[range] Concept check failing with custom C++20 range support

From: Jared McIntyre (jmcintyre_at_[hidden])
Date: 2022-09-19 23:30:44

I've been toying around with a pull request that was entered against
boost::range to support integration of boost range_v2 and C++20 ranges to
see how flexible it would be:

https://github.com/boostorg/range/pull/123

I have a need to continue to use boost::any_range in some of our
interfaces, but it would be nice to switch to using C++20 ranges under the
covers. A lot of it seems to work well with the type definitions from the
pull request, but I'm running into an issue with concept check on line 228
or any_iterator_wrapper.hpp specifically when using the transform adaptor.
I don't have the same issue with boost::adaptors::transformed. What has me
confused is that I think that the result views::transform should be forward
traversal, but I'm not really clear on exactly why it is failing. I created
a test if anyone wants to play around with it in a compiler and help me
understand what is going on:

https://godbolt.org/z/1hMsofbrv

Switching between the two commented out base_range declarations will get
you different results. Here is the code for anyone that just wants to look
at it here:

#include <boost/range.hpp>
#include <boost/range/adaptors.hpp>
#include <boost/range/any_range.hpp>
#include <ranges>

#if 1
namespace boost::range_detail {
template< std::ranges::common_range C >
struct range_mutable_iterator< C > {
typedef std::ranges::iterator_t< C > type;
};
template< std::ranges::common_range C >
struct range_const_iterator_helper< C > {
typedef std::ranges::iterator_t< C const > type;
};
}
#endif

int main() {
std::vector<int> vec(5, 0);
auto mul2 = [](int i) { return i * 2; };
auto base_range = vec | std::views::transform(mul2);
//auto base_range = vec | boost::adaptors::transformed(mul2);
boost::any_range<int, boost::forward_traversal_tag> new_range = base_range;
}