From 8a2235011962ac9eb7e2794053e4081611cf729e Mon Sep 17 00:00:00 2001
From: Vinnie Falco <vinnie.falco@gmail.com>
Date: Sat, 3 Jun 2017 15:36:56 -0700
Subject: [PATCH] Tidy up traits

fix #406
---
 CHANGELOG.md                              |   1 +
 include/beast/core/detail/type_traits.hpp | 134 ++++-----
 include/beast/core/type_traits.hpp        | 326 +++++++++++++++++++---
 include/beast/http/detail/type_traits.hpp |  32 +++
 include/beast/http/type_traits.hpp        |  75 +++--
 test/core/type_traits.cpp                 |  21 +-
 6 files changed, 445 insertions(+), 144 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 234a73c3..deebdcd5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@ Version 48
 
 * Make buffer_prefix_view public
 * Remove detail::sync_ostream
+* Tidy up core type traits
 
 API Changes:
 
diff --git a/include/beast/core/detail/type_traits.hpp b/include/beast/core/detail/type_traits.hpp
index 13511848..065e3a00 100644
--- a/include/beast/core/detail/type_traits.hpp
+++ b/include/beast/core/detail/type_traits.hpp
@@ -16,6 +16,31 @@
 #include <type_traits>
 #include <string>
 
+// A few workarounds to keep things working
+
+namespace boost {
+namespace asio {
+
+// for has_get_io_service
+class io_service;
+
+// for is_dynamic_buffer
+template<class Allocator>
+class basic_streambuf;
+
+namespace detail {
+
+// for is_buffer_sequence
+template<class Buffer, class Buffers>
+class consuming_buffers;
+
+} // detail
+
+} // asio
+} // boost
+
+//------------------------------------------------------------------------------
+
 namespace beast {
 namespace detail {
 
@@ -32,6 +57,11 @@ struct make_void
 template<class... Ts>
 using void_t = typename make_void<Ts...>::type;
 
+template<class T>
+inline
+void
+accept_rv(T){}
+
 template<class... Ts>
 inline
 void
@@ -109,8 +139,7 @@ is_invocable_test(C&& c, long, A&& ...a);
 */
 /** @{ */
 template<class C, class F>
-struct is_invocable
-    : std::false_type
+struct is_invocable : std::false_type
 {
 };
 
@@ -145,72 +174,57 @@ using ConstBufferSequence =
 using MutableBufferSequence =
     BufferSequence<boost::asio::mutable_buffer>;
 
-template<class T, class BufferType>
-class is_buffer_sequence
+template<class T, class B, class = void>
+struct is_buffer_sequence : std::false_type {};
+
+template<class T, class B>
+struct is_buffer_sequence<T, B, void_t<decltype(
+    std::declval<typename T::value_type>(),
+    std::declval<typename T::const_iterator&>() =
+        std::declval<T const&>().begin(),
+    std::declval<typename T::const_iterator&>() =
+        std::declval<T const&>().end(),
+        (void)0)>> : std::integral_constant<bool,
+    std::is_convertible<typename T::value_type, B>::value &&
+#if 0
+    std::is_base_of<std::bidirectional_iterator_tag,
+        typename std::iterator_traits<
+            typename T::const_iterator>::iterator_category>::value
+#else
+    // workaround:
+    // boost::asio::detail::consuming_buffers::const_iterator
+    // is not bidirectional
+    std::is_base_of<std::forward_iterator_tag,
+        typename std::iterator_traits<
+            typename T::const_iterator>::iterator_category>::value
+#endif
+        >
 {
-    template<class U, class R = std::is_convertible<
-        typename U::value_type, BufferType> >
-    static R check1(int);
-    template<class>
-    static std::false_type check1(...);
-    using type1 = decltype(check1<T>(0));
-
-    template<class U, class R = std::is_base_of<
-    #if 0
-        std::bidirectional_iterator_tag,
-            typename std::iterator_traits<
-                typename U::const_iterator>::iterator_category>>
-    #else
-        // workaround:
-        // boost::asio::detail::consuming_buffers::const_iterator
-        // is not bidirectional
-        std::forward_iterator_tag,
-            typename std::iterator_traits<
-                typename U::const_iterator>::iterator_category>>
-    #endif
-    static R check2(int);
-    template<class>
-    static std::false_type check2(...);
-    using type2 = decltype(check2<T>(0));
-
-    template<class U, class R = typename
-        std::is_convertible<decltype(
-            std::declval<U>().begin()),
-                typename U::const_iterator>::type>
-    static R check3(int);
-    template<class>
-    static std::false_type check3(...);
-    using type3 = decltype(check3<T>(0));
-
-    template<class U, class R = typename std::is_convertible<decltype(
-        std::declval<U>().end()),
-            typename U::const_iterator>::type>
-    static R check4(int);
-    template<class>
-    static std::false_type check4(...);
-    using type4 = decltype(check4<T>(0));
-
-public:
-    using type = std::integral_constant<bool,
-        std::is_copy_constructible<T>::value &&
-        std::is_destructible<T>::value &&
-        type1::value && type2::value &&
-        type3::value && type4::value>;
 };
 
