Warning when compiling type_traits/detail/config.hpp with -Wundef on clang

[jredmondson]

When compiling with -Wundef on clang we get the following warning when using/including type_traits/detail/config.hpp:

In file included from /x/boost/iostreams/stream.hpp:20:
In file included from /x/boost/iostreams/detail/select.hpp:32:
In file included from /x/boost/type_traits/is_base_and_derived.hpp:12:
In file included from x/boost/type_traits/intrinsics.hpp:16:
 [x/boost/type_traits/detail/config.hpp:85:52:  [0m [0;1;31merror:  [0m [1m'__clang_major___WORKAROUND_GUARD' is not defined, evaluates to 0 [-Werror,-Wundef] [0m
      && !BOOST_WORKAROUND(BOOST_MSVC, < 1900) && !BOOST_WORKAROUND(__clang_major__, <= 4)
 [0;1;32m                                                   ^
 [0m [1m/x/boost/config/workaround.hpp:246:10:  [0m [0;1;30mnote:  [0mexpanded from macro 'BOOST_WORKAROUND' [0m
       ((symbol ## _WORKAROUND_GUARD + 0 == 0) &&     \
 [0;1;32m         ^
 [0m [1m<scratch space>:17:1:  [0m [0;1;30mnote:  [0mexpanded from here [0m
__clang_major___WORKAROUND_GUARD

I'm not sure that the usage of this in type_traits/detail/config.hpp is correct, but the issue is that __clang_major__ + WORKAROUND_GUARD is being compared here without first checking that it has been defined and thus I get an error in environments where I am forced to use a strict clang (no undefs). My fix for this in a 1.70 base is similar to #186 in that the following can be added to config/workaround.hpp (tested in a UE4 plugin project that includes Boost libs and headers):

#ifndef __clang_major__
#define __clang_major___WORKAROUND_GUARD 1
#else
#define __clang_major___WORKAROUND_GUARD 0
#endif

Similar change to any other versions should also work. Thanks!

[Andor233]

I got same problem, thank you bro