No Diagnostic Required
From cppreference.com
                    
                                        
                    
                    
                                                            
                    "No diagnostic required" indicates that some phraseology is ill-formed according to the language rules, but a compiler need not issue any diagnostic or error message. Usually, the reason is that trying to detect these situations would result in prohibitively long compile times.
If such a program is executed, the behavior is undefined.
See also
|   C documentation for No Diagnostic Required 
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