-//------------------------------------------------------------------------------
+#if 0
+// workaround:
+// boost::asio::detail::consuming_buffers::const_iterator
+// is not bidirectional
+template<class Buffer, class Buffers, class B>
+struct is_buffer_sequence<
+    boost::asio::detail::consuming_buffers<Buffer, Buffers>>
+        : std::true_type
+{
+};     
+#endif
 
 template<class B1, class... Bn>
 struct is_all_const_buffer_sequence
     : std::integral_constant<bool,
-        is_buffer_sequence<B1, boost::asio::const_buffer>::type::value &&
+        is_buffer_sequence<B1, boost::asio::const_buffer>::value &&
         is_all_const_buffer_sequence<Bn...>::value>
 {
 };
 
 template<class B1>
 struct is_all_const_buffer_sequence<B1>
-    : is_buffer_sequence<B1, boost::asio::const_buffer>::type
+    : is_buffer_sequence<B1, boost::asio::const_buffer>
 {
 };
 
@@ -345,14 +359,4 @@ public:
 } // detail
 } // beast
 
-namespace boost {
-namespace asio {
-
-// for is_dynamic_buffer
-template<class Allocator>
-class basic_streambuf;
-
-} // asio
-} // boost
-
 #endif
diff --git a/include/beast/core/type_traits.hpp b/include/beast/core/type_traits.hpp
index 87ff89b8..4596c260 100644
--- a/include/beast/core/type_traits.hpp
+++ b/include/beast/core/type_traits.hpp
@@ -21,31 +21,86 @@ namespace beast {
 //
 //------------------------------------------------------------------------------
 
-/// Determine if `T` meets the requirements of @b ConstBufferSequence.
+/** Determine if `T` meets the requirements of @b ConstBufferSequence.
+
+    @par Example
+    Use with `static_assert`:
+    @code
+        template<class ConstBufferSequence>
+        void f(ConstBufferSequence const& buffers)
+        {
+            static_assert(is_const_buffer_sequence<ConstBufferSequence>::value,
+                "ConstBufferSequence requirements not met");
+            ...
+        }
+    @endcode
+    Use with `std::enable_if`
+    @code
+        template<class ConstBufferSequence>
+        typename std::enable_if<is_const_buffer_sequence<ConstBufferSequence>::value>::type
+        f(ConstBufferSequence const& buffers);
+    @endcode
+*/
 template<class T>
 #if BEAST_DOXYGEN
 struct is_const_buffer_sequence : std::integral_constant<bool, ...>
 #else
 struct is_const_buffer_sequence :
-    detail::is_buffer_sequence<T,
-        boost::asio::const_buffer>::type
+    detail::is_buffer_sequence<T, boost::asio::const_buffer>
 #endif
 {
 };
 
