get_lowest_layer is a type alias:

fix #941, fix #1016

`get_lowest_layer` is now a type alias for the
lowest layer instead of a struct with a nested type.

Actions required:

* Replace instances of `typename get_lowest_layer<T>::type`
  with `get_lowest_layer<T>`.

CHANGELOG.md

@@ -7,6 +7,16 @@ Version 158:
 * DynamicBuffer input areas are not mutable
 * Tidy up some documentation
 
+API Changes:
+
+* get_lowest_layer is a type alias
+
+Actions required:
+
+* Replace instances of `typename get_lowest_layer<T>::type`
+  with `get_lowest_layer<T>`.
+
+
 --------------------------------------------------------------------------------
 
 Version 157:

doc/qbk/09_releases.qbk

@@ -160,7 +160,9 @@ to update to the latest Boost release.
   Actions required: do not attempt to write to input areas of dynamic
   buffers.
 
-
+* ([\issue 941]) `get_lowest_layer` is now a type alias.
+  Actions required: Replace instances of `typename get_lowest_layer<T>::type`
+  with `get_lowest_layer<T>`.
 
 [heading Boost 1.66]
 

include/boost/beast/core/buffered_read_stream.hpp

@@ -111,8 +111,7 @@ public:
         typename std::remove_reference<Stream>::type;
 
     /// The type of the lowest layer.
-    using lowest_layer_type =
+    using lowest_layer_type = get_lowest_layer<next_layer_type>;
-        typename get_lowest_layer<next_layer_type>::type;
 
     /** Move constructor.
 

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

@@ -255,6 +255,21 @@ max_all(U0 u0, U1 u1, UN... un)
 
 //------------------------------------------------------------------------------
 
+template<class T, class = void>
+struct get_lowest_layer_helper
+{
+    using type = T;
+};
+
+template<class T>
+struct get_lowest_layer_helper<T,
+    void_t<typename T::lowest_layer_type>>
+{
+    using type = typename T::lowest_layer_type;
+};
+
+//------------------------------------------------------------------------------
+
 //
 // buffer concepts
 //

include/boost/beast/core/type_traits.hpp

@@ -122,10 +122,10 @@ struct has_get_executor<T, beast::detail::void_t<decltype(
     (void)0)>> : std::true_type {};
 #endif
 
-/** Returns `T::lowest_layer_type` if it exists, else `T`
+/** Alias for `T::lowest_layer_type` if it exists, else `T`
 
-    This will contain a nested `type` equal to `T::lowest_layer_type`
+    This will be a type alias for `T::lowest_layer_type`
-    if it exists, else `type` will be equal to `T`.
+    if it exists, else it will be an alias for `T`.
 
     @par Example
 
@@ -136,7 +136,7 @@ struct has_get_executor<T, beast::detail::void_t<decltype(
     struct stream_wrapper
     {
         using next_layer_type = typename std::remove_reference<Stream>::type;
-        using lowest_layer_type = typename get_lowest_layer<stream_type>::type;
+        using lowest_layer_type = get_lowest_layer<stream_type>;
     };
     @endcode
 
@@ -146,25 +146,15 @@ struct has_get_executor<T, beast::detail::void_t<decltype(
     /// Alias for `std::true_type` if `T` wraps another stream
     template<class T>
     using is_stream_wrapper : std::integral_constant<bool,
-        ! std::is_same<T, typename get_lowest_layer<T>::type>::value> {};
+        ! std::is_same<T, get_lowest_layer<T>>::value> {};
     @endcode
 */
 #if BOOST_BEAST_DOXYGEN
 template<class T>
 struct get_lowest_layer;
 #else
-template<class T, class = void>
-struct get_lowest_layer
-{
-    using type = T;
-};
-
 template<class T>
-struct get_lowest_layer<T, detail::void_t<
+using get_lowest_layer = typename detail::get_lowest_layer_helper<T>::type;
-    typename T::lowest_layer_type>>
-{
-    using type = typename T::lowest_layer_type;
-};
 #endif
 
 /** Determine if `T` meets the requirements of @b AsyncReadStream.

include/boost/beast/websocket/stream.hpp

@@ -225,8 +225,7 @@ public:
         typename std::remove_reference<NextLayer>::type;
 
     /// The type of the lowest layer.
-    using lowest_layer_type =
+    using lowest_layer_type = get_lowest_layer<next_layer_type>;
-        typename get_lowest_layer<next_layer_type>::type;
 
     /// The type of the executor associated with the object.
     using executor_type = typename next_layer_type::executor_type;

test/beast/core/type_traits.cpp

@@ -71,8 +71,7 @@ struct F3
     using next_layer_type =
         typename std::remove_reference<F>::type;
 
-    using lowest_layer_type = typename
+    using lowest_layer_type = get_lowest_layer<next_layer_type>;
-        get_lowest_layer<next_layer_type>::type;
 };
 
 template<class F>
@@ -81,18 +80,18 @@ struct F4
     using next_layer_type =
         typename std::remove_reference<F>::type;
 
-    using lowest_layer_type = typename
+    using lowest_layer_type =
-        get_lowest_layer<next_layer_type>::type;
+        get_lowest_layer<next_layer_type>;
 };
 
-BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F1>::type, F1>::value);
+BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F1>, F1>::value);
-BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F2>::type, F2>::value);
+BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F2>, F2>::value);
-BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F3<F1>>::type, F1>::value);
+BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F3<F1>>, F1>::value);
-BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F3<F2>>::type, F2>::value);
+BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F3<F2>>, F2>::value);
-BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F4<F1>>::type, F1>::value);
+BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F4<F1>>, F1>::value);
-BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F4<F2>>::type, F2>::value);
+BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F4<F2>>, F2>::value);
-BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F4<F3<F1>>>::type, F1>::value);
+BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F4<F3<F1>>>, F1>::value);
-BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F4<F3<F2>>>::type, F2>::value);
+BOOST_STATIC_ASSERT(std::is_same<get_lowest_layer<F4<F3<F2>>>, F2>::value);
 
 //
 // min_all, max_all