[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 #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #3)
> (In reply to Patrick Palka from comment #1)
> > Does this also mean the iterator's default ctor needs a
> > default_initializable<_Vs...[0]> constraint?
> Aside: I've just noticed the iterator should be renamed to _Iterator.
Oops, will fix.  IIUC even though iterator is a reserved name (due to
std::iterator) we can't safely use it here since it could cause name lookup
ambiguity in valid code, e.g.

  struct A { using iterator = void; }
  struct B : concat_view<...>, A { using type = iterator; };