[cfe-dev] [llvm-dev] problem (and fix) with -fms-extensions

[Marc Espie via cfe-dev]

On Wed, May 17, 2017 at 03:55:06PM -0700, Reid Kleckner wrote:
> 
>    On Wed, May 17, 2017 at 3:22 PM, Marc Espie <[1]espie at nerim.net> wrote:
> 
>      On Wed, May 17, 2017 at 01:01:07PM -0700, Reid Kleckner wrote:
>      >
>      >Â  Â  I'd rather not add new pre-defined macros if we can avoid it.
>      There are
>      >Â  Â  already too many. You can probably detect this situation
>      with:
>      >Â  Â  #if defined(_MSC_EXTENSIONS) && !defined(_WCHAR_T_DEFINED)
>      >
>      Was this added recently ?
>      There is no _MSC_EXTENSIONS in the clang I'm using...
> 
>    Looks like we only define it for Windows targets.
>    How about __is_identifier?

Or, what's the problem with moving _MSC_EXTENSIONS to be for all targets.


Look, this stuff is highly specific, I doubt many people are going to
use -fms-extensions who don't know what they're doing.
E.g., the exposure to _MSC_EXTENSIONS will be minimal.

Having a single define will mean replacing (for us)
#ifndef __cplusplus
typedef int                     __wchar_t;
#endif

with
#if !defined(__cplusplus) && !defined(_MSC_EXTENSIONS)
typedef int                     __wchar_t;
#endif

which is fairly acceptable.

Playing with __is_identifier is rather more awful, because there is no
simple construct, we can't simply write:
#if !defined(__cplusplus) || __is_identifier(__wchar_t)
because  this *also* has to work with other compilers, some of which
do not understand __is_identifier at all, so it can't be a single test
then.

It would become some atrocity like

#if !defined(__cplusplus)
# if defined(__is_identifier)
#  if __is_identifier(__wchar_t)
#  else
typedef int	__wchar_t;
#  endif
# else
typedef int	__wchar_t;
# endif
#endif

And that has about zero chance to ever be committed in our tree,
especially since this is in a md _types.h file, so this construct x10 times
in the tree.