[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 #2 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Patrick Palka from comment #1)
> Does this also mean the iterator's default ctor needs a
> default_initializable<_Vs...[0]> constraint?

The variant constructor already has such a constraint.