* The library triggers a static_assert correctly if a callable
can't be called with a subset of its arguments but it seems like this is
not enough to make the compiler stop causing the generation
of a 0 - 1 -> std::size_t::max length index sequence which obviously
causes infinite work.
* Reproducible with:
```
cti::make_continuable<std::string>([](cti::promise<std::string> promise) {
promise.set_value("hello");
}).then([](int num) {
//
});
```
* Closes#21
* wait is implemented by a atomic and default condition variable.
* wait_for and wait_until are expensive since we can't assume
anything about the environment thus we have to allocate
a persistent frame.
* Introduce cancellation_result to represent a cancelled async task
* Add cancellation unit tests
* This doesn't allow cancellation of continuables, it is meant
for treating the special state action canceled on the receiver side.
Cancellation of a chain is still up to the user.
* Can be used to specialize the asynchronous control flow
on immediate available asynchronous results mostly returned by:
- make_ready_continuable
- make_exceptional_continuable
* Usable to avoid longer unnecessary synchronous callback nestings.
* cti::query_arg_t was renamed into cti::unpack_arg_t.
* The continuation overload nowreturns result<Args...> rather
than std::tuple<Args...>.
* Add is_ready optimizations to make_exceptional_continuable.
* This allows cheap cancellation of the control flow by passing
a default constructed `exception_t` object to `set_exception`.
* Since possible representatives like
- `std::exception_ptr`
- `std::error_code`
- `std::error_condition`
can be default constructed we have to test them anyway before possibly
rethrowing them.
* This makes it possible to use promise_base for optional
promises directly rather than wrapping it as optional<promise_base<...>>.
* Invalidate the promise_base after its first usage.
* Expose an `operator bool()` to make the validility accessible.
* Remove the no longer needed private promise_no_init_arg_t tag.
* Makes it possible to specify an executor in addition to
the arguments passed to async.
* The reason why async should not support this directly is
that it should be closely modelled to std::async.
