Re: [PROPOSED 2/2] assert-h: prefer to ‘verify’

[Bruno Haible <bruno@clisp.org>]

> > Where it’s easy, prefer ‘static_assert’ to ‘verify’,
> 
> Compiling a current GNU gettext with MSVC 14, I get this compilation error:

I got this working, by using preprocessor tricks, inspired by
  <https://github.com/Hirrolot/awesome-c-preprocessor>
  <https://stackoverflow.com/questions/11761703/>
  <https://groups.google.com/g/comp.std.c/c/d-6Mj5Lko_s?pli=1>
  <https://stackoverflow.com/questions/5134523/>


2022-09-14  Bruno Haible  <bruno@clisp.org>

	verify: Avoid syntax error due to static_assert with MSVC 14 in C++.
	* lib/verify.h (static_assert): For MSVC in C++ mode, use a definition
	that supports both the two-arguments and the one-argument syntax.

diff --git a/lib/verify.h b/lib/verify.h
index a0d597f3d4..0f5f0edc86 100644
--- a/lib/verify.h
+++ b/lib/verify.h
@@ -229,7 +229,22 @@ template <int w>
 # if (!defined static_assert \
       && __STDC_VERSION__ < 202311 \
       && __cpp_static_assert < 201411 && __GNUG__ < 6)
-#  define static_assert _Static_assert /* C11 requires this #define.  */
+#  if defined __cplusplus && _MSC_VER >= 1900 && !defined __clang__
+/* MSVC 14 in C++ mode supports the two-arguments static_assert but not
+   the one-argument static_assert, and it does not support _Static_assert.
+   We have to play preprocessor tricks to distinguish the two cases.
+   Since the MSVC preprocessor is not ISO C compliant (cf.
+   <https://stackoverflow.com/questions/5134523/>), the solution is specific
+   to MSVC.  */
+#   define _GL_EXPAND(x) x
+#   define _GL_SA1(a1) static_assert ((a1), "static assertion failed")
+#   define _GL_SA2 static_assert
+#   define _GL_SA3 static_assert
+#   define _GL_SA_PICK(x1,x2,x3,x4,...) x4
+#   define static_assert(...) _GL_EXPAND(_GL_SA_PICK(__VA_ARGS__,_GL_SA3,_GL_SA2,_GL_SA1)) (__VA_ARGS__)
+#  else
+#   define static_assert _Static_assert /* C11 requires this #define. */
+#  endif
 # endif
 #endif