[Bug libstdc++/119415] flat_set::insert_range may not handle C++20 common-range

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119415
--- Comment #14 from 康桓瑋 <hewillk at gmail dot com> ---
I believe the correct way should be:

else if constexpr (ranges::common_range<_Rg> && 
                   requires { 
                     requires 
                        derived_from<typename
iterator_traits<ranges::iterator_t<_Rg>>::iterator_category,
                                     input_iterator_tag>; 
                    }
                  )
// ...

But it isn't lovely to put it all in a single condition.