Move mp_void to detail/mp_void.hpp

include/boost/mp11/detail/mp_void.hpp

@@ -0,0 +1,32 @@
+#ifndef BOOST_MP11_DETAIL_MP_VOID_HPP_INCLUDED
+#define BOOST_MP11_DETAIL_MP_VOID_HPP_INCLUDED
+
+//  Copyright 2015-2017 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
+
+namespace boost
+{
+namespace mp11
+{
+
+// mp_void<T...>
+namespace detail
+{
+
+template<class... T> struct mp_void_impl
+{
+    using type = void;
+};
+
+} // namespace detail
+
+template<class... T> using mp_void = typename detail::mp_void_impl<T...>::type;
+
+} // namespace mp11
+} // namespace boost
+
+#endif // #ifndef BOOST_MP11_DETAIL_MP_VOID_HPP_INCLUDED

include/boost/mp11/function.hpp

@@ -14,6 +14,7 @@
 #include <boost/mp11/detail/mp_count.hpp>
 #include <boost/mp11/detail/mp_plus.hpp>
 #include <boost/mp11/detail/mp_min_element.hpp>
+#include <boost/mp11/detail/mp_void.hpp>
 #include <type_traits>
 
 namespace boost
@@ -22,17 +23,7 @@ namespace mp11
 {
 
 // mp_void<T...>
-namespace detail
-{
-
-template<class... T> struct mp_void_impl
-{
-    using type = void;
-};
-
-} // namespace detail
-
-template<class... T> using mp_void = typename detail::mp_void_impl<T...>::type;
+//   in detail/mp_void.hpp
 
 // mp_and<T...>
 #if BOOST_WORKAROUND( BOOST_MSVC, < 1910 )