...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
The type of the immediate executor associated with this object.
using immediate_executor_type = implementation-defined;
If a class derived from boost::beast::async_base
is a completion handler,
then the associated immediage executor of the derived class will be this
type.