From 59dfeec8157c419fabfb2a26f2412547d9a18d85 Mon Sep 17 00:00:00 2001
From: Kohei Takahashi <flast@flast.jp>
Date: Tue, 8 Sep 2015 00:18:37 +0900
Subject: [PATCH] Add workarounds for older MSVC.

---
 .../fusion/algorithm/iteration/detail/fold.hpp      | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/include/boost/fusion/algorithm/iteration/detail/fold.hpp b/include/boost/fusion/algorithm/iteration/detail/fold.hpp
index f7fa995b..22cb6ecd 100644
--- a/include/boost/fusion/algorithm/iteration/detail/fold.hpp
+++ b/include/boost/fusion/algorithm/iteration/detail/fold.hpp
@@ -59,13 +59,18 @@ namespace boost { namespace fusion
 
         template<int SeqSize, typename It, typename State, typename F>
         struct BOOST_PP_CAT(result_of_it_,BOOST_FUSION_FOLD_NAME)<SeqSize,It,State,F
-          , typename boost::enable_if_has_type<typename State::type>::type>
+            // MSVC9 issues a compile error as a partial specialization is ambiguous.
+          , typename boost::enable_if_has_type<
+                typename boost::disable_if_c<SeqSize == 0, State>::type::type
+            >::type>
           : BOOST_PP_CAT(result_of_it_,BOOST_FUSION_FOLD_NAME)<
                 SeqSize-1
               , typename result_of::BOOST_FUSION_FOLD_IMPL_NEXT_IT_FUNCTION<It>::type
               , boost::result_of<
                     F(
-                        typename State::type const&,
+                        typename add_reference<typename add_const<
+                            typename State::type
+                        >::type>::type,
                         BOOST_FUSION_FOLD_IMPL_INVOKE_IT_META_TRANSFORM(It const)
                     )
                 >
@@ -103,7 +108,9 @@ namespace boost { namespace fusion
                 typename result_of::BOOST_FUSION_FOLD_IMPL_NEXT_IT_FUNCTION<It>::type
               , boost::result_of<
                     F(
-                        typename State::type const&,
+                        typename add_reference<typename add_const<
+                            typename State::type
+                        >::type>::type,
                         BOOST_FUSION_FOLD_IMPL_INVOKE_IT_META_TRANSFORM(It const)
                     )
                 >