From 95f2a76c90d0b0bd660504fd16dd0c2df09442c6 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sat, 18 Dec 2021 07:43:33 +0200
Subject: [PATCH] Specialize std::basic_common_reference to enable
 std::common_reference and std::common_reference_with

---
 include/boost/core/detail/string_view.hpp | 29 +++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/include/boost/core/detail/string_view.hpp b/include/boost/core/detail/string_view.hpp
index fedfbc9..c8343c8 100644
--- a/include/boost/core/detail/string_view.hpp
+++ b/include/boost/core/detail/string_view.hpp
@@ -31,6 +31,10 @@
 #if !defined(BOOST_NO_CXX17_HDR_STRING_VIEW)
 # include <string_view>
 #endif
+#if BOOST_CXX_VERSION >= 202000L
+# include <type_traits>
+# include <version>
+#endif
 
 namespace boost
 {
@@ -1216,4 +1220,29 @@ typedef basic_string_view<char8_t> u8string_view;
 } // namespace core
 } // namespace boost
 
+// std::common_reference support
+// needed for iterators that have reference=string_view and value_type=std::string
+
+#if BOOST_CXX_VERSION >= 202000L && defined(__cpp_lib_concepts) && __cpp_lib_concepts >= 201907L
+
+template<class Ch, class A, template<class> class Q1, template<class> class Q2>
+struct std::basic_common_reference<
+    boost::core::basic_string_view<Ch>,
+    std::basic_string<Ch, std::char_traits<Ch>, A>,
+    Q1, Q2>
+{
+    using type = boost::core::basic_string_view<Ch>;
+};
+
+template<class Ch, class A, template<class> class Q1, template<class> class Q2>
+struct std::basic_common_reference<
+    std::basic_string<Ch, std::char_traits<Ch>, A>,
+    boost::core::basic_string_view<Ch>,
+    Q1, Q2>
+{
+    using type = boost::core::basic_string_view<Ch>;
+};
+
+#endif
+
 #endif  // #ifndef BOOST_CORE_STRING_VIEW_HPP_INCLUDED