This issue has multiple aspects. I will gather them here since they are semantically connected.

The problem

Brittle boost::wave::need_cppXX(..) usage

The need_cppXX() functions return true for exactly the chosen language version. Consequently, all places wanting a branch being active for version XX and following have to do need_cppXX() || need_cppYY() || need_cppZZ() || ... Besides being suboptimal code code readability, it is also easy to forget extending one of those constructs when adding a language version.

In my recent contributions I found such an existing bug by chance.

Managing the C++ language version in 2 ways with the programmatic API and the BOOST_WAVE_SUPPORT_CPPXX macros

Code readability and maintaining suffer in my opinion due to those constructs all over the plase:

#if BOOST_WAVE_SUPPORT_CPPXX != 0
	// Do something requiring at least version XX
#endif
#if BOOST_WAVE_SUPPORT_CPPYY != 0
 	// Do something requiring at least version YY
#endif

Proposal

Better express desired logic for "active starting from C++ version XX"

Introduce functions boost::wave::need_at_least_cppXX(..):

• Those are always true given the user chooses a language version >= XX. Those would allow replacing all the or chaining cases with a single expressive function.
• Maybe we can even remove the need_cppXX functions. I can't think of a proper use case for them. This is optional. If there are use cases I do not see right now, they can also stay.

include/boost/wave/language_support.hpp always supports all C++ versions independent of BOOST_WAVE_SUPPORT_CPPXX macros

The only reason for the BOOST_WAVE_SUPPORT_CPPXX I can think of is performance optimization. This does in my opinion not really matter for what happens in include/boost/wave/language_support.hpp.
Having the enum and selection/check functions for all C++ versions and features always available is the precondition for at some point writing code no longer using the BOOST_WAVE_SUPPORT_CPPXX macros pattern.

This part of the proposal can be rejected and we can still implement need_at_least_cppXX(..). But we should decide first, as both parts of the proposal touch include/boost/wave/language_support.hpp.