Spirit X3 1.61.0 error repro

repro.cpp

#include <iostream>
#include <string>
#include <boost/spirit/home/x3.hpp>
#include <boost/fusion/include/std_pair.hpp>

using namespace std;
namespace x3 = boost::spirit::x3;

struct repro_string_parser : x3::parser<repro_string_parser>
{
    using attribute_type = string;

    template <typename Iterator, typename Attribute>
    bool parse(Iterator& first, Iterator const& last, x3::unused_type, x3::unused_type, Attribute& attr) const
    {
        attr = "repro";
        return true;
    }
};

auto const repro_string = repro_string_parser{};

struct repro_empty_parser : x3::parser<repro_empty_parser>
{
    using attribute_type = x3::unused_type;

    template <typename Iterator, typename Attribute>
    bool parse(Iterator& first, Iterator const& last, x3::unused_type, x3::unused_type, Attribute& attr) const
    {
        return true;
    }
};

auto const repro_empty = repro_empty_parser{};

x3::rule<class repro_tag, pair<string, string>> const repro_rule = "repro";
auto const repro_rule_def = repro_string > repro_empty > repro_string;
BOOST_SPIRIT_DEFINE(repro_rule);

int main()
{
    string input = "";
    pair<string, string> result;
    x3::parse(input.begin(), input.end(), repro_rule, result);
    
    // Expected output: repro repro
    cout << result.first << " " << result.second << endl;
    return 0;
}

The above is to demonstrate a compilation error (at line 16) when compiled with Boost 1.61.0. The repro builds without error and produces the expected output with Boost 1.60.0.

@djowel I'm not sure if you saw my email on the Boost Spirit mailing list, but this break appears related to boostorg/spirit@a8e391b. Do you have any advice on how to fix this in the example parser with the x3::unused_type attribute type? Thanks!