Add first draft of is_constructible.

include/boost/type_traits/is_constructible.hpp

@@ -0,0 +1,51 @@
+
+//  (C) Copyright John Maddock 2015.
+//  Use, modification and distribution are subject to 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).
+//
+//  See http://www.boost.org/libs/type_traits for most recent version including documentation.
+
+#ifndef BOOST_TT_IS_CONSTRUCTIBLE_HPP_INCLUDED
+#define BOOST_TT_IS_CONSTRUCTIBLE_HPP_INCLUDED
+
+#include <boost/type_traits/integral_constant.hpp>
+#include <boost/detail/workaround.hpp>
+
+#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES) && !defined(BOOST_NO_CXX11_DECLTYPE) && !BOOST_WORKAROUND(BOOST_MSVC, < 1800)
+
+#include <boost/type_traits/detail/yes_no_type.hpp>
+#include <boost/type_traits/detail/decl.hpp>
+
+namespace boost{
+
+   namespace detail{
+
+      struct is_constructible_imp
+      {
+         template<typename T, typename ...TheArgs, typename = decltype(T(tt_declval<TheArgs>()...))>
+         static boost::type_traits::yes_type test(int);
+
+         template<typename, typename>
+         static boost::type_traits::no_type test(...);
+      };
+
+   }
+
+   template <class T, class ...Args> struct is_constructible : public integral_constant<bool, sizeof(detail::is_constructible_imp::test<T, Args...>(0)) == sizeof(boost::type_traits::yes_type)>{};
+
+#else
+
+#include <boost/type_traits/is_convertible.hpp>
+#include <boost/type_traits/is_default_constructible.hpp>
+
+namespace boost{
+
+   // We don't know how to implement this:
+   template <class T, class U = void> struct is_constructible : public is_convertible<U, T>{};
+   template <class T> struct is_constructible<T, void> : public is_default_constructible<T>{};
+#endif
+
+} // namespace boost
+
+#endif // BOOST_TT_IS_ASSIGNABLE_HPP_INCLUDED