[PATCH v2] Define alignof_slot using _Alignof when using C11 or newer

[Khem Raj <raj.khem@gmail.com>]

WG14 N2350 made very clear that it is an UB having type definitions
within "offsetof" [1]. This patch enhances the implementation of macro
alignof_slot to use builtin "_Alignof" to avoid undefined behavior on
when using std=c11 or newer. Make exceptions for gcc < 4.9 and
clang < 8 where there is bug [3] in _Alignof implementation

clang 16+ has started to flag this [2]

Fixes build when using -std >= gnu11 and using clang16+

[1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
[2] https://reviews.llvm.org/D133574
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023

Signed-off-by: Khem Raj <raj.khem@gmail.com>
---
v1->v2:
- Exclude gcc < 4.9 and clang < 8 even though they may have C11 support

 ChangeLog     |  5 +++++
 lib/alignof.h | 10 +++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index fb467a3c14..33c0a0bb8a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2023-01-15  Khem Raj  <raj.khem@gmail.com>
+
+	* lib/alignof.h (alignof_slot): Use _Alignof when using C11 or newer
+	standard barring GCC < 4.9 and clang < 8.
+
 2023-01-14  Bruno Haible  <bruno@clisp.org>
 
 	error, verror tests: Fix link error when the package uses libintl.
diff --git a/lib/alignof.h b/lib/alignof.h
index 505ad97aa4..962c8f31f4 100644
--- a/lib/alignof.h
+++ b/lib/alignof.h
@@ -28,8 +28,16 @@
 #if defined __cplusplus
   template <class type> struct alignof_helper { char __slot1; type __slot2; };
 # define alignof_slot(type) offsetof (alignof_helper<type>, __slot2)
-#else
+/* GCC releases before GCC 4.9 had a bug in _Alignof.  See GCC bug 52023
+   <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023>.
+   clang versions < 8.0.0 have the same bug.  */
+#elif (!defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 \
+       || (defined __GNUC__ && __GNUC__ < 4 + (__GNUC_MINOR__ < 9) \
+           && !defined __clang__) \
+       || (defined __clang__ && __clang_major__ < 8))
 # define alignof_slot(type) offsetof (struct { char __slot1; type __slot2; }, __slot2)
+#else
+# define alignof_slot(type) _Alignof(type)
 #endif
 
 /* alignof_type (TYPE)
-- 
2.39.0