Fixes #195: Only allowing typedef iterator_concept in C++20

[TristanFloch]

Fixes #195.

I was a bit surprised that there were no better way to check if the standard was C++20. Please correct me if there is.

[theodelrieu]

Any news on this?

[TristanFloch]

Any news on this?

None yet.

[pdimov]

The relatively new BOOST_CXX_VERSION is what one uses nowadays, if no appropriate Config or feature macro exists.

[pdimov]

In this case, the feature test macro that controls whether <iterator> has the new tags and iterator_concept seems to be __cpp_lib_ranges. (<iterator> must have been included beforehand.)

[igaztanaga]

In this case, the feature test macro that controls whether <iterator> has the new tags and iterator_concept seems to be __cpp_lib_ranges. (<iterator> must have been included beforehand.)

In theory, for C++20 features we could also check if header exists and if so, check for __cpp_lib_ranges, I guess.

[pdimov]

Checking for header existence is pretty useless in practice, as the header might exist but be empty or generate an #error. Fortunately, there's no need for that here, because <iterator> always exists.