[boost] [system] Clang CI builds broken because of constexpr

Subject: [boost] [system] Clang CI builds broken because of constexpr
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2018-07-17 11:52:26

Hi,

I'm seeing these errors in Travis CI builds with Clang 3.8, 3.9, 4.0 and 
5.0:

./boost/system/error_code.hpp:232:41: error: constexpr constructor never 
produces a constant expression [-Winvalid-constexpr]
         BOOST_SYSTEM_CONSTEXPR explicit std_category( 
boost::system::error_category const * pc ): pc_( pc )
                                         ^
./boost/system/error_code.hpp:232:41: note: non-constexpr constructor 
'error_category' cannot be used in a constant expression
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/system_error:69:5: 
note: declared here
     error_category() noexcept;
     ^

I'm assuming this is because the compiler uses libstdc++ from gcc 4.8, 
in which error_category() is not marked constexpr. Can this be fixed 
somehow?