Boost.X3: does not compile with std::optional<std::string> - boost-spirit-x3

Question

The following parser does not compile when I use std::optional<std::string> for values of a -lexeme[+alpha] rule: something breaks in the attribute management.
The grammar works fine if instead of std::string I use int as a base type, it also works if I use twice std::string (i.e., no std::optional). However it does not work with boost::optional either. In the real grammar there is a difference between the absence of the string, and an empty string: I want to use std::optional (or its Boost predecessor).
The full example is available on Coliru, but here are the relevant bits:
struct pair_t
{
std::string first;
std::optional<std::string> second;
};
BOOST_FUSION_ADAPT_STRUCT(pair_t, first, second)
const auto pair_rule = lexeme[+alpha] >> -lexeme[+alpha] >> eoi;
for (std::string i: {"ab", "ab cd"})
{
auto res = pair_t{};
auto first = i.cbegin();
auto last = i.cend();
auto r = x3::phrase_parse(first, last, pair_rule, space, res);
if (r && first == last)
std::cout << i << ": " << res << '\n';
else
std::cout << i << ": failed\n";
}
My compiler reports:
clang++-mp-5.0 -std=c++17 -isystem /opt/local/include/ x3-optional.cc && ./a.out
In file included from x3-optional.cc:8:
In file included from /opt/local/include/boost/spirit/home/x3.hpp:14:
In file included from /opt/local/include/boost/spirit/home/x3/auxiliary.hpp:11:
In file included from /opt/local/include/boost/spirit/home/x3/auxiliary/any_parser.hpp:17:
/opt/local/include/boost/spirit/home/x3/support/traits/move_to.hpp:180:9: error: no matching function
for call to 'move_to'
detail::move_to(std::move(src), dest
^~~~~~~~~~~~~~~
/opt/local/include/boost/spirit/home/x3/char/char_parser.hpp:31:29: note: in instantiation of
function template specialization 'boost::spirit::x3::traits::move_to<const char &,
std::__1::basic_string<char> >' requested here
x3::traits::move_to(*first, attr);
^