[boost] [config] Positive feature macros, r2

Subject: [boost] [config] Positive feature macros, r2
From: Peter Dimov (lists_at_[hidden])
Date: 2018-01-25 16:51:31

I'd like to revisit the topic of Boost.Config feature macros, specifically 
that in my opinion, positive feature macros (BOOST_CXX17_FEATURE) make more 
sense than negative macros (BOOST_NO_CXX17_FEATURE) in the current 
environment.

Negative macros made sense in the past when they signified bugs and 
deficiencies in the compiler as measured against a static standard. 
Nowadays, the standard, such as it is, is in constant motion and macros 
indicate merely a current state of development for both the compiler and the 
standard.

The arguments for positive macros are the same as last time:

- adding a new negative macro requires changes to all legacy compilers that 
do not and will never implement the feature; a positive macro only requires 
changes to active compilers.

- positive macros are a better match for the standard feature macros, which 
are positive. When we see a standard feature macro being set, we typically 
would set ours as well. This means that the amount of changes required to 
add a new macro is further reduced; we only need to touch compiler-specific 
config files if the compiler either does not implement feature macros, or 
sets them incorrectly (in our considered opinion.)

TL;DR we should switch to positive feature macros in Boost.Config, and 
delaying this switch just accumulates more unnecessary code in Boost.Config.