Boost 1.63 karma problem converting a char vector to string

The version of Boost.Spirit.Karma included with Boost 1.63.0 appears to have
a problem converting a char vector to a string when building with Apple
Clang 8.0 using the -std=c++14 (or -std=gnu++14) option. Consider the
following example:

  #include <boost/spirit/include/karma.hpp>
  #include <boost/fusion/include/adapt_struct.hpp>

  struct foo {
    std::string a, b;
  };

  BOOST_FUSION_ADAPT_STRUCT(
    foo,
    (std::string, a)
    (std::string, b)
  )

  int main()
  {
    using namespace boost::spirit::karma;

    typedef rule<std::back_insert_iterator&lt;std::string>, foo(),
space_type>
      rule_t;

    rule_t r
      =   +char_ << '(' << string << ')'
      |   skip[string] << string
      ;

    foo f = {"a", "b"};
    std::string s;
    generate_delimited(std::back_inserter(s), r, space, f);
  }

When compiled with the -std=c++14 and -stdlib=libc++ options, the following
error is emitted:

  boost/fusion/container/vector/vector.hpp:168:19: error: 
      no matching constructor for initialization of 'std::__1::vector<char,
      std::__1::allocator&lt;char> >'
                : elem(std::forward<U>(rhs))
                  ^    ~~~~~~~~~~~~~~~~~~~~

(along with a stack of several others, of course, which I've omitted for
brevity)

The problem construct appears to be "+char_". The error goes away if it is
replaced with "string"; but the error also goes away if the alternative is
removed from the rule.

This code worked fine with the Boost.Spirit.Karma included with Boost
1.62.0. Can anyone suggest a workaround that would make -std=c++14 usable?

Braden




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