From 480183059e7be08f7d6249b99d29a7db91d3222f Mon Sep 17 00:00:00 2001
From: Joel de Guzman <djowel@gmail.com>
Date: Thu, 13 Oct 2011 00:23:40 +0000
Subject: [PATCH] + Fixes Ticket #6016 using a two-level specialization such
 that specialization is preferred over SFINAE. + Removes tag_of_fallback. It's
 not needed anymore. The proper way is to specialize tag_of.

[SVN r74934]
---
 include/boost/fusion/support/tag_of.hpp     | 31 +++++++++++----------
 include/boost/fusion/support/tag_of_fwd.hpp |  3 --
 2 files changed, 16 insertions(+), 18 deletions(-)

diff --git a/include/boost/fusion/support/tag_of.hpp b/include/boost/fusion/support/tag_of.hpp
index 1c5a26e5..a3fef3ba 100644
--- a/include/boost/fusion/support/tag_of.hpp
+++ b/include/boost/fusion/support/tag_of.hpp
@@ -46,28 +46,29 @@ namespace boost { namespace fusion
     namespace detail
     {
         BOOST_MPL_HAS_XXX_TRAIT_DEF(fusion_tag)
+
+        template <typename Sequence, typename Active>
+        struct tag_of_impl
+          : mpl::if_<fusion::detail::is_mpl_sequence<Sequence>,
+              mpl::identity<mpl_sequence_tag>,
+              mpl::identity<non_fusion_tag> >::type
+        {};
+
+        template <typename Sequence>
+        struct tag_of_impl<
+            Sequence
+          , typename boost::enable_if<detail::has_fusion_tag<Sequence> >::type>
+        {
+            typedef typename Sequence::fusion_tag type;
+        };
     }
 
     namespace traits
     {
-        template <typename Sequence, typename Active>
-        struct tag_of_fallback
-        {
-            typedef non_fusion_tag type;
-        };
-
         template <typename Sequence, typename Active>
         struct tag_of
-          : mpl::if_< fusion::detail::is_mpl_sequence<Sequence>,
-              mpl::identity<mpl_sequence_tag>,
-              tag_of_fallback<Sequence> >::type
+            : boost::fusion::detail::tag_of_impl<Sequence, Active>
         {};
-
-        template <typename Sequence>
-        struct tag_of<Sequence, typename boost::enable_if<detail::has_fusion_tag<Sequence> >::type>
-        {
-            typedef typename Sequence::fusion_tag type;
-        };
     }
 
     namespace detail
diff --git a/include/boost/fusion/support/tag_of_fwd.hpp b/include/boost/fusion/support/tag_of_fwd.hpp
index dfc0e01c..ba434d93 100644
--- a/include/boost/fusion/support/tag_of_fwd.hpp
+++ b/include/boost/fusion/support/tag_of_fwd.hpp
@@ -12,9 +12,6 @@ namespace boost { namespace fusion
 {
     namespace traits
     {
-        template <typename Sequence, typename Active = void>
-        struct tag_of_fallback;
-
         template<typename T, typename Active = void>
         struct tag_of;
     }