Fix all async initiating functions with use_awaitable:

Add trait:
    is_completion_token_for

Fixes for
    buffered_read_stream

Add tests for
    basic_stream
    buffered_read_stream
    flat_stream
    http (all)
    websocket (all)

resolves #1920

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

@@ -10,6 +10,8 @@
 #ifndef BOOST_BEAST_DETAIL_IS_INVOCABLE_HPP
 #define BOOST_BEAST_DETAIL_IS_INVOCABLE_HPP
 
+#include <boost/asio/async_result.hpp>
+#include <boost/type_traits/make_void.hpp>
 #include <type_traits>
 #include <utility>
 
@@ -51,6 +53,26 @@ struct is_invocable<C, R(A...)>
 };
 /** @} */
 
+template<class CompletionToken, class Signature, class = void>
+struct is_completion_token_for : std::false_type
+{
+};
+
+struct any_initiation
+{
+    template<class...AnyArgs>
+    void operator()(AnyArgs&&...);
+};
+
+template<class CompletionToken, class R, class...Args>
+struct is_completion_token_for<
+    CompletionToken, R(Args...), boost::void_t<decltype(
+        boost::asio::async_initiate<CompletionToken, R(Args...)>(
+            any_initiation(), std::declval<CompletionToken&>())
+        )>> : std::true_type
+{
+};
+
 } // detail
 } // beast
 } // boost

include/boost/beast/core/impl/buffered_read_stream.hpp

@@ -110,7 +110,7 @@ struct run_read_op
     operator()(
         ReadHandler&& h,
         buffered_read_stream* s,
-        Buffers const& b)
+        Buffers const* b)
     {
         // If you get an error on the following line it means
         // that your handler does not meet the documented type
@@ -124,7 +124,7 @@ struct run_read_op
         read_op<
             Buffers,
             typename std::decay<ReadHandler>::type>(
-                std::forward<ReadHandler>(h), *s, b);
+                std::forward<ReadHandler>(h), *s, *b);
     }
 };
 
@@ -153,7 +153,7 @@ async_write_some(
     static_assert(net::is_const_buffer_sequence<
         ConstBufferSequence>::value,
             "ConstBufferSequence type requirements not met");
-    static_assert(detail::is_invocable<WriteHandler,
+    static_assert(detail::is_completion_token_for<WriteHandler,
         void(error_code, std::size_t)>::value,
             "WriteHandler type requirements not met");
     return next_layer_.async_write_some(buffers,
@@ -233,7 +233,7 @@ async_read_some(
             typename ops::run_read_op{},
             handler,
             this,
-            buffers);
+            &buffers);
 }
 
 } // beast