sender and sender_to

template<class S>
  concept sender =
    move_constructible<remove_cvref_t<S>> &&
    !requires {
      typename sender_traits<remove_cvref_t<S>>::__unspecialized; // exposition only
    };

template<class S, class R>
  concept sender_to =
    sender<S> &&
    receiver<R> &&
    requires (S&& s, R&& r) {
      execution::connect((S&&) s, (R&&) r);
    };

None of these operations shall introduce data races as a result of concurrent invocations of those functions from different threads.

A sender type's destructor shall not block pending completion of the submitted function objects.

[Note: The ability to wait for completion of submitted function objects may be provided by the associated execution context. —end note]

typed_sender

A sender is typed if it declares what types it sends through a receiver's channels. The typed_sender concept is defined as:

template<template<template<class...> class Tuple, template<class...> class Variant> class>
  struct has-value-types; // exposition only

template<template<class...> class Variant>
  struct has-error-types; // exposition only

template<class S>
  concept has-sender-types = // exposition only
    requires {
      typename has-value-types<S::template value_types>;
      typename has-error-types<S::template error_types>;
      typename bool_constant<S::sends_done>;
    };

template<class S>
  concept typed_sender =
    sender<S> &&
    has-sender-types<sender_traits<remove_cvref_t<S>>>;