Made add_loss_layer constructor more flexible. Now you can construct

from objects that are implicitly convertible to a loss details object
just like you can for computational layers.

dlib/dnn/core.h

@@ -2190,11 +2190,25 @@ namespace dlib
         {
         }
 
+        template <typename T, typename ...U>
+        struct disable_forwarding_constr 
+        {
+            const static bool value = std::is_constructible<LOSS_DETAILS,T>::value;
+        };
         template <typename ...T>
+        struct disable_forwarding_constr<add_loss_layer<T...>>
+        {
+            const static bool value = true;
+        };
+
+        template <
+            typename ...T, 
+            typename = typename std::enable_if<!disable_forwarding_constr<typename std::remove_reference<T>::type...>::value>::type
+            >
         add_loss_layer(
-            T ...args
+            T&& ...args
         ) : 
-            subnetwork(std::move(args)...)
+            subnetwork(std::forward<T>(args)...)
         {
         }
 

dlib/dnn/core_abstract.h

@@ -660,7 +660,7 @@ namespace dlib
         ); 
         /*!
             ensures
-                - #loss_details() == layer_det
+                - #loss_details() == loss_details_type(layer_det)
                 - #subnet()       == subnet_type(args)
         !*/
 
@@ -671,16 +671,19 @@ namespace dlib
         );
         /*!
             ensures
-                - #loss_details() == layer_det
+                - #loss_details() == loss_details_type(layer_det)
                 - #subnet()       == subnet_type(args)
         !*/
 
         template <typename ...T>
         add_loss_layer(
-            T ...args
+            T&& ...args
         ); 
         /*!
             ensures
+                - This version of the constructor is only called if loss_details_type can't
+                  be constructed from the first thing in args.  In this case, the args are
+                  simply passed on to the sub layers in their entirety.
                 - #loss_details() == loss_details_type()
                 - #subnet()       == subnet_type(args)
         !*/