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

[redi at gcc dot gnu.org]

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

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Patrick Palka from comment #1)
> Does this also mean the iterator's default ctor needs a
> default_initializable<_Vs...[0]> constraint?

Yes, I think the default ctor should get deleted if the constraint isn't met.

Aside: I've just noticed the iterator should be renamed to _Iterator.