--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2009-05-31  Bruno Haible  <bruno@clisp.org>
+
+	* lib/alignof.h (alignof_slot, alignof_type, alignof): Document
+	restriction due to compiler bugs.
+	Reported by Eric Blake.
+
 2009-05-31  Simon Josefsson  <simon@josefsson.org>
             Bruno Haible  <bruno@clisp.org>
 

--- a/lib/alignof.h
+++ b/lib/alignof.h
@@ -21,7 +21,9 @@
 #include <stddef.h>
 
 /* Determine the alignment of a structure slot (field) of a given type,
-   at compile time.  Note that the result depends on the ABI.  */
+   at compile time.  Note that the result depends on the ABI.
+   Note: The result cannot be used as a value for an 'enum' constant,
+   due to bugs in HP-UX 10.20 cc and AIX 3.2.5 xlc.  */
 #if defined __cplusplus
   template <class type> struct alignof_helper { char __slot1; type __slot2; };
 # define alignof_slot(type) offsetof (alignof_helper<type>, __slot2)
@@ -33,7 +35,9 @@
    Note that this is not necessarily the same as alignof_slot(type).
    For example, with GNU C on x86 platforms: alignof_type(double) = 8, but
    - when -malign-double is not specified:  alignof_slot(double) = 4,
-   - when -malign-double is specified:      alignof_slot(double) = 8.  */
+   - when -malign-double is specified:      alignof_slot(double) = 8.
+   Note: The result cannot be used as a value for an 'enum' constant,
+   due to bugs in HP-UX 10.20 cc and AIX 3.2.5 xlc.  */
 #if defined __GNUC__
 # define alignof_type __alignof__
 #else
@@ -42,7 +46,8 @@
 
 /* alignof is an alias for alignof_slot semantics, since that's what most
    callers need.
-   Note: The result cannot be used as a value for an 'enum' constant,  */
+   Note: The result cannot be used as a value for an 'enum' constant,
+   due to bugs in HP-UX 10.20 cc and AIX 3.2.5 xlc.  */
 #define alignof alignof_slot
 
 #endif /* _ALIGNOF_H */