std::hash (std::error_code)
From cppreference.com
                    
                                        
                    < cpp | error | error code
                    
                                                            
                    |   Defined in header <system_error>
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|   struct hash<error_code>;  | 
(since C++11) | |
The template specialization of std::hash for std::error_code allows users to obtain hashes of objects of type std::error_code.