[iter] Add hb_bool() and make hb_filter default to it for predicate

src/hb-algs.hh

@@ -41,6 +41,12 @@ static HB_UNUSED const struct hb_identity_ft
   operator () (const T& v) const { return v; }
 } hb_identity;
 
+static HB_UNUSED const struct hb_bool_ft
+{
+  template <typename T> bool
+  operator () (const T& v) const { return bool (v); }
+} hb_bool;
+
 template <typename T1, typename T2>
 struct hb_pair_t
 {

src/hb-iter.hh

@@ -281,9 +281,9 @@ struct hb_filter_iter_factory_t
   Pred p;
   Proj f;
 };
-template <typename Pred, typename Proj = const hb_identity_ft&>
+template <typename Pred = const hb_bool_ft&, typename Proj = const hb_identity_ft&>
 inline hb_filter_iter_factory_t<Pred, Proj>
-hb_filter (Pred&& p, Proj&& f = hb_identity)
+hb_filter (Pred&& p = hb_bool, Proj&& f = hb_identity)
 { return hb_filter_iter_factory_t<Pred, Proj> (p, f); }
 
 /* hb_zip() */

src/test-iter.cc

@@ -84,7 +84,8 @@ test_iterator (Iter it)
 
   assert (*it == it[0]);
 
-  if (it.is_random_access_iterator) {}
+  static_assert (true || it.is_random_access_iterator, "");
+  static_assert (true || it.is_sorted_iterator, "");
 }
 
 template <typename Iterable,
@@ -134,5 +135,12 @@ main (int argc, char **argv)
   hb_array_t<hb_vector_t<int> > pa;
   pa->as_array ();
 
+  s
+  | hb_map (hb_identity)
+  | hb_filter ()
+  | hb_filter (hb_bool)
+  | hb_filter (hb_bool, hb_identity)
+  ;
+
   return 0;
 }