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diff --git a/boost/type_traits/detail/common_type_impl.hpp b/boost/type_traits/detail/common_type_impl.hpp
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+++ b/boost/type_traits/detail/common_type_impl.hpp
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+#ifndef BOOST_TYPE_TRAITS_DETAIL_COMMON_TYPE_IMPL_HPP_INCLUDED
+#define BOOST_TYPE_TRAITS_DETAIL_COMMON_TYPE_IMPL_HPP_INCLUDED
+
+//
+//  Copyright 2015 Peter Dimov
+//
+//  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
+//
+
+#include <boost/type_traits/detail/common_arithmetic_type.hpp>
+#include <boost/type_traits/detail/composite_pointer_type.hpp>
+#include <boost/type_traits/detail/composite_member_pointer_type.hpp>
+#include <boost/type_traits/type_identity.hpp>
+#include <boost/type_traits/is_class.hpp>
+#include <boost/type_traits/is_union.hpp>
+#include <boost/type_traits/is_convertible.hpp>
+#include <boost/type_traits/is_pointer.hpp>
+#include <boost/type_traits/is_member_pointer.hpp>
+#include <boost/type_traits/conditional.hpp>
+
+namespace boost
+{
+
+namespace type_traits_detail
+{
+
+// the arguments to common_type_impl have already been passed through decay<>
+
+template<class T, class U> struct common_type_impl;
+
+// same type
+
+template<class T> struct common_type_impl<T, T>
+{
+    typedef T type;
+};
+
+// one of the operands is a class type, try conversions in both directions
+
+template<class T, class U> struct ct_class
+{
+    BOOST_STATIC_CONSTANT( bool, ct = boost::is_class<T>::value || boost::is_union<T>::value );
+    BOOST_STATIC_CONSTANT( bool, cu = boost::is_class<U>::value || boost::is_union<U>::value );
+
+    BOOST_STATIC_CONSTANT( bool, value = ct || cu );
+};
+
+template<class T, class U> struct common_type_impl3;
+
+template<class T, class U> struct common_type_class: public boost::conditional<
+
+    boost::is_convertible<T, U>::value && !boost::is_convertible<U, T>::value,
+    boost::type_identity<U>,
+
+    typename boost::conditional<
+
+        boost::is_convertible<U, T>::value && !boost::is_convertible<T, U>::value,
+        boost::type_identity<T>,
+
+        common_type_impl3<T, U>
+    >::type
+>::type
+{
+};
+
+template<class T, class U> struct common_type_impl: public boost::conditional<
+    ct_class<T, U>::value,
+    common_type_class<T, U>,
+    common_type_impl3<T, U> >::type
+{
+};
+
+// pointers
+
+template<class T, class U> struct common_type_impl4;
+
+template<class T, class U> struct common_type_impl3: public boost::conditional<
+    boost::is_pointer<T>::value || boost::is_pointer<U>::value,
+    composite_pointer_type<T, U>,
+    common_type_impl4<T, U> >::type
+{
+};
+
+// pointers to members
+
+template<class T, class U> struct common_type_impl5;
+
+template<class T, class U> struct common_type_impl4: public boost::conditional<
+    boost::is_member_pointer<T>::value || boost::is_member_pointer<U>::value,
+    composite_member_pointer_type<T, U>,
+    common_type_impl5<T, U> >::type
+{
+};
+
+// arithmetic types (including class types w/ conversions to arithmetic and enums)
+
+template<class T, class U> struct common_type_impl5: public common_arithmetic_type<T, U>
+{
+};
+
+} // namespace type_traits_detail
+
+} // namespace boost
+
+#endif // #ifndef BOOST_TYPE_TRAITS_DETAIL_COMMON_TYPE_IMPL_HPP_INCLUDED