[Spirit Devel] [ADAPT_ADT and Karma] ADAPT_ADT causes grammars with variants to not compile in karma

I am trying to output a variant in using the ADAPT_ADT macro and Karma.  It
seems to be a problem with the adt_proxy class that causes the variant
conversion to fail.  I can easily get similar code working with
ADAPT_STRUCT.
The following fails to compile.

typedef const boost::variant<std::string,unsigned int> const_variant_type;

struct y
{
  const_variant_type& get() const;
  void set(const_variant_type&);
};

BOOST_FUSION_ADAPT_ADT(y,(const_variant_type,const_variant_type,obj.get(),obj.set(val)))

int main()
{
  using namespace boost::spirit::karma;
  using namespace boost::phoenix;
  boost::spirit::karma::rule<std::string::iterator,std::string()> a;
  boost::spirit::karma::rule<std::string::iterator,unsigned int()> b;
  boost::spirit::karma::rule<std::string::iterator,y()> Y;
  Y = (a | b)[_1 = at_c<0>(_val)];
}

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#include <boost/spirit/include/karma.hpp>
#include <boost/fusion/adapted/adt/adapt_adt.hpp>
#include <boost/spirit/include/phoenix_fusion.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/spirit/include/phoenix_stl.hpp>
#include <boost/spirit/home/support/adapt_adt_attributes.hpp>
#include <boost/variant/variant.hpp>
#include <string>

typedef const boost::variant<std::string,unsigned int> const_variant_type;

struct y
{
  const_variant_type& get() const;
  void set(const_variant_type&); 
};

BOOST_FUSION_ADAPT_ADT(y,(const_variant_type,const_variant_type,obj.get(),obj.set(val)))

int main()
{
  using namespace boost::spirit::karma;
  using namespace boost::phoenix;
  boost::spirit::karma::rule<std::string::iterator,std::string()> a;
  boost::spirit::karma::rule<std::string::iterator,unsigned int()> b;
  boost::spirit::karma::rule<std::string::iterator,y()> Y;
  Y = (a | b)[_1 = at_c<0>(_val)];
}