...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
template< typename E, typename... Args> constexpr constraint< is_same< typename decay< E >::type, boost::system::error_code >::value||is_same< typename decay< E >::type, std::exception_ptr >::value, cancellation_type_t >::type operator()( const E & e, Args && ...) const;