[map] add keys()

src/hb-map.hh

@@ -212,11 +212,18 @@ struct hb_hashmap_t
    */
   auto iter() const HB_AUTO_RETURN
   (
-    + hb_array_t<item_t> (items, mask + 1)
+    + hb_array (items, mask + 1)
     | hb_filter (&item_t::is_real)
     | hb_map (&item_t::get_pair)
   )
 
+  auto keys() const HB_AUTO_RETURN
+  (
+	+ iter()
+    | hb_map (&hb_pair_t<K, V>::first)
+  )
+  // | hb_map ([&] (item_t i) { return i.key; })
+
   protected:
 
   unsigned int bucket_for (K key) const

src/test-iter.cc

@@ -210,6 +210,10 @@ main (int argc, char **argv)
   + hb_iter (m)
   | hb_map ([] (map_pair_t p) { return p.first * p.second; });
 
+	using map_key_t = decltype (*m.keys());
+	+ hb_iter (m.keys ())
+	| hb_filter ([] (map_key_t k) { return k < 42; });
+
   unsigned int temp1 = 10;
   unsigned int temp2 = 0;
   hb_map_t *result =