-/// Determine if `T` meets the requirements of @b MutableBufferSequence.
+/** Determine if `T` meets the requirements of @b MutableBufferSequence.
+
+    @par Example
+    Use with `static_assert`:
+    @code
+        template<class MutableBufferSequence>
+        void f(MutableBufferSequence const& buffers)
+        {
+            static_assert(is_const_buffer_sequence<MutableBufferSequence>::value,
+                "MutableBufferSequence requirements not met");
+            ...
+        }
+    @endcode
+    Use with `std::enable_if`
+    @code
+        template<class MutableBufferSequence>
+        typename std::enable_if<is_mutable_buffer_sequence<MutableBufferSequence>::value>::type
+        f(MutableBufferSequence const& buffers);
+    @endcode
+*/
 template<class T>
 #if BEAST_DOXYGEN
 struct is_mutable_buffer_sequence : std::integral_constant<bool, ...>
 #else
 struct is_mutable_buffer_sequence :
-    detail::is_buffer_sequence<T,
-        boost::asio::mutable_buffer>::type
+    detail::is_buffer_sequence<T, boost::asio::mutable_buffer>
 #endif
 {
 };
 
-/// Determine if `T` meets the requirements of @b DynamicBuffer.
+/** Determine if `T` meets the requirements of @b DynamicBuffer.
+
+    @par Example
+    Use with `static_assert`:
+    @code
+        template<class DynamicBuffer>
+        void f(DynamicBuffer& buffer)
+        {
+            static_assert(is_dynamic_buffer<DynamicBuffer>::value,
+                "DynamicBuffer requirements not met");
+            ...
+        }
+    @endcode
+    Use with `std::enable_if`
+    @code
+        template<class DynamicBuffer>
+        typename std::enable_if<is_dynamic_buffer<DynamicBuffer>::value>::type
+        f(DynamicBuffer const& buffer);
+    @endcode
+*/
 #if BEAST_DOXYGEN
 template<class T>
 struct is_dynamic_buffer : std::integral_constant<bool, ...> {};
@@ -54,8 +109,7 @@ template<class T, class = void>
 struct is_dynamic_buffer : std::false_type {};
 
 template<class T>
-struct is_dynamic_buffer<T, beast::detail::void_t<
-        decltype(
+struct is_dynamic_buffer<T, detail::void_t<decltype(
     std::declval<std::size_t&>() =
         std::declval<T const&>().size(),
     std::declval<std::size_t&>() =
@@ -64,7 +118,7 @@ struct is_dynamic_buffer<T, beast::detail::void_t<
         std::declval<T const&>().capacity(),
     std::declval<T&>().commit(std::declval<std::size_t>()),
     std::declval<T&>().consume(std::declval<std::size_t>()),
-        (void)0)>> : std::integral_constant<bool,
+        (void)0)> > : std::integral_constant<bool,
     is_const_buffer_sequence<
         typename T::const_buffers_type>::value &&
     is_mutable_buffer_sequence<
@@ -93,32 +147,77 @@ struct is_dynamic_buffer<
 //
 //------------------------------------------------------------------------------
 
-/// Determine if `T` meets the requirements of @b CompletionHandler.
+/** Determine if `T` meets the requirements of @b CompletionHandler.
+
+    This metafunction will be equivalent to `std::true_type` if `T`
+    meets the requirements for a completion handler callable with
+    the given signature.
+
+    @par Example
+    @code
+    struct handler
+    {
+        void operator()(error_code&);
+    };
+
+    static_assert(is_completion_handler<handler, void(error_code&)>::value,
+        "Not a completion handler");
+    @endcode
+*/
 template<class T, class Signature>
 #if BEAST_DOXYGEN
 using is_completion_handler = std::integral_constant<bool, ...>;
 #else
 using is_completion_handler = std::integral_constant<bool,
     std::is_copy_constructible<typename std::decay<T>::type>::value &&
-        detail::is_invocable<T, Signature>::value>;
+    detail::is_invocable<T, Signature>::value>;
 #endif
 
-
 //------------------------------------------------------------------------------
 //
 // Stream concepts
 //
 //------------------------------------------------------------------------------
 
