use std::is_reference<T>::value instead of std::is_reference_v<T> as it seems to be unsupported on OSX-LLVM yet

include/NiklasRosenstein/iterator.hpp

@@ -72,7 +72,7 @@ namespace niklasrosenstein {
 
     template <typename Y, bool move_value>
     struct _holder_type<Y, true, move_value> {
-      static_assert(std::is_reference_v<Y>, "not a reference type");
+      static_assert(std::is_reference<Y>::value, "not a reference type");
       std::add_pointer_t<std::remove_reference_t<Y>> value;
       _holder_type() : value(nullptr) { }
       _holder_type(Y value_) : value(value_) { }
@@ -81,7 +81,7 @@ namespace niklasrosenstein {
     };
 
     template <typename T, bool move_value>
-    using holder_type = _holder_type<T, std::is_reference_v<T>, move_value>;
+    using holder_type = _holder_type<T, std::is_reference<T>::value, move_value>;
 
   } // namespace iterator
   } // namespace detail