Use SFINAE on return values

CHANGELOG

@@ -1,3 +1,6 @@
 1.0.0-b6
 
+* Use SFINAE on return values
+* Use beast::error_code instead of nested types
+
 --------------------------------------------------------------------------------

TODO.txt

@@ -1,9 +1,5 @@
 * Add writer::prepare(msg&) interface to set Content-Type
 
-General:
-* Use SFINAE on return values (search for "class =")
-* Remove http,websocket error_code types and use the one in <beast/error.hpp>
-
 Boost.Http
 * Use enum instead of bool in isRequest
 * move version to a subclass of message

include/beast/core/to_string.hpp

@@ -25,15 +25,16 @@ namespace beast {
     @return A string representing the contents of the input area.
 
     @note This function participates in overload resolution only if
-    the streambuf parameter meets the requirements of @b `Streambuf`.
+    the buffers parameter meets the requirements of @b `ConstBufferSequence`.
 */
-template<class ConstBufferSequence
-#if ! GENERATING_DOCS
-    ,class = std::enable_if<is_ConstBufferSequence<
-        ConstBufferSequence>::value>
-#endif
->
+template<class ConstBufferSequence>
+#if GENERATING_DOCS
 std::string
+#else
+typename std::enable_if<
+    is_ConstBufferSequence<ConstBufferSequence>::value,
+        std::string>::type
+#endif
 to_string(ConstBufferSequence const& buffers)
 {
     using boost::asio::buffer_cast;

include/beast/http/basic_headers.hpp

@@ -401,10 +401,9 @@ public:
         If a field value already exists the new value will be
         extended as per RFC2616 Section 4.2.
     */
-    template<class T,
-        class = typename std::enable_if<
-            ! std::is_constructible<boost::string_ref, T>::value>::type>
-    void
+    template<class T>
+    typename std::enable_if<
+        ! std::is_constructible<boost::string_ref, T>::value>::type
     insert(boost::string_ref name, T const& value)
     {
         insert(name,
@@ -414,7 +413,7 @@ public:
     /** Replace a field value.
 
         The current field value, if any, is removed. Then the
-        specified value is inserted as if by insert(field, value).
+        specified value is inserted as if by `insert(field, value)`.
     */
     void
     replace(boost::string_ref const& name,
@@ -423,12 +422,11 @@ public:
     /** Replace a field value.
 
         The current field value, if any, is removed. Then the
-        specified value is inserted as if by insert(field, value).
+        specified value is inserted as if by `insert(field, value)`.
     */
-    template<class T,
-        class = typename std::enable_if<
-            ! std::is_constructible<boost::string_ref, T>::value>::type>
-    void
+    template<class T>
+    typename std::enable_if<
+        ! std::is_constructible<boost::string_ref, T>::value>::type
     replace(boost::string_ref const& name, T const& value)
     {
         replace(name,