Simplify some type traits:

Several type traits are simplified and optimized to
reduce compile time and memory consumption.

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

CHANGELOG.md

@@ -2,6 +2,7 @@ Version 191:
 
 * Add bind_front_handler
 * Use bind_front_handler
+* Simplify some type traits
 
 --------------------------------------------------------------------------------
 

include/boost/beast/core/detail/type_traits.hpp

@@ -12,6 +12,7 @@
 
 #include <boost/beast/core/error.hpp>
 #include <boost/asio/buffer.hpp>
+#include <boost/mp11/function.hpp>
 #include <boost/type_traits.hpp>
 #include <iterator>
 #include <tuple>
@@ -90,34 +91,6 @@ accept_rv(T){}
 
 //------------------------------------------------------------------------------
 
-template<unsigned N, class T, class... Tn>
-struct repeat_tuple_impl
-{
-    using type = typename repeat_tuple_impl<
-        N - 1, T, T, Tn...>::type;
-};
-
-template<class T, class... Tn>
-struct repeat_tuple_impl<0, T, Tn...>
-{
-    using type = std::tuple<T, Tn...>;
-};
-
-template<unsigned N, class T>
-struct repeat_tuple
-{
-    using type =
-        typename repeat_tuple_impl<N-1, T>::type;
-};
-
-template<class T>
-struct repeat_tuple<0, T>
-{
-    using type = std::tuple<>;
-};
-
-//------------------------------------------------------------------------------
-
 template<class R, class C, class ...A>
 auto
 is_invocable_test(C&& c, int, A&& ...a)
@@ -307,16 +280,9 @@ struct is_all_const_buffer_sequence<B>
 {
 };
 
-template<class... Bn>
+//
-struct common_buffers_type
+// Returns the iterator type of a buffer sequence
-{
+//
-    using type = typename std::conditional<
-        boost::is_convertible<std::tuple<Bn...>,
-            typename repeat_tuple<sizeof...(Bn),
-                boost::asio::mutable_buffer>::type>::value,
-                    boost::asio::mutable_buffer,
-                        boost::asio::const_buffer>::type;
-};
 
 template<class B>
 struct buffer_sequence_iterator
@@ -326,6 +292,60 @@ struct buffer_sequence_iterator
             std::declval<B const&>()));
 };
 
+template<>
+struct buffer_sequence_iterator<
+    boost::asio::const_buffer>
+{
+    using type = boost::asio::const_buffer const*;
+};
+
+template<>
+struct buffer_sequence_iterator<
+    boost::asio::mutable_buffer>
+{
+    using type = boost::asio::mutable_buffer const*;
+};
+
+//
+// Returns the value type of the iterator of a buffer sequence
+//
+
+template<class B>
+struct buffer_sequence_value_type
+{
+    using type = typename std::iterator_traits<
+        typename buffer_sequence_iterator<B>::type
+            >::value_type;
+};
+
+template<>
+struct buffer_sequence_value_type<
+    boost::asio::const_buffer const*>
+{
+    using type = boost::asio::const_buffer;
+};
+
+template<>
+struct buffer_sequence_value_type<
+    boost::asio::mutable_buffer const*>
+{
+    using type = boost::asio::mutable_buffer;
+};
+
+//
+
+template<class... Bn>
+struct common_buffers_type
+{
+    using type = typename std::conditional<
+        mp11::mp_and<
+            boost::is_convertible<
+                typename buffer_sequence_value_type<Bn>::type,
+                boost::asio::mutable_buffer>...>::value,
+            boost::asio::mutable_buffer,
+            boost::asio::const_buffer>::type;
+};
+
 // Types that meet the requirements,
 // for use with std::declval only.
 struct StreamHandler

test/beast/core/buffers_cat.cpp

@@ -224,7 +224,7 @@ public:
         using boost::beast::buffers_suffix;
         using boost::asio::buffer;
         using boost::asio::const_buffer;
-   
+
         char out[64];
         std::array<const_buffer, 2> buffers{
             {buffer("Hello, "), buffer("world!")}};
@@ -283,8 +283,8 @@ public:
             mutable_buffer,
             decltype(buffers_cat(
                 std::declval<mutable_buffer>(),
-                std::declval<user_defined>(),
+                std::declval<mutable_buffer>(),
-                std::declval<mutable_buffer>()
+                std::declval<std::vector<mutable_buffer>>()
                     ))::value_type>::value);
 
         // Ensure that concatenating mixed buffer