assert-h: static_assert is a keyword in C23

* m4/assert_h.m4 (gl_ASSERT_H): Also test for static_assert
keyword a la C23, and define HAVE_C_STATIC_ASSERT if so.
If not, arrange for config.h to #define static_assert
by including <assert.h>, and then do "#undef assert"
so that the assert macro still needs an explicit include.
This should be safe even on very old hosts, as assert.h
has been re-includable for decades.
* tests/tests-assert.c: New test.
* modules/assert-h-tests (Files, Makefile.am): Add it.

1 files changed, 23 insertions, 14 deletions

diff --git a/doc/posix-headers/assert.texi b/doc/posix-headers/assert.texi
index 3392a1691a..f3d50533f9 100644
--- a/doc/posix-headers/assert.texi
+++ b/doc/posix-headers/assert.texi
@@ -10,28 +10,37 @@ See also the Gnulib modules @code{assert} and @code{verify}.
 Portability problems fixed by Gnulib:
 @itemize
 @item
-On older platforms @code{static_assert} and @code{_Static_assert} do
-not allow the second string-literal argument to be omitted.  For
-example, GCC versions before 9.1 do not support the single-argument
-@code{static_assert} that was standardized by C2x and C++17.
+On older C platforms @code{<assert.h>} must be included before using
+@code{static_assert}.  For example, GCC versions before 13 do not
+support the @code{static_assert} keyword that was standardized by C23.
 @item
-Even-older platforms do not support @code{static_assert} or
-@code{_Static_assert} at all.  For example, GCC versions before 4.6 do
-not support @code{_Static_assert}, and G++ versions before 4.3 do not
-support @code{static_assert}, which was standardized by C11 and C++11.
+On older platforms @code{static_assert} does not allow the second
+string-literal argument to be omitted.  For example, GCC versions
+before 9.1 do not support the single-argument @code{static_assert}
+that was standardized by C23 and C++17.
+@item
+Even-older platforms do not support @code{static_assert} at all.
+For example, GCC versions before 4.6 and G++ versions before 4.3
+do not support the two-argument form, which was standardized
+by C11 and C++11.
+@item
+Older C platforms might not support the obsolescent
+@code{_Static_assert} keyword or macro.
+This portability problem should not matter with code using this
+module, as such code should use @code{static_assert} instead.
 @end itemize
 
 Portability problems not fixed by Gnulib:
 @itemize
 @item
-C @code{_Static_assert} and C++ @code{static_assert}
-are keywords that can be used without including @code{<assert.h>}.
-The Gnulib substitutes are macros that require including @code{<assert.h>}.
-@item
-The C @code{static_assert} and @code{_Static_assert} can also
+A @code{static_assert} can also
 be used within a @code{struct} or @code{union} specifier, in place of
 an ordinary declaration of a member of the struct or union.  The
-Gnulib substitute can be used only as an ordinary declaration.
+Gnulib substitute can be used only as an ordinary declaration
+in code intended to be portable to C99 or earlier.
+@item
+In C23 and C++11 and later, @code{static_assert} is a keyword.
+In C11 and C17 it is a macro.  Any Gnulib substitute is also a macro.
 @item
 In C99 and later, @code{assert} can be applied to any scalar expression.
 In C89, the argument to @code{assert} is of type @code{int}.