diff --git a/include/boost/system/detail/error_code.hpp b/include/boost/system/detail/error_code.hpp
index b934a8d..5d263b0 100644
--- a/include/boost/system/detail/error_code.hpp
+++ b/include/boost/system/detail/error_code.hpp
@@ -393,25 +393,25 @@ public:
     //
 
     template<class E, class N1 = void, class N2 = typename detail::enable_if<std::is_error_code_enum<E>::value>::type>
-    inline friend bool operator==( error_code const & lhs, E rhs ) BOOST_NOEXCEPT
+    BOOST_SYSTEM_CONSTEXPR inline friend bool operator==( error_code const & lhs, E rhs ) BOOST_NOEXCEPT
     {
         return lhs == make_error_code( rhs );
     }
 
     template<class E, class N1 = void, class N2 = typename detail::enable_if<std::is_error_code_enum<E>::value>::type>
-    inline friend bool operator==( E lhs, error_code const & rhs ) BOOST_NOEXCEPT
+    BOOST_SYSTEM_CONSTEXPR inline friend bool operator==( E lhs, error_code const & rhs ) BOOST_NOEXCEPT
     {
         return make_error_code( lhs ) == rhs;
     }
 
     template<class E, class N1 = void, class N2 = typename detail::enable_if<std::is_error_code_enum<E>::value>::type>
-    inline friend bool operator!=( error_code const & lhs, E rhs ) BOOST_NOEXCEPT
+    BOOST_SYSTEM_CONSTEXPR inline friend bool operator!=( error_code const & lhs, E rhs ) BOOST_NOEXCEPT
     {
         return !( lhs == rhs );
     }
 
     template<class E, class N1 = void, class N2 = typename detail::enable_if<std::is_error_code_enum<E>::value>::type>
-    inline friend bool operator!=( E lhs, error_code const & rhs ) BOOST_NOEXCEPT
+    BOOST_SYSTEM_CONSTEXPR inline friend bool operator!=( E lhs, error_code const & rhs ) BOOST_NOEXCEPT
     {
         return !( lhs == rhs );
     }