...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
template< class Arg0, class... Args, [role red error.class-detail-template.1][role red error.class-detail-template.2] = typename std::enable_if< ! std::is_convertible<Arg0, error_code>::value>::type> void operator()( Arg0&&, Args&& ...);