Support for std::piecewise_construct.

include/boost/unordered/detail/fwd.hpp

@@ -1,5 +1,5 @@
 
-// Copyright (C) 2008-2011 Daniel James.
+// Copyright (C) 2008-2016 Daniel James.
 // Distributed under the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
@@ -11,12 +11,46 @@
 #pragma once
 #endif
 
+#if defined(BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT)
+// Already defined.
+#elif defined(BOOST_LIBSTDCXX11)
+// https://github.com/gcc-mirror/gcc/blob/gcc-4_6-branch/libstdc++-v3/include/bits/stl_pair.h#L70
+#   if BOOST_LIBSTDCXX_VERSION > 40600
+#       define BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT 1
+#   endif
+#elif defined(_LIBCPP_VERSION)
+// https://github.com/llvm-mirror/libcxx/blob/release_30/include/utility#L206
+#   if LIBCPP_VERSION >= 3000
+#       define BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT 1
+#   endif
+#elif defined(BOOST_MSVC)
+// Apparently C++11 standard supported in Visual Studio 2012
+// https://msdn.microsoft.com/en-us/library/hh567368.aspx#stl
+// 2012 = VC+11 = BOOST_MSVC 1700 Hopefully!
+#   if BOOST_MSVC >= 1700
+#       define BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT 1
+#   endif
+#endif
+
+#if !defined(BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT)
+#define BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT 0
+#endif
+
+#if BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT
+#include <utility>
+#endif
+
 namespace boost
 {
 namespace unordered
 {
+#if BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT
+    using std::piecewise_construct_t;
+    using std::piecewise_construct;
+#else
     struct piecewise_construct_t {};
     const piecewise_construct_t piecewise_construct = piecewise_construct_t();
+#endif
 }
 }