std::chrono::weekday_indexed::ok
From cppreference.com
                    
                                        
                    < cpp | chrono | weekday indexed
                    
                                                            
                    | constexpr bool ok() const noexcept; | (since C++20) | |
Checks if the weekday object and the index stored in *this are both valid.
Return value
true if weekday().ok() == true and index() is in the range [1, 5]. Otherwise false.