BLI: use forwarding reference in Map

The is necessary when Map.add_or_modify is called with callbacks that
return a reference.

source/blender/blenlib/BLI_map.hh

@@ -1016,7 +1016,7 @@ class Map {
           return;
         }
         else {
-          auto return_value = create_value(value_ptr);
+          auto &&return_value = create_value(value_ptr);
           slot.occupy_no_value(std::forward<ForwardKey>(key), hash);
           occupied_and_removed_slots_++;
           return return_value;

source/blender/blenlib/tests/BLI_map_test.cc

@@ -295,6 +295,24 @@ TEST(map, AddOrModify)
   EXPECT_EQ(map.lookup(1), 15.0f);
 }
 
+TEST(map, AddOrModifyReference)
+{
+  Map<int, std::unique_ptr<int>> map;
+  auto create_func = [](std::unique_ptr<int> *value) -> int & {
+    new (value) std::unique_ptr<int>(new int{10});
+    return **value;
+  };
+  auto modify_func = [](std::unique_ptr<int> *value) -> int & {
+    **value += 5;
+    return **value;
+  };
+  EXPECT_EQ(map.add_or_modify(1, create_func, modify_func), 10);
+  int &a = map.add_or_modify(1, create_func, modify_func);
+  EXPECT_EQ(a, 15);
+  a = 100;
+  EXPECT_EQ(*map.lookup(1), 100);
+}
+
 TEST(map, AddOverwrite)
 {
   Map<int, float> map;