diff --git a/include/boost/config/stdlib/libcpp.hpp b/include/boost/config/stdlib/libcpp.hpp
index b3055921..1d824a73 100644
--- a/include/boost/config/stdlib/libcpp.hpp
+++ b/include/boost/config/stdlib/libcpp.hpp
@@ -158,6 +158,11 @@
 #  define BOOST_NO_CXX20_HDR_SEMAPHORE
 #endif
 
+# if(_LIBCPP_VERSION < 9000) && !defined(BOOST_NO_CXX20_HDR_SOURCE_SPAN)
+// as_writable_bytes is missing.
+#  define BOOST_NO_CXX20_HDR_SOURCE_SPAN
+#endif
+
 #else
 #define BOOST_NO_CXX17_STD_INVOKE      // Invoke support is incomplete (no invoke_result)
 #define BOOST_NO_CXX17_HDR_EXECUTION