Re: _LIBCXX_DEBUG value initialized singular iterators assert failures in std algorithms

[Jonathan Wakely <jwakely@redhat.com>]

On Mon, 11 Mar 2024 at 20:07, Maciej Miera <maciej.miera@gmail.com> wrote:
>
> Hello,
>
> I have tried to introduce an extra level of safety to my codebase and utilize _GLIBCXX_DEBUG in my test builds in order to catch faulty iterators.
> However, I have encountered the following problem: I would like to utilize singular, value-initialized iterators as an arbitrary "null range”.
> However, this leads to failed assertions in std:: algorithms taking such range.
>
> Consider the following code sample with find_if:
>
> #include <map>
> #include <algorithm>
> #include <iterator>
>
> #ifndef __cpp_lib_null_iterators
> #warning "Not standard compliant"
> #endif
>
> int main()
> {
>     std::multimap<char, int>::iterator it1{};
>     std::multimap<char, int>::iterator it2{};
>
>     (void) (it1==it2); // OK
>     (void) std::find_if(
>         it1, it2, [](const auto& el) { return el.second == 8;});
> }
>
> Compiled with -std=c++20 and -D_GLIBCXX_DEBUG it produces the warning "Not standard compliant"
> and the execution results in the following assert failure:
>
> /opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/stl_algo.h:3875:
> In function:
>     constexpr _IIter std::find_if(_IIter, _IIter, _Predicate) [with _IIter =
>     gnu_debug::_Safe_iterator<_Rb_tree_iterator<pair<const char, int> >,
>     debug::multimap<char, int>, bidirectional_iterator_tag>; _Predicate =
>     main()::<lambda(const auto:16&)>]
>
> The question is though: is it by design, or is it just a mere oversight? The warning actually suggest the first option.
> If it is an intentional design choice, could you provide some rationale behind it, please?

The macro was not defined because the C++14 rule wasn't implemented
for debug mode, but that should have been fixed for GCC 11, according
to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98466 and
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70303
So we should be able to define macro now, except maybe it wasn't fixed
for the RB tree containers.