glib provides G_STATIC_ASSERT(cond) which expands to a typedef.  That's useful in global scope, but cannot be used, eg, in macros expanding to an expression or statement.  For example, from gatomic.h:

# define g_atomic_int_get(atomic) \
 ((void) sizeof (gchar [sizeof (*(atomic)) == sizeof (gint) ? 1 : -1]), \
  (g_atomic_int_get) ((volatile gint G_GNUC_MAY_ALIAS *) (volatile void *) (atomic)))


The first line in there is essentially and expression version of G_STATIC_ASSERT().  So I suggest we add the following macro (and use it in gatomic.h too):

#define G_STATIC_ASSERT_EXPR(expr) ((void) sizeof (char[(expr) ? 1: -1]))