-/// Determine if `T` has the `get_io_service` member.
-template<class T>
+/** Determine if `T` has the `get_io_service` member.
+
+    @par Example
+    @code
+    struct stream
+    {
+        boost::asio::io_service& get_io_service();
+    };
+
+    static_assert(has_get_io_service<stream>::value,
+        "Missing get_io_service member");
+    @endcode
+*/
 #if BEAST_DOXYGEN
+template<class T>
 struct has_get_io_service : std::integral_constant<bool, ...>{};
 #else
-using has_get_io_service = typename detail::has_get_io_service<T>::type;
+template<class T, class = void>
+struct has_get_io_service : std::false_type {};
+
+template<class T>
+struct has_get_io_service<T, beast::detail::void_t<decltype(
+    detail::accept_rv<boost::asio::io_service&>(
+        std::declval<T&>().get_io_service()),
+    (void)0)>> : std::true_type {};
 #endif
 
-/// Returns `T::lowest_layer_type` if it exists, else `T`
+/** Returns `T::lowest_layer_type` if it exists, else `T`
+
+    @par Example
+    @code
+    template<class Stream>
+    struct stream_wrapper
+    {
+        using next_layer_type = typename std::remove_reference<Stream>::type;
+        using lowest_layer_type = typename get_lowest_layer<stream_type>::type;
+    };
+    @endcode
+*/
 #if BEAST_DOXYGEN
 template<class T>
 struct get_lowest_layer;
@@ -137,52 +236,213 @@ struct get_lowest_layer<T, detail::void_t<
 };
 #endif
 
-/// Determine if `T` meets the requirements of @b AsyncReadStream.
-template<class T>
+/** Determine if `T` meets the requirements of @b AsyncReadStream.
+
+    @par Example
+    Use with `static_assert`:
+    @code
+        template<class AsyncReadStream>
+        void f(AsyncReadStream& stream)
+        {
+            static_assert(is_async_read_stream<AsyncReadStream>::value,
+                "AsyncReadStream requirements not met");
+            ...
+        }
+    @endcode
+    Use with `std::enable_if`
+    @code
+        template<class AsyncReadStream>
+        typename std::enable_if<is_async_read_stream<AsyncReadStream>::value>::type
+        f(AsyncReadStream& stream);
+    @endcode
+*/
 #if BEAST_DOXYGEN
+template<class T>
 struct is_async_read_stream : std::integral_constant<bool, ...>{};
 #else
-using is_async_read_stream = typename detail::is_async_read_stream<T>::type;
+template<class T, class = void>
+struct is_async_read_stream : std::false_type {};
+
+template<class T>
+struct is_async_read_stream<T, detail::void_t<decltype(
+    std::declval<T>().async_read_some(
+        std::declval<detail::MutableBufferSequence>(),
+        std::declval<detail::ReadHandler>()),
+            (void)0)>> : std::integral_constant<bool,
+    has_get_io_service<T>::value
+        > {};
 #endif
 
-/// Determine if `T` meets the requirements of @b AsyncWriteStream.
-template<class T>
+/** Determine if `T` meets the requirements of @b AsyncWriteStream.
+
+    @par Example
+    Use with `static_assert`:
+    @code
+        template<class AsyncWriteStream>
+        void f(AsyncWriteStream& stream)
+        {
+            static_assert(is_async_write_stream<AsyncWriteStream>::value,
+                "AsyncWriteStream requirements not met");
+            ...
+        }
+    @endcode
+    Use with `std::enable_if`
+    @code
+        template<class AsyncWriteStream>
+        typename std::enable_if<is_async_write_stream<AsyncWriteStream>::value>::type
+        f(AsyncWriteStream& stream);
+    @endcode
+*/
 #if BEAST_DOXYGEN
+template<class T>
 struct is_async_write_stream : std::integral_constant<bool, ...>{};
 #else
-using is_async_write_stream = typename detail::is_async_write_stream<T>::type;
+template<class T, class = void>
+struct is_async_write_stream : std::false_type {};
+
+template<class T>
+struct is_async_write_stream<T, detail::void_t<decltype(
+    std::declval<T>().async_write_some(
+        std::declval<detail::ConstBufferSequence>(),
+        std::declval<detail::WriteHandler>()),
+            (void)0)>> : std::integral_constant<bool,
+    has_get_io_service<T>::value
+        > {};
 #endif
 
