UB in timer constructor

[klemens-morgenstern]

This line

corosio/include/boost/corosio/detail/timer_service.hpp

Line 903 in 07fab88

return *timer_service_access::get_scheduler(static_cast<io_context&>(ctx))

assumes that a capy::execution_context is always a corosio::io_context. This is not enforced, and execution_context can be default constructed.

capy::execution_context ec;
timer t(ec);

There's nothing obviously wrong with this code yet it'll cause a SIGSEV.

There are three solutions:

1. make execution_context abstract so it cannot be constructed this way
2. throw an exception if it's not an io_context, so it cannot run the scheduler
3. implicitly construct a thread and let it run the scheduler (what asio does).

Note that Nr.3 would allow us to put an asio::io_context into an capy::any_executor. That would enable users to add corosio functionality to an existing asio app without rewriting everything.