Fix a subtle parameter expansion issue in sync info (#155)

It's mildly unfortunate that the function for recording the event on a
SyncInfo is called "AddSynchronizationPoint". Thus we need an explicit
`if constexpr` to stop the recursion such that the event record is
actually avoided when we don't need it. C'est la vie.

include/hydrogen/SynchronizeAPI.hpp

@@ -20,38 +20,19 @@ void AddSynchronizationPoint(SyncInfo<D> const &master,
         // synchronization points. Skip "other" call recursively with the rest.
         if (master.Stream() == other.Stream())
         {
-            AddSynchronizationPoint(master, others...);
+            if constexpr (sizeof...(others) > 0UL)
+                AddSynchronizationPoint(master, others...);
             return;
         }
     }
 #endif // HYDROGEN_HAVE_GPU
 
     AddSynchronizationPoint(master);
-    int dummy[] = {(details::AddSyncPoint(master, other),
-                    details::AddSyncPoint(master, others), 0)...};
+    int dummy[] = {(details::AddSyncPoint(master, other), 0),
+                   (details::AddSyncPoint(master, others), 0)...};
     (void)dummy;
 }
 
-// Specialization of the above function for two arguments
-template <Device D, Device D2>
-void AddSynchronizationPoint(SyncInfo<D> const &master,
-                             SyncInfo<D2> const &other)
-{
-#ifdef HYDROGEN_HAVE_GPU
-    if constexpr (D == Device::GPU && D == D2)
-    {
-        // When the two streams are the same, there is no need to create
-        // synchronization points.
-        if (master.Stream() == other.Stream())
-        {
-            return;
-        }
-    }
-#endif // HYDROGEN_HAVE_GPU
-
-    AddSynchronizationPoint(master);
-}
-
 template <Device D, Device... Ds>
 void AllWaitOnMaster(SyncInfo<D> const &master, SyncInfo<Ds> const &...others)
 {