The C++2a standard will introduce the <version> header, which contains all the “feature macros” indicating features supported by the standard library. So if you live in the year 2035 and compile for a lot of different compilers, all of which support at least the bare bones of C++2a, <version> might soon be your best friend.

Suppose C++2b introduces std::experimental::superoptional<T>, which is like optional but even better; and then C++2c moves it from std::experimental into the main std namespace. Our future self will write this code:

#include <version>  // must exist because C++2a
#if __cpp_lib_superoptional >= 202601L
 #include <superoptional>
 namespace super = std;
#elif __cpp_lib_experimental_superoptional >= 202301L
 #include <experimental/superoptional>
 namespace super = std::experimental;
#else
 #error "I didn't find anything appropriate for namespace super"
#endif

and now we can use super::superoptional<T>.

In C++2a, including any library header (other than <version>) prior to testing the corresponding __cpp_lib_feature_flag is an antipattern, as far as I can tell — because #includeing an unsupported header is allowed to blow up, and frequently does blow up in practice. So you have to test first and #include second.

Testing __has_include(<any_header>) is redundant for any standard header whose existence is implied by a __cpp_lib_feature_flag; you can just test the feature flag instead.

Testing __has_include() will still be useful if you’re checking for a non-standard header, e.g.

#include <version>
#if __cpp_lib_superoptional >= 202601L
 #include <superoptional>
 namespace super = std;
#elif __cpp_lib_experimental_superoptional >= 202301L
 #include <experimental/superoptional>
 namespace super = std::experimental;
#elif __has_include(<boost/superoptional.hpp>)
 #include <boost/superoptional.hpp>
 namespace super = boost::superoptional;
#else
 #error "I didn't find anything appropriate for namespace super"
#endif

Of course this new idiom works only for C++2a-and-later programmers, which is none of us right now… but by the year 2035 will be most of us.

One action you can take today to prepare for C++2a is to audit your codebase’s include directories for any text files named VERSION and move those files elsewhere!