[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 #19 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
Updated resolution to use __has_input_iter_cat instead of
__cpp17_input_iterator.