-/// Determine if `T` meets the requirements of @b SyncReadStream.
-template<class T>
+/** Determine if `T` meets the requirements of @b SyncReadStream.
+
+    @par Example
+    Use with `static_assert`:
+    @code
+        template<class SyncReadStream>
+        void f(SyncReadStream& stream)
+        {
+            static_assert(is_sync_read_stream<SyncReadStream>::value,
+                "SyncReadStream requirements not met");
+            ...
+        }
+    @endcode
+    Use with `std::enable_if`
+    @code
+        template<class SyncReadStream>
+        typename std::enable_if<is_sync_read_stream<SyncReadStream>::value>::type
+        f(SyncReadStream& stream);
+    @endcode
+*/
 #if BEAST_DOXYGEN
+template<class T>
 struct is_sync_read_stream : std::integral_constant<bool, ...>{};
 #else
-using is_sync_read_stream = typename detail::is_sync_read_stream<T>::type;
+template<class T, class = void>
+struct is_sync_read_stream : std::false_type {};
+
+template<class T>
+struct is_sync_read_stream<T, detail::void_t<decltype(
+    std::declval<std::size_t&>() = std::declval<T>().read_some(
+        std::declval<detail::MutableBufferSequence>()),
+    std::declval<std::size_t&>() = std::declval<T>().read_some(
+        std::declval<detail::MutableBufferSequence>(),
+        std::declval<boost::system::error_code&>()),
+            (void)0)>> : std::integral_constant<bool,
+    has_get_io_service<T>::value
+        > {};
 #endif
 
-/// Determine if `T` meets the requirements of @b SyncWriterStream.
-template<class T>
+/** Determine if `T` meets the requirements of @b SyncWriterStream.
+
+    @par Example
+    Use with `static_assert`:
+    @code
+        template<class SyncReadStream>
+        void f(SyncReadStream& stream)
+        {
+            static_assert(is_sync_read_stream<SyncReadStream>::value,
+                "SyncReadStream requirements not met");
+            ...
+        }
+    @endcode
+    Use with `std::enable_if`
+    @code
+        template<class SyncReadStream>
+        typename std::enable_if<is_sync_read_stream<SyncReadStream>::value>::type
+        f(SyncReadStream& stream);
+    @endcode
+*/
 #if BEAST_DOXYGEN
+template<class T>
 struct is_sync_write_stream : std::integral_constant<bool, ...>{};
 #else
-using is_sync_write_stream = typename detail::is_sync_write_stream<T>::type;
+template<class T, class = void>
+struct is_sync_write_stream : std::false_type {};
+
+template<class T>
+struct is_sync_write_stream<T, detail::void_t<decltype(
+    std::declval<std::size_t&>() = std::declval<T&>().write_some(
+        std::declval<detail::ConstBufferSequence>()),
+    std::declval<std::size_t&>() = std::declval<T&>().write_some(
+        std::declval<detail::ConstBufferSequence>(),
+        std::declval<boost::system::error_code&>()),
+            (void)0)>> : std::integral_constant<bool,
+    has_get_io_service<T>::value
+        > {};
 #endif
 
-/// Determine if `T` meets the requirements of @b AsyncStream.
-template<class T>
+/** Determine if `T` meets the requirements of @b AsyncStream.
+
+    @par Example
+    Use with `static_assert`:
+    @code
+        template<class AsyncStream>
+        void f(AsyncStream& stream)
+        {
+            static_assert(is_async_stream<AsyncStream>::value,
+                "AsyncStream requirements not met");
+            ...
+        }
+    @endcode
+    Use with `std::enable_if`
+    @code
+        template<class AsyncStream>
+        typename std::enable_if<is_async_stream<AsyncStream>::value>::type
+        f(AsyncStream& stream);
+    @endcode
+*/
 #if BEAST_DOXYGEN
+template<class T>
 struct is_async_stream : std::integral_constant<bool, ...>{};
 #else
+template<class T>
 using is_async_stream = std::integral_constant<bool,
     is_async_read_stream<T>::value && is_async_write_stream<T>::value>;
 #endif
 
