From 21609491500d03e980885b3c3d49084e9637ce08 Mon Sep 17 00:00:00 2001
From: Nikita Kniazev <nok.raven@gmail.com>
Date: Fri, 12 Oct 2018 00:34:20 +0300
Subject: [PATCH] A slightly cleaner N3031 workaround

---
 .../container/map/detail/value_at_key_impl.hpp  |  8 +++-----
 .../container/vector/detail/value_at_impl.hpp   |  8 +++-----
 include/boost/fusion/support/config.hpp         | 17 ++++++++++-------
 3 files changed, 16 insertions(+), 17 deletions(-)

diff --git a/include/boost/fusion/container/map/detail/value_at_key_impl.hpp b/include/boost/fusion/container/map/detail/value_at_key_impl.hpp
index da6259e6..d53a8f02 100644
--- a/include/boost/fusion/container/map/detail/value_at_key_impl.hpp
+++ b/include/boost/fusion/container/map/detail/value_at_key_impl.hpp
@@ -24,12 +24,10 @@ namespace boost { namespace fusion
         struct value_at_key_impl<map_tag>
         {
             template <typename Sequence, typename Key>
-            struct apply
-            {
-                typedef typename BOOST_FUSION_IDENTIFIED_TYPE((
+            struct apply : BOOST_FUSION_DECLTYPE_N3031((
                     boost::declval<Sequence>().get_val(mpl::identity<Key>())
-                )) type;
-            };
+                ))
+            {};
         };
     }
 }}
diff --git a/include/boost/fusion/container/vector/detail/value_at_impl.hpp b/include/boost/fusion/container/vector/detail/value_at_impl.hpp
index a2dd5fcd..f29c0e14 100644
--- a/include/boost/fusion/container/vector/detail/value_at_impl.hpp
+++ b/include/boost/fusion/container/vector/detail/value_at_impl.hpp
@@ -47,12 +47,10 @@ namespace boost { namespace fusion
         struct value_at_impl<vector_tag>
         {
             template <typename Sequence, typename N>
-            struct apply
-            {
-                typedef typename BOOST_FUSION_IDENTIFIED_TYPE((
+            struct apply : BOOST_FUSION_DECLTYPE_N3031((
                     vector_detail::value_at_impl<N::value>(boost::declval<Sequence*>())
-                )) type;
-            };
+                ))
+            {};
         };
     }
 }}
diff --git a/include/boost/fusion/support/config.hpp b/include/boost/fusion/support/config.hpp
index 21630f3d..ca3ce126 100644
--- a/include/boost/fusion/support/config.hpp
+++ b/include/boost/fusion/support/config.hpp
@@ -97,17 +97,20 @@ namespace std
 #endif
 
 
-// Workaround for compiler which doesn't compile decltype(expr)::type.
-// It expects decltype(expr) deduced as mpl::identity<T>.
+// Workaround for compilers not implementing N3031 (DR743 and DR950).
 #if BOOST_WORKAROUND(BOOST_MSVC, BOOST_TESTED_AT(1913)) || \
     BOOST_WORKAROUND(BOOST_GCC, < 40700) || \
     defined(BOOST_CLANG) && (__clang_major__ == 3 && __clang_minor__ == 0)
-#   include <boost/mpl/identity.hpp>
-#   define BOOST_FUSION_IDENTIFIED_TYPE(parenthesized_expr) \
-        boost::mpl::identity<decltype parenthesized_expr>::type::type
+namespace boost { namespace fusion { namespace detail
+{
+    template <typename T>
+    using type_alias_t = T;
+}}}
+#   define BOOST_FUSION_DECLTYPE_N3031(parenthesized_expr) \
+        boost::fusion::detail::type_alias_t<decltype parenthesized_expr>
 #else
-#   define BOOST_FUSION_IDENTIFIED_TYPE(parenthesized_expr) \
-        decltype parenthesized_expr ::type
+#   define BOOST_FUSION_DECLTYPE_N3031(parenthesized_expr) \
+        decltype parenthesized_expr
 #endif