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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:

#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

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, ...)

# 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> ?

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