YAMAMOTO Mitsuharu wrote:
> With the above macros, XUNTAG (a, type) is expanded to:
>
>    ((void *) ((char *) (0) - (type) + (intptr_t) XLI (a)))
>
> It is relative to the NULL pointer and clang seems to treat its
> dereference as an undefined behavior.

Thanks for diagnosing that.  This is arguably a bug in clang, but it's easy 
enough to work around as you suggest.  I reproduced the problem with Fedora 21 
x86-64 (clang 3.5.0) and installed the attached patch, which fixed it for me.