Late fix for Boost PP

From: Edward Diener (eldiener_at_[hidden])
Date: 2019-11-12 16:32:31

I had added support in the Boost PP library in C20 mode ( __cplusplus > 
201703L ) for the __VA_OPT__ construct and enhanced an internal function 
used fairly extensively in the library to use this functionality in c20 
mode when __VA_OPT__ was supported. I had tested this against the 
compilers which I knew could operate in c20 mode, gcc and clang, and all 
was well. I just discovered that the vc++ compiler can also work in c20 
mode in Boost PP as defined above if you pass the '/std:c++latest' and 
the '/Zc:__cplusplus' command line switches and that vc++, in its usual 
non-standard way, blows up badly when doing so using the preprocessor 
library because it can not handle the rather simple code that checks for 
__VA_OPT__ support in c20 mode.

I have a simple fix for this which is to turn off the __VA_OPT__ support 
and __VA_OPT__ support checking in c20 mode specifically for vc++ and 
removes which I have pushed to 'develop'. Can I push this fix to the 
'master' branch now ? The fix needs to get in the coming release or else 
vc++ programmers specifying both the '/std:c++latest' and the 
'/Zc:__cplusplus' command line switched will not be able to use Boost PP 
and the many libraries which use Boost PP in the upcoming release.

I apologize for the seeming aggressiveness of this change, which 
incorporated the c20 __VA_OPT__ support internally in some much-used 
Boost PP functionality when c20 mode is being used, but I did not 
anticipate that it would blow up on me, although maybe I should have. I 
can also remove that internal dependency with a very simple one-line 
change. I realize that Boost PP stability has to be maintained, even in 
c20 mode for the few compilers which support that mode and apologize 
again that I have endangered it there.