Disable operator_brackets_proxy assignment if reference is not assignable.

Use SFINAE to disable operator_brackets_proxy assignment operator is reference
is not assignable from the assignment argument. Also fix the iterator type
in std::declval in noexcept specification.

Author: Lastique

include/boost/iterator/iterator_facade.hpp

@@ -360,9 +360,14 @@ class operator_brackets_proxy
 
     template< typename T >
     typename std::enable_if<
-        !std::is_same<
-            operator_brackets_proxy< Iterator >,
-            typename std::remove_cv< typename std::remove_reference< T >::type >::type
+        detail::conjunction<
+            detail::negation<
+                std::is_same<
+                    operator_brackets_proxy< Iterator >,
+                    typename std::remove_cv< typename std::remove_reference< T >::type >::type
+                >
+            >,
+            std::is_assignable< reference, T&& >
         >::value,
         operator_brackets_proxy&
     >::type operator= (T&& val) noexcept(noexcept(*std::declval< Iterator const& >() = std::declval< T&& >()))