Re: [boost] Clang 600 and BOOST_SYSTEM_CONSTEXPR causes error_code.hpp compile failure.

Subject: Re: [boost] Clang 600 and BOOST_SYSTEM_CONSTEXPR causes error_code.hpp compile failure.
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2018-07-07 16:13:33

On Thu, Jul 5, 2018 at 8:12 AM, Paul A. Bristow via Boost <
boost_at_[hidden]> wrote:

> Having failed to build Boost libraries using Clang 6.0.0 using b2/bjam on
> Windows, I am now trying instead to use Codeblocks on
> Windows 10 to build static libraries.
>
> It comes to grief with these messages (the berbose gory details can be
> posted if anyone can help).
>
> In file included from I:\modular-boost\boost/system/system_error.hpp:14:
> I:\modular-boost\boost/system/error_code.hpp:409:88: error: variable does
> not have a constant initializer
> template<class T> BOOST_SYSTEM_REQUIRE_CONST_INIT system_error_category
> cat_holder<T>::system_category_instance;
>
>              ^~~~~~~~~~~~~~~~~~~~~~~~
> I:\modular-boost\boost/system/error_code.hpp:409:19: note: required by
> 'require_constant_initialization' attribute here
> template<class T> BOOST_SYSTEM_REQUIRE_CONST_INIT system_error_category
> cat_holder<T>::system_category_instance;
>                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> I:\modular-boost\boost/system/error_code.hpp:392:44: note: expanded from
> macro 'BOOST_SYSTEM_REQUIRE_CONST_INIT'
> # define BOOST_SYSTEM_REQUIRE_CONST_INIT [[clang::require_constant_
> initialization]]
>                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~~~~~~~~
> This seems to be the ultimate cause
>
> I:\modular-boost\boost/system/error_code.hpp:226:41: note: non-constexpr
> constructor 'error_category' cannot be used in a constant
> expression
>         BOOST_SYSTEM_CONSTEXPR explicit std_category(
> boost::system::error_category const * pc ): pc_( pc )
>
> Is this because BOOST_SYSTEM_CONSTEXPR should not be defined?
>
> I note this in error_code.hpp
>
> #if defined(__GNUC__) && (__GNUC__ == 7 || __GNUC__ == 8) && __cplusplus
> >= 201700L
> // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83835
> #  undef BOOST_SYSTEM_HAS_CONSTEXPR
> #endif
>
> should this  undef list also include clang, and if so which versions 600?
>

I *suspect* that this is a red herring - but not having access to a Windows
box, I don't know that.

* Clang defines  __GNUC__ to 4 (for compatibility reasons), so this #ifdef
should never fire under clang.
* This block of code is to work around a bug in gcc that clang doesn't have.

-- Marshall