From 23de20c23049245d1e7d8a215167eff66dda719f Mon Sep 17 00:00:00 2001
From: jzmaddock <john@johnmaddock.co.uk>
Date: Fri, 5 Mar 2021 12:59:22 +0000
Subject: [PATCH] Older libsc++ versions have incomplete <span>.

---
 include/boost/config/stdlib/libcpp.hpp | 5 +++++
 1 file changed, 5 insertions(+)

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