[Bug libstdc++/71004] recursive_directory_iterator does not have a user-provided default ctor

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71004

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Eric Fiselier from comment #1)
>     recursive_directory_iterator it;
>     assert(it.recursion_pending() == false);
>     assert(it.recursion_pending() == true);

N.B. This test is undefined, those observer functions have preconditions that
they are not called on the end iterator.