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: Peter Dimov (lists_at_[hidden])
Date: 2018-07-08 14:42:55

> John Maddock wrote:
>
> > 1>C:\Program Files (x86)\Microsoft Visual Studio 
> > 14.0\VC\include\system_error(168,18):  note: declared here
> > 1>        /* constexpr */ error_category() _NOEXCEPT      // TRANSITION
>
> This commented-out constexpr (in the 2015 headers) is the problem, thanks. 
> Now how do we work around it...

The problem here is that Clang can pick up and use any MSVC headers it finds 
depending on who knows what search algorithm, and to work around the 
commented-out constexpr I need to detect that it uses the 2015 STL (because 
the 2017 STL would presumably have the constexpr, although I haven't checked 
all of its flavors.)

I'm not sure that checking _MSC_VER would be enough, as I suspect that Clang 
doesn't take the compatibility version into account when searching for 
headers to use. There's _CPPLIB_VER, but I can't find what values correspond 
to what versions. Boost.Config says that certain things appeared in 520, 
540, 610, 650, but what releases do these correspond to?

In addition, Microsoft stopped increasing _CPPLIB_VER at some point 
(starting with 2017?), then added their own macro (a welcome addition) but I 
can't recall how that was called, when it was added, and can find no info on 
that either. :-)

It ain't easy.