### Receiver concepts

A receiver represents the continuation of an asynchronous operation. An asynchronous operation may complete with a (possibly empty) set of values, an error, or it may be cancelled. A receiver has three principal operations corresponding to the three ways an asynchronous operation may complete: `set_value`, `set_error`, and `set_done`. These are collectively known as a receiver’s completion-signal operations.

    template<class T, class E = exception_ptr>
    concept receiver =
      move_constructible<remove_cvref_t<T>> &&
      constructible_from<remove_cvref_t<T>, T> &&
      requires(remove_cvref_t<T>&& t, E&& e) {
        { execution::set_done(std::move(t)) } noexcept;
        { execution::set_error(std::move(t), (E&&) e) } noexcept;
      };

    template<class T, class... An>
    concept receiver_of =
      receiver<T> &&
      requires(remove_cvref_t<T>&& t, An&&... an) {
        execution::set_value(std::move(t), (An&&) an...);
      };

The receiver’s completion-signal operations have semantic requirements that are collectively known as the receiver contract, described below:

- None of a receiver’s completion-signal operations shall be invoked before `execution::start` has been called on the operation state object that was returned by `execution::connect` to connect that receiver to a sender.
- Once `execution::start` has been called on the operation state object, exactly one of the receiver’s completion-signal operations shall complete non-exceptionally before the receiver is destroyed.
- If `execution::set_value` exits with an exception, it is still valid to call `execution::set_error` or `execution::set_done` on the receiver.

Once one of a receiver’s completion-signal operations has completed non-exceptionally, the receiver contract has been satisfied.