#### basic_signal_set::async_wait

Start an asynchronous operation to wait for a signal to be delivered.

    template<
        typename SignalToken = DEFAULT>
    DEDUCED async_wait(
        SignalToken && token = DEFAULT);

This function may be used to initiate an asynchronous wait against the signal set. It is an initiating function for an asynchronous operation, and always returns immediately.

For each call to `async_wait()`, the completion handler will be called exactly once. The completion handler will be called when:

- One of the registered signals in the signal set occurs; or
- The signal set was cancelled, in which case the handler is passed the error code `asio::error::operation_aborted`.

###### Parameters

token

  The completion token that will be used to produce a completion handler, which will be called when the wait completes. Potential completion tokens include `use_future`, `use_awaitable`, `yield_context`, or a function object with the correct completion signature. The function signature of the completion handler must be:

      void handler(
        const asio::error_code& error, // Result of operation.
        int signal_number // Indicates which signal occurred.
      );

  Regardless of whether the asynchronous operation completes immediately or not, the completion handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `post`.

###### Completion Signature

    void(asio::error_code, int)

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following `cancellation_type` values:

- `cancellation_type::terminal`
- `cancellation_type::partial`
- `cancellation_type::total`