为什么Boost Variant使用模板构造函数而不是Boost::beast::websocket::stream的移

我正在尝试将boost::beast::websocket::stream<T>(针对2个特定的T(封装在boost::variant中，以允许对TLS([T = boost::asio::ssl::stream<boost::asio::ip::tcp::socket>](和非TLS([T = boost::asio::ip::tcp::socket](websocket进行相同的处理。我陷入了编译失败的困境。

我能想出的最简单的失败例子是：

#include <boost/asio/ssl.hpp>
#include <boost/beast/core.hpp>
#include <boost/beast/websocket.hpp>
#include <boost/variant.hpp>
using tcp = boost::asio::ip::tcp;
namespace ws = boost::beast::websocket;
namespace ssl = boost::asio::ssl;
using base_ws = boost::variant<
ws::stream<tcp::socket>, ws::stream<ssl::stream<tcp::socket>>>;
class test
{
public:
static void init( tcp::socket &&  socket )
{
ssl::context  ctx{ ssl::context::tlsv12_server };
ws::stream<ssl::stream<tcp::socket>>  s{ std::move( socket ), ctx };
base_ws{ std::move( s ) };
}
};
int main()
{}

编译失败，出现错误：

In file included from /server/src/ws_server.cpp:9:
In file included from /include/boost/beast/websocket.hpp:18:
In file included from /include/boost/beast/websocket/stream.hpp:3455:
/include/boost/beast/websocket/impl/stream.ipp:47:7: error: no matching constructor for initialization of 'boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> >'
: stream_(std::forward<Args>(args)...)
^       ~~~~~~~~~~~~~~~~~~~~~~~~
/include/boost/variant/detail/initializer.hpp:122:27: note: in instantiation of function template specialization 'boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> > >::stream<boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> > > >' requested here
new(dest) value_T( boost::detail::variant::move(operand) );
^
/include/boost/variant/variant.hpp:1687:28: note: in instantiation of member function 'boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, boost::mpl::l_iter<boost::mpl::list2<boost::beast::websocket::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> >, boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> > > > > >::initializer_node, mpl_::int_<1> >, boost::mpl::l_iter<boost::mpl::list1<boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> > > > > >::initializer_node::initialize' requested here
initializer::initialize(
^
/include/boost/variant/variant.hpp:1858:9: note: in instantiation of function template specialization 'boost::variant<boost::beast::websocket::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> >, boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> > > >::convert_construct<boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> > > >' requested here
convert_construct( detail::variant::move(operand), 1L);
^
/server/src/ws_server.cpp:372:20: note: in instantiation of function template specialization 'boost::variant<boost::beast::websocket::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> >, boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> > > >::variant<boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> > > >' requested here
return base_ws( std::move( s ) );
^
/include/boost/asio/ssl/stream.hpp:64:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> > >' to 'const boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp> >' for 1st argument
class stream :
^
/include/boost/asio/ssl/stream.hpp:98:3: note: candidate constructor template not viable: requires 2 arguments, but 1 was provided
stream(Arg&& arg, context& ctx)
^

我会提供godbolt，但它超时了，而且coliru没有提供boost/asio/ssl.hpp所需的openssl。

根据这里，boost::variant应该接受T &&，并使用它在内部实例化T。

根据这里，boost::beast::websocket::stream确实有一个move构造函数。

move构造函数更具体，那么编译器为什么选择第一个注释中所示的可变构造函数(stream<...>(呢？

boost::beast::websocket::stream<
boost::asio::ssl::stream<
boost::asio::basic_stream_socket<boost::asio::ip::tcp>
>
>::stream<
boost::beast::websocket::stream<
boost::asio::ssl::stream<
boost::asio::basic_stream_socket<boost::asio::ip::tcp>
>
>
>

我使用的是Boost 1.66.0和Clang 7。