stddef: fix compilation error in C++ mode on MSVC

[Bruno Haible]

When building a testdir with C++ tests on MSVC 14, I see this error:

source='../../gltests/test-stddef-c++2.cc' object='test-stddef-c++2.obj' 
libtool=no \
DEPDIR=.deps depmode=msvc7 /bin/sh ../../build-aux/depcomp \
/home/bruno/msvc/compile cl -nologo -DHAVE_CONFIG_H -DEXEEXT=\".exe\" -I. 
-I../../gltests -I..  -DGNULIB_STRICT_CHECKING=1 -DIN_GNULIB_TESTS=1 -I. 
-I../../gltests -I.. -I../../gltests/.. -I../gllib -I../../gltests/../gllib 
-D_WIN32_WINNT=_WIN32_WINNT_WINXP -I/usr/local/msvc64/include  -MD -c -o 
test-stddef-c++2.obj `cygpath -w '../../gltests/test-stddef-c++2.cc'`
test-stddef-c++2.cc
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\cstddef(27): 
error C2874: using-declaration causes a multiple declaration of 
'std::rpl_max_align_t'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\cstddef(24): 
note: see declaration of 'std::rpl_max_align_t'
make[4]: *** [Makefile:9886: test-stddef-c++2.obj] Error 2

So, rpl_max_align_t is apparently making trouble. But the max_align_t override
is not needed on this platform, in C++ mode, since the original max_align_t
already has alignment 8.


2019-12-11  Bruno Haible  <address@hidden>

        stddef: Fix compilation error in C++ mode on MSVC.
        * lib/stddef.in.h (max_align_t): With MSVC in C++ mode, don't define it;
        instead, include <cstddef>.

diff --git a/lib/stddef.in.h b/lib/stddef.in.h
index e01f41b..e6c601a 100644
--- a/lib/stddef.in.h
+++ b/lib/stddef.in.h
@@ -83,20 +83,26 @@
 
 /* Some platforms lack max_align_t.  The check for _GCC_MAX_ALIGN_T is
    a hack in case the configure-time test was done with g++ even though
-   we are currently compiling with gcc.  */
-#if ! (@HAVE_MAX_ALIGN_T@ || defined _GCC_MAX_ALIGN_T)
-# if !GNULIB_defined_max_align_t
+   we are currently compiling with gcc.
+   On MSVC, max_align_t is defined only in C++ mode, after <cstddef> was
+   included.  Its definition is good since it has an alignment of 8 (on x86
+   and x86_64).  */
+#if defined _MSC_VER && defined __cplusplus
+# include <cstddef>
+#else
+# if ! (@HAVE_MAX_ALIGN_T@ || defined _GCC_MAX_ALIGN_T)
+#  if !GNULIB_defined_max_align_t
 /* On the x86, the maximum storage alignment of double, long, etc. is 4,
    but GCC's C11 ABI for x86 says that max_align_t has an alignment of 8,
    and the C11 standard allows this.  Work around this problem by
    using __alignof__ (which returns 8 for double) rather than _Alignof
    (which returns 4), and align each union member accordingly.  */
-#  ifdef __GNUC__
-#   define _GL_STDDEF_ALIGNAS(type) \
-      __attribute__ ((__aligned__ (__alignof__ (type))))
-#  else
-#   define _GL_STDDEF_ALIGNAS(type) /* */
-#  endif
+#   ifdef __GNUC__
+#    define _GL_STDDEF_ALIGNAS(type) \
+       __attribute__ ((__aligned__ (__alignof__ (type))))
+#   else
+#    define _GL_STDDEF_ALIGNAS(type) /* */
+#   endif
 typedef union
 {
   char *__p _GL_STDDEF_ALIGNAS (char *);
@@ -104,8 +110,9 @@ typedef union
   long double __ld _GL_STDDEF_ALIGNAS (long double);
   long int __i _GL_STDDEF_ALIGNAS (long int);
 } rpl_max_align_t;
-#  define max_align_t rpl_max_align_t
-#  define GNULIB_defined_max_align_t 1
+#   define max_align_t rpl_max_align_t
+#   define GNULIB_defined_max_align_t 1
+#  endif
 # endif
 #endif