vec_iterator member typedef iterator_concept should only be defined in C++20

[theodelrieu]

Hello,

After upgrading Boost to 1.77.0, I had compiler errors when trying to combine boost::container::flat_map and range-v3.

I'm using apple-clang 13, in C++17 mode.

vec_iterator added the following line, with no #ifdef around:

typedef std::contiguous_iterator_tag                                             iterator_concept;

Problem is that std::contiguous_iterator_tag is only defined in C++20, and it relies on a forward declaration found in boost/move/detail/iterator_traits.hpp (which is illegal IIRC).

Since range-v3 provides emulated concepts, it has to check the inheritence hierarchy of these tags, which requires them to be defined. As it makes use of iterator_concept the compiler is not happy:

/Users/theo/.conan/data/range-v3/0.11.0-r3/_/_/package/5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9/include/concepts/concepts.hpp:878:13: error: incomplete type >'std::contiguous_iterator_tag' used in type trait expression
          META_IS_BASE_OF(U, T) &&

[igaztanaga]

I've tried to detect if the standard library defines std::contiguous_iterator_tag in commit before forward declaring it in the commit:

boostorg/move@78f26da

and then, following your suggestion, defined vec_iterator::iterator_concept when std::contiguous_iterator tag it's defined:

19e68ee

Let me know if this fix helps with the issue.

[igaztanaga]

I've just checked that with the mentioned commits on Boost.Move and Boost.Container makes range-v3 happy with boost::container::vector in C++17 and C++20 modes.