Can't forward single-argument constructors to sinks

[steinar+boost@…]

CasparCG has code like this:

`

class sink_backend : public boost::log::sinks::basic_formatted_sink_backend<char> {

std::function<void(std::string line)> formatted_line_sink_;

public:

sink_backend(std::function<void(std::string line)> formatted_line_sink)

: formatted_line_sink_(std::move(formatted_line_sink))

{ }

void consume(const boost::log::record_view& rec, const std::string& formatted_message) {

try {

formatted_line_sink_(formatted_message);

} catch (...) {

std::cerr << "[sink_backend] Error while consuming formatted message: " << formatted_message << std::endl << std::endl;

}

}

};

[…]

auto sink = boost::make_shared<sink_type>(std::move(formatted_line_sink));

`

This fails with a long spew of template errors, evidently because for single-argument constructors, only variants with named parameters are considered (sync_frontend.hpp, BOOST_LOG_SINK_CTOR_FORWARD_INTERNAL_1). This is a regression from 1.59.0.

Adding a dummy int parameter and setting it to 0 makes the code compile.