[core] make actor self reference optional - again

- uses a different method of constructor selection that also works on Apple clang < 8.2.

1 files changed, 7 insertions, 2 deletions

diff --git a/include/mbgl/actor/actor.hpp b/include/mbgl/actor/actor.hpp
index 6610421eb5..a0df19208e 100644
--- a/include/mbgl/actor/actor.hpp
+++ b/include/mbgl/actor/actor.hpp
@@ -50,13 +50,18 @@ class Actor : public util::noncopyable {
 public:
 
     // Enabled for Objects with a constructor taking ActorRef<Object> as the first parameter
-    template <typename U = Object, class... Args,
-            typename std::enable_if<std::is_constructible<U, ActorRef<Object>, Args...>::value>::type...>
+    template <typename U = Object, class... Args, typename std::enable_if<std::is_constructible<U, ActorRef<U>, Args...>::value>::type * = nullptr>
     Actor(Scheduler& scheduler, Args&&... args_)
             : mailbox(std::make_shared<Mailbox>(scheduler)),
               object(self(), std::forward<Args>(args_)...) {
     }
 
+    // Enabled for plain Objects
+    template<typename U = Object, class... Args, typename std::enable_if<!std::is_constructible<U, ActorRef<U>, Args...>::value>::type * = nullptr>
+    Actor(Scheduler& scheduler, Args&& ... args_)
+            : mailbox(std::make_shared<Mailbox>(scheduler)), object(std::forward<Args>(args_)...) {
+    }
+
     ~Actor() {
         mailbox->close();
     }