tidy up type traits

Author: vinniefalco

include/boost/buffers/detail/type_traits.hpp

@@ -18,6 +18,15 @@ namespace boost {
 namespace buffers {
 namespace detail {
 
+#if defined(__cpp_lib_concepts) && __cpp_lib_concepts >= 201907
+
+template<class T>
+using is_bidirectional_iterator = std::is_bidirectional_iterator<T>;
+
+#else
+
+// C++11 version of is_bidirectional_iterator
+
 // Alias for true_type if T is a BidirectionalIterator
 template<class T, class = void>
 struct is_bidirectional_iterator : std::false_type
@@ -88,6 +97,8 @@ struct is_bidirectional_iterator<T, void_t<decltype(
 {
 };
 
+#endif
+
 } // detail
 } // buffers
 } // boost

include/boost/buffers/range.hpp

@@ -79,6 +79,8 @@ constexpr struct
     }
 } begin {};
 
+//------------------------------------------------------------------------------
+
 /** Return an iterator to the end of the buffer sequence.
 */
 constexpr struct