Suppress unused typedef warning when using clang.

asio/include/asio/detail/handler_type_requirements.hpp

diff --git a/asio/include/asio/detail/handler_type_requirements.hpp b/asio/include/asio/detail/handler_type_requirements.hpp
index 64b80a1..04bab88 100644
--- a/asio/include/asio/detail/handler_type_requirements.hpp
+++ b/asio/include/asio/detail/handler_type_requirements.hpp
@@ -53,8 +53,10 @@
 # include "asio/handler_type.hpp"
 #endif // defined(ASIO_ENABLE_HANDLER_TYPE_REQUIREMENTS)
 
-// Newer gcc needs special treatment to suppress unused typedef warnings.
-#if defined(__GNUC__)
+// Newer gcc, clang need special treatment to suppress unused typedef warnings.
+#if defined(__clang__) && (__clang_major__ >= 7)
+# define ASIO_UNUSED_TYPEDEF __attribute__((__unused__))
+#elif defined(__GNUC__)
 # if ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)) || (__GNUC__ > 4)
 #  define ASIO_UNUSED_TYPEDEF __attribute__((__unused__))
 # endif // ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)) || (__GNUC__ > 4)