Why does boost::spirit::unicode::char_ no longer work with UTF-8 char* strings?

With boost version 1.60 I could use #define BOOST_SPIRIT_UNICODE and boost::spirit::unicode::char_ to process UTF-8 input strings without any further preprocessing. With boost version 1.72 this fails with an exception.

The solution seems to be to use boost::u8_to_u32_iterator and let spirit work with wide strings. But why did it work so flawlessly in the earlier version and if possible how can I reactivate the old behavior?

Here is some sample code:

#define BOOST_SPIRIT_UNICODE
#include <boost/spirit/include/qi.hpp>

int main()
{
   typedef std::string::const_iterator iterator_type;
   namespace qi = boost::spirit::qi;
   namespace unicode = boost::spirit::unicode;

   std::string input("\"Test ⏳\"");
   qi::rule<iterator_type, std::string(), unicode::space_type> quoted_string = qi::lexeme['"' >> +(unicode::char_ - '"') >> '"'];

   iterator_type iter = input.begin();
   iterator_type end = input.end();
   std::string output;
   bool r = phrase_parse(iter, end, quoted_string, unicode::space, output);

   if (r && iter == end)
      std::cout << "successfully parsed " << input << " to " << output << std::endl;
   else
      std::cout << "failed to parse " << input << std::endl;

   return 0;
}