are deleted.