Tidy up read_size_helper and dynamic buffers

fix #376

test/core/buffer_test.hpp

@@ -9,6 +9,8 @@
 #define BEAST_TEST_BUFFER_TEST_HPP
 
 #include <beast/core/type_traits.hpp>
+#include <beast/core/detail/read_size_helper.hpp>
+#include <beast/core/detail/type_traits.hpp>
 #include <boost/asio/buffer.hpp>
 #include <algorithm>
 #include <type_traits>
@@ -83,6 +85,32 @@ size_rev_post(ConstBufferSequence const& buffers)
     return n;
 }
 
+namespace has_read_size_helper
+{
+    using beast::detail::read_size_helper;
+
+    template<class T, class = void>
+    struct trait : std::false_type {};
+
+    template<class T>
+    struct trait<T, decltype(
+        std::declval<std::size_t&>() =
+            read_size_helper(std::declval<T>(), 0),
+        (void)0)> : std::true_type
+    {
+    };
+}
+
+// Make sure read_size_helper works
+template<class DynamicBuffer>
+inline
+void
+check_read_size_helper()
+{
+    static_assert(has_read_size_helper::trait<DynamicBuffer>::value,
+            "Missing read_size_helper for dynamic buffer");
+}
+
 } // test
 } // beast