-/// Determine if `T` meets the requirements of @b SyncStream.
-template<class T>
+/** Determine if `T` meets the requirements of @b SyncStream.
+
+
+    @par Example
+    Use with `static_assert`:
+    @code
+        template<class SyncStream>
+        void f(SyncStream& stream)
+        {
+            static_assert(is_sync_stream<SyncStream>::value,
+                "SyncStream requirements not met");
+            ...
+        }
+    @endcode
+    Use with `std::enable_if`
+    @code
+        template<class SyncStream>
+        typename std::enable_if<is_sync_stream<SyncStream>::value>::type
+        f(SyncStream& stream);
+    @endcode
+*/
 #if BEAST_DOXYGEN
+template<class T>
 struct is_sync_stream : std::integral_constant<bool, ...>{};
 #else
+template<class T>
 using is_sync_stream = std::integral_constant<bool,
     is_sync_read_stream<T>::value && is_sync_write_stream<T>::value>;
 #endif
diff --git a/include/beast/http/detail/type_traits.hpp b/include/beast/http/detail/type_traits.hpp
index afc6f681..d27f6fbc 100644
--- a/include/beast/http/detail/type_traits.hpp
+++ b/include/beast/http/detail/type_traits.hpp
@@ -16,6 +16,9 @@ namespace http {
 template<bool isRequest, class Fields>
 struct header;
 
+template<bool, class, class>
+struct message;
+
 namespace detail {
 
 template<class T>
@@ -32,6 +35,35 @@ public:
 template<class T>
 using is_header = typename is_header_impl<T>::type;
 
+struct fields_model
+{
+    string_view method() const;
+    string_view reason() const;
+    string_view target() const;
+};
+
+template<class T, class = beast::detail::void_t<>>
+struct has_value_type : std::false_type {};
+
+template<class T>
+struct has_value_type<T, beast::detail::void_t<
+    typename T::value_type
+        > > : std::true_type {};
+
+template<class T, class = beast::detail::void_t<>>
+struct has_content_length : std::false_type {};
+
+template<class T>
+struct has_content_length<T, beast::detail::void_t<decltype(
+    std::declval<T>().content_length()
+        )> > : std::true_type
+{
+    static_assert(std::is_convertible<
+        decltype(std::declval<T>().content_length()),
+            std::uint64_t>::value,
+        "Writer::content_length requirements not met");
+};
+
 } // detail
 } // http
 } // beast
diff --git a/include/beast/http/type_traits.hpp b/include/beast/http/type_traits.hpp
index 6aa51b2a..20a74930 100644
--- a/include/beast/http/type_traits.hpp
+++ b/include/beast/http/type_traits.hpp
@@ -12,6 +12,7 @@
 #include <beast/core/error.hpp>
 #include <beast/core/string_view.hpp>
 #include <beast/core/type_traits.hpp>
+#include <beast/http/detail/type_traits.hpp>
 #include <boost/asio/buffer.hpp>
 #include <boost/optional.hpp>
 #include <type_traits>
@@ -20,43 +21,23 @@
 namespace beast {
 namespace http {
 
-template<bool, class, class>
-struct message;
+/** Determine if `T` meets the requirements of @b Body.
 
-namespace detail {
+    This metafunction is equivalent to `std::true_type`
+    if `T` has a nested type named `value_type`.
 
-struct fields_model
-{
-    string_view method() const;
-    string_view reason() const;
-    string_view target() const;
-};
+    @tparam T The body type to test.
 
-template<class T, class = beast::detail::void_t<>>
-struct has_value_type : std::false_type {};
-
-template<class T>
-struct has_value_type<T, beast::detail::void_t<
-    typename T::value_type
-        > > : std::true_type {};
-
-template<class T, class = beast::detail::void_t<>>
-struct has_content_length : std::false_type {};
-
-template<class T>
-struct has_content_length<T, beast::detail::void_t<decltype(
-    std::declval<T>().content_length()
-        )> > : std::true_type
-{
-    static_assert(std::is_convertible<
-        decltype(std::declval<T>().content_length()),
-            std::uint64_t>::value,
-        "Writer::content_length requirements not met");
-};
-
-} // detail
-
-/// Determine if `T` meets the requirements of @b Body.
+    @par Example
+    @code
+    template<bool isRequest, class Body, class Fields>
+    void check_body(message<isRequest, Body, Fields> const&)
+    {
+        static_assert(is_body<Body>::value,
+            "Body requirements not met");
+    }
+    @endcode
+*/
 template<class T>
 #if BEAST_DOXYGEN
 struct is_body : std::integral_constant<bool, ...>{};
@@ -68,11 +49,20 @@ using is_body = detail::has_value_type<T>;
 
     This metafunction is equivalent to `std::true_type` if:
 
-    @li @b T has a nested type named `reader`
+    @li `T` has a nested type named `reader`
 
     @li The nested type meets the requirements of @b BodyReader.
 
