[meta] Add hb_declval() macro

src/hb-iter.hh

@@ -173,7 +173,7 @@ struct _hb_is_iterator
 };
 template<typename Iter, typename Item>
 struct hb_is_iterator { enum {
-  value = sizeof (int) == sizeof (hb_declval<_hb_is_iterator<Iter> > () (hb_declval<Iter*> ())) }; };
+  value = sizeof (int) == sizeof (hb_declval (_hb_is_iterator<Iter>) (hb_declval (Iter*))) }; };
 #define hb_is_iterator(Iter, Item) hb_is_iterator<Iter, Item>::value
 
 #define hb_is_sorted_iterator(Iter, Item) \

src/hb-meta.hh

@@ -36,6 +36,7 @@
 
 
 template <typename T> static inline T hb_declval ();
+#define hb_declval(T) (hb_declval<T> ())
 
 template <typename T> struct hb_match_const { typedef T type; enum { value = false }; };
 template <typename T> struct hb_match_const<const T> { typedef T type; enum { value = true }; };