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

[tkaminsk at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119415

--- Comment #4 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
If want to support user-defined containers, I think we should check if
iterator_traits<It>::iterator category exists, before calling insert(Iterator,
Iterator) overload.

This will prevent hard-errors from older code, that do not handle C++20
iterators. In case where they are supported, users can add insert_range
overload.