Replace some usage of std::forward to static_cast.

For more details, See inline comment of include/boost/fusion/support/config.hpp .

include/boost/fusion/support/config.hpp

@@ -9,6 +9,7 @@
 #define FUSION_SUPPORT_CONFIG_01092014_1718
 
 #include <boost/config.hpp>
+#include <boost/detail/workaround.hpp>
 
 #ifndef BOOST_FUSION_GPU_ENABLED
 #define BOOST_FUSION_GPU_ENABLED BOOST_GPU_ENABLED
@@ -39,4 +40,32 @@ namespace boost { namespace fusion { namespace detail
 #define BOOST_FUSION_BARRIER_BEGIN namespace barrier {
 #define BOOST_FUSION_BARRIER_END   }
 
+
+#if BOOST_WORKAROUND(BOOST_MSVC, BOOST_TESTED_AT(1900))
+// All of rvalue-reference ready MSVC don't perform implicit conversion from
+// fundamental type to rvalue-reference of another fundamental type [1].
+//
+// Following example doesn't compile
+//
+//   int i;
+//   long &&l = i; // sigh..., std::forward<long&&>(i) also fail.
+//
+// however, following one will work.
+//
+//   int i;
+//   long &&l = static_cast<long &&>(i);
+//
+// OK, now can we replace all usage of std::forward to static_cast? -- I say NO!
+// All of rvalue-reference ready Clang doesn't compile above static_cast usage [2], sigh...
+//
+// References:
+// 1. https://connect.microsoft.com/VisualStudio/feedback/details/1037806/implicit-conversion-doesnt-perform-for-fund
+// 2. http://llvm.org/bugs/show_bug.cgi?id=19917
+//
+// Tentatively, we use static_cast to forward if run under MSVC.
+#   define BOOST_FUSION_FWD_ELEM(type, value) static_cast<type&&>(value)
+#else
+#   define BOOST_FUSION_FWD_ELEM(type, value) std::forward<type>(value)
+#endif
+
 #endif