[Bug libstdc++/115218] The conversion constructor of concat_view::iterator always default-constructs variant

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115218

--- Comment #9 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Patrick Palka from comment #8)
> Fixed, thanks!

The fix for LWG 4082 is the missing viewable_range constraint for one pack
case.