make boost::implicit_cast constexpr (fixes #25)

include/boost/implicit_cast.hpp

@@ -6,6 +6,11 @@
 #ifndef BOOST_IMPLICIT_CAST_DWA200356_HPP
 #define BOOST_IMPLICIT_CAST_DWA200356_HPP
 
+#include <boost/config.hpp>
+#ifdef BOOST_HAS_PRAGMA_ONCE
+# pragma once
+#endif
+
 namespace boost {
 
 namespace detail {
@@ -23,7 +28,7 @@ template<class T> struct icast_identity
 // The use of identity creates a non-deduced form, so that the
 // explicit template argument must be supplied
 template <typename T>
-inline T implicit_cast (typename boost::detail::icast_identity<T>::type x) {
+BOOST_CONSTEXPR inline T implicit_cast (typename boost::detail::icast_identity<T>::type x) {
     return x;
 }