...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Convert the specified executor to the inner executor type, then use that to construct the adapted executor.
template< typename OtherExecutor> executor_with_default( const OtherExecutor & ex, typename constraint< is_convertible< OtherExecutor, InnerExecutor >::value >::type = 0);