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

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

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

--- Comment #10 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to 康桓瑋 from comment #9)
> (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.
Where does that constraint come from? Since views::concat is "just" a CPO, not
a range adaptor object, I don't think viewable_range is implied?