id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
12142	Including optional/optional_fwd.hpp then type_traits.hpp	Tony Lewis <tonyelewis@…>	akrzemi1	"1.61.0 beta introduces a new problem: the following code has started failing to compile since 1.60.0:

{{{#!cpp
#include <boost/optional/optional_fwd.hpp>
#include <boost/type_traits.hpp>
}}}

This appears to be caused by a change to `optional_fwd.hpp` so that it now includes `boost/config/suffix.hpp`. It turns out that that already had the potential to cause problems in 1.60.0, eg the following fails to compile:

{{{#!cpp
#include <boost/config/suffix.hpp>
#include <boost/type_traits.hpp>
}}}

Does Boost.Config allow other Boost libraries to include `boost/config/suffix.hpp` directly? If so, I think Config needs to fix the above problem. If not, `optional_fwd.hpp` needs to be changed to include `boost/config.hpp` not `boost/config/suffix.hpp` (and in that case it might be good to do the following).
 1. scan for any other includes in any other Boost libraries
 1. fix them
 1. move `suffix.hpp` and similar in a `detail` subdirectory.

I'm seeing equivalent errors on both Clang 3.6.2-1 and GCC 5.2.1.

Command lines:

{{{
clang++ -std=c++11 -isystem /opt/boost_1_61_0_b1_clang_build/include a.cpp
g++     -std=c++11 -isystem /opt/boost_1_61_0_b1_gcc_build/include   a.cpp
}}}"	Bugs	closed	Boost 1.62.0	optional	Boost 1.61.0	Regression	fixed	config/suffix.hpp,type_traits,optional