[boost] clang and variadic macro warnings revisited

Subject: [boost] clang and variadic macro warnings revisited
From: Edward Diener (eldiener_at_[hidden])
Date: 2014-08-17 13:56:39

After making pull requests for a number of Boost libraries in order to 
reduce the warnings from compilation from clang using the Boost PP 
library, John Maddock suggested a specific solution, as Niall Douglas 
had suggested a general one before, that changes in the Boost PP library 
could get rid of these extraneous warnings. Originally I thought this 
would be too difficult and intrusive in Boost PP but after reading 
John's specific suggestion I decided to try to implement it.

The suggestion John made was to surround code in Boost PP which uses 
variadic macros with:

# ifdef __clang__
# pragma clang diagnostic push
# pragma clang diagnostic ignored "-Wvariadic-macros"
# endif

// variadic macros

# ifdef __clang__
# pragma clang diagnostic pop
# endif

This was actually easier than expected since all code in Boost PP which 
uses variadic macros is surrounded by:

# if BOOST_PP_VARIADICS
#endif // eventually

Of course there are many files changed by this but overall it is very 
straightforward. This technique does work for the diagnostic 
"-Wvariadic-macros". Hooray !

Unfortunately the second situation involving warnings, where clang warns 
about:

warning: empty macro arguments are a C99 feature [-Wc99-extensions]

coming from what I only see as a specific place in Boost PP in 
variadic/elem.hpp is not conducive to the same technique. In other words 
surrounding that one macro with:

# ifdef __clang__
# pragma clang diagnostic push
# pragma clang diagnostic ignored "-Wc99-extensions"
# endif

#define BOOST_PP_VARIADIC_ELEM(n, ...) 
BOOST_PP_CAT(BOOST_PP_VARIADIC_ELEM_, n)(__VA_ARGS__,)

# ifdef __clang__
# pragma clang diagnostic pop
# endif

does not eliminate the warning at all ( I have reported this on the 
clang developers mailing list ). And since that macro is heavily used in 
Boost PP this warning occurs quite often when compiling Boost library 
code using Boost PP wih clang.

The only other intrusive Boost PP solution to this problem is to treat 
clang like gcc as far as Boost PP variadic macro code is concerned: if 
the end-user compiles with one of the 'std=' dialects which explicitly 
supports variadic macros, such as 'std=c99', 'std=c++0x, 'std=c++11', 
then variadic macros are turned on in Boost PP for clang, else they are 
turned off. Do realize that both gcc and clang support variadic macros 
even when one of the 'std=' dialects are not chosen, and that in fact 
clang has always supported variadic macros. Also changing Boost PP 
support for clang and variadic macros would be a regression from the way 
it is now where it is always supported.

In retrospect, when I changed Boost PP from never supporting variadic 
macros for clang ( Paul's original conservative choice ) to always 
supporting variadic macros for clang, I gave in to an outcry that clang 
users should have variadic macro support in Boost PP. It would have been 
wiser to follow the gcc method in Boost PP. But that is past and we need 
a good solution now.

Thoughts ? Solutions ? Brickbats <g> ?