add lowest_layer() that's also present in ssl::stream

include/boost/wintls/stream.hpp

@@ -43,6 +43,11 @@ class stream {
   /// The type of the next layer.
   using next_layer_type = typename std::remove_reference<NextLayer>::type;
 
+#if !defined(BOOST_ASIO_NO_EXTENSIONS)
+  /// The type of the lowest layer.
+  typedef typename next_layer_type::lowest_layer_type lowest_layer_type;
+#endif
+
   /// The type of the executor associated with the object.
   using executor_type = typename std::remove_reference<next_layer_type>::type::executor_type;
 
@@ -100,6 +105,34 @@ class stream {
     return next_layer_;
   }
 
+#if !defined(BOOST_ASIO_NO_EXTENSIONS)
+  /// Get a reference to the lowest layer.
+  /**
+   * This function returns a reference to the lowest layer in a stack of
+   * stream layers.
+   *
+   * @return A reference to the lowest layer in the stack of stream layers.
+   * Ownership is not transferred to the caller.
+   */
+  lowest_layer_type& lowest_layer()
+  {
+	  return next_layer_.lowest_layer();
+  }
+
+  /// Get a reference to the lowest layer.
+  /**
+   * This function returns a reference to the lowest layer in a stack of
+   * stream layers.
+   *
+   * @return A reference to the lowest layer in the stack of stream layers.
+   * Ownership is not transferred to the caller.
+   */
+  const lowest_layer_type& lowest_layer() const
+  {
+	  return next_layer_.lowest_layer();
+  }
+#endif
+
   /** Set SNI hostname
    *
    * Sets the SNI hostname the client will use for requesting and