     @tparam T The body type to test.
+
+    @par Example
+    @code
+    template<bool isRequest, class Body, class Fields>
+    void check_can_serialize(message<isRequest, Body, Fields> const&)
+    {
+        static_assert(is_body_reader<Body>::value,
+            "Cannot serialize Body, no reader");
+    @endcode
 */
 #if BEAST_DOXYGEN
 template<class T>
@@ -95,20 +85,27 @@ struct is_body_reader<T, beast::detail::void_t<
         typename T::reader::const_buffers_type>::value &&
     std::is_constructible<typename T::reader,
         message<true, T, detail::fields_model> const& >::value
-        >
-{
-};
+        > {};
 #endif
 
 /** Determine if a @b Body type has a writer.
 
     This metafunction is equivalent to `std::true_type` if:
 
-    @li @b T has a nested type named `writer`
+    @li `T` has a nested type named `writer`
 
     @li The nested type meets the requirements of @b BodyWriter.
 
     @tparam T The body type to test.
+
+    @par Example
+    @code
+    template<bool isRequest, class Body, class Fields>
+    void check_can_parse(message<isRequest, Body, Fields>&)
+    {
+        static_assert(is_body_writer<Body>::value,
+            "Cannot parse Body, no writer");
+    @endcode
 */
 #if BEAST_DOXYGEN
 template<class T>
diff --git a/test/core/type_traits.cpp b/test/core/type_traits.cpp
index d9ffba1f..4706155b 100644
--- a/test/core/type_traits.cpp
+++ b/test/core/type_traits.cpp
@@ -10,6 +10,7 @@
 
 #include <boost/asio/ip/tcp.hpp>
 #include <boost/asio/streambuf.hpp>
+#include <boost/asio/detail/consuming_buffers.hpp>
 
 namespace beast {
 
@@ -125,10 +126,16 @@ BOOST_STATIC_ASSERT(! is_completion_handler<H, void(void)>::value);
 // stream concepts
 //
 
-//namespace {
+namespace {
 
 using stream_type = boost::asio::ip::tcp::socket;
 
+struct not_a_stream
+{
+    void
+    get_io_service();
+};
+
 BOOST_STATIC_ASSERT(has_get_io_service<stream_type>::value);
 BOOST_STATIC_ASSERT(is_async_read_stream<stream_type>::value);
 BOOST_STATIC_ASSERT(is_async_write_stream<stream_type>::value);
@@ -137,12 +144,12 @@ BOOST_STATIC_ASSERT(is_sync_read_stream<stream_type>::value);
 BOOST_STATIC_ASSERT(is_sync_write_stream<stream_type>::value);
 BOOST_STATIC_ASSERT(is_sync_stream<stream_type>::value);
 
-BOOST_STATIC_ASSERT(! has_get_io_service<int>::value);
-BOOST_STATIC_ASSERT(! is_async_read_stream<int>::value);
-BOOST_STATIC_ASSERT(! is_async_write_stream<int>::value);
-BOOST_STATIC_ASSERT(! is_sync_read_stream<int>::value);
-BOOST_STATIC_ASSERT(! is_sync_write_stream<int>::value);
+BOOST_STATIC_ASSERT(! has_get_io_service<not_a_stream>::value);
+BOOST_STATIC_ASSERT(! is_async_read_stream<not_a_stream>::value);
+BOOST_STATIC_ASSERT(! is_async_write_stream<not_a_stream>::value);
+BOOST_STATIC_ASSERT(! is_sync_read_stream<not_a_stream>::value);
+BOOST_STATIC_ASSERT(! is_sync_write_stream<not_a_stream>::value);
 
-//} // (anonymous)
+} // (anonymous)
 
 } // beast