Cannot export boost::iostreams::source implementation on MSVC

[mathstuf@…]

The following fails on MSVC 10 (VS2010) and MSVC 11 (VS2012) at least:

#include <boost/iostreams/concepts.hpp> class declspec(dllexport) mystream : public boost::iostreams::source {};

with the error:

error C2338: (is_convertible<Mode, two_sequence>::value)

in concepts.hpp line 47. The same think happens with a sink. Are custom sources and sinks not meant to be exported from a library?