diff --git a/include/boost/system/error_code.hpp b/include/boost/system/error_code.hpp
index 920d87e..81965fb 100644
--- a/include/boost/system/error_code.hpp
+++ b/include/boost/system/error_code.hpp
@@ -442,9 +442,19 @@ inline const error_category & generic_category() BOOST_SYSTEM_NOEXCEPT
 namespace detail
 {
 
+#if defined(BOOST_SYSTEM_SOURCE)
+
+// clang++ requires a strictly matching declaration
+BOOST_SYSTEM_DECL extern system_error_category system_category_instance;
+BOOST_SYSTEM_DECL extern generic_error_category generic_category_instance;
+
+#else
+
 extern system_error_category system_category_instance;
 extern generic_error_category generic_category_instance;
 
+#endif
+
 } // namespace detail
 
 constexpr const error_category & system_category() BOOST_SYSTEM_NOEXCEPT