diff --git a/include/asio/error.hpp b/include/asio/error.hpp
index 596c9b9860..23e50bb812 100644
--- a/include/asio/error.hpp
+++ b/include/asio/error.hpp
@@ -357,9 +357,10 @@ inline asio::error_code make_error_code(misc_errors e)
 
 // boostify: non-boost code starts here
 namespace detail {
-
+#if !defined(ASIO_ERROR_NUMBER_TO_CONDITION_DECLARED)
+#define ASIO_ERROR_NUMBER_TO_CONDITION_DECLARED
 ASIO_DECL std::error_condition error_number_to_condition(int ev);
-
+#endif
 } // namespace detail
 // boostify: non-boost code ends here
 } // namespace error
diff --git a/include/asio/impl/error_code.ipp b/include/asio/impl/error_code.ipp
index 69ec24ea17..cfb84dc9f5 100644
--- a/include/asio/impl/error_code.ipp
+++ b/include/asio/impl/error_code.ipp
@@ -35,7 +35,11 @@ namespace asio {
 namespace error {
 namespace detail {
 
+    // Forward declaration required when error.hpp has not been included.
+#if !defined(ASIO_ERROR_NUMBER_TO_CONDITION_DECLARED)
+#define ASIO_ERROR_NUMBER_TO_CONDITION_DECLARED
 ASIO_DECL std::error_condition error_number_to_condition(int ev);
+#endif
 
 } // namespace detail
 } // namespace error