[exec.snd.concepts]

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namespace std::execution { template<auto> concept is-constant = true; / exposition only template<class Sndr> concept is-sender = / exposition only derived_from<typename Sndr::sender_concept, sender_t>; template<class Sndr> concept enable-sender = / exposition only is-sender<Sndr> || is-awaitable<Sndr, env-promise<env<>; / [exec.awaitable] template<class Sndr> inline constexpr bool enable_sender = enable-sender<Sndr>; template<class Sndr> consteval bool is-dependent-sender-helper() try { / exposition only get_completion_signatures<Sndr>(); return false; } catch (dependent_sender_error&) { return true; } template<class Sndr> concept sender = enable_sender<remove_cvref_t<Sndr>> && requires (const remove_cvref_t<Sndr>& sndr) { { get_env(sndr) } -> queryable; } && move_constructible<remove_cvref_t<Sndr>> && constructible_from<remove_cvref_t<Sndr>, Sndr>; template<class Sndr, class. Env> concept sender_in = sender<Sndr> && (sizeof.(Env) <= 1) && (queryable<Env> &&.) && is-constant<get_completion_signatures<Sndr, Env..>()>; template<class Sndr> concept dependent_sender = sender<Sndr> && bool_constant<is-dependent-sender-helper<Sndr>()>::value; template<class Sndr, class Rcvr> concept sender_to = sender_in<Sndr, env_of_t<Rcvr>> && receiver_of<Rcvr, completion_signatures_of_t<Sndr, env_of_t<Rcvr>> && requires (Sndr&& sndr, Rcvr&& rcvr) { connect(std::forward<Sndr>(sndr), std::forward<Rcvr>(rcvr)); }; }

For a type Sndr, if sender<Sndr> is true and dependent_sender<Sndr> is false, then Sndr is a non-dependent sender ([exec.async.ops]).

Given a subexpression sndr, let Sndr be decltype(sndr)) and let rcvr be a receiver with an associated environment whose type is Env.

A completion operation is a permissible completion for Sndr and Env if its completion signature appears in the argument list of the specialization of completion_signatures denoted by completion_signatures_of_t<Sndr, Env>.

Sndr and Env model sender_in<Sndr, Env> if all the completion operations that are potentially evaluated by connecting sndr to rcvr and starting the resulting operation state are permissible completions for Sndr and Env.

Remarks: Pursuant to [namespace.std], users may specialize enable_sender to true for cv-unqualified program-defined types that model sender, and false for types that do not.

Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.

The exposition-only concepts sender-of and sender-in-of define the requirements for a sender type that completes with a given unique set of value result types.

namespace std::execution { template<class. As> using value-signature = set_value_t(As..); / exposition only template<class Sndr, class SetValue, class. Env> concept sender-in-of-impl = / exposition only sender_in<Sndr, Env..> && MATCHING-SIG(SetValue, / see [exec.general] gather-signatures<set_value_t, / see [exec.cmplsig] completion_signatures_of_t<Sndr, Env..>, value-signature, type_identity_t>); template<class Sndr, class Env, class. Values> concept sender-in-of = / exposition only sender-in-of-impl<Sndr, set_value_t(Values..), Env>; template<class Sndr, class. Values> concept sender-of = / exposition only sender-in-of-impl<Sndr, set_value_t(Values..)>; }

Let sndr be an expression such that decltype(sndr)) is Sndr.

The type tag_of_t<Sndr> is as follows:

(6.1)
If the declaration auto& [tag, data, ..children] = sndr; would be well-formed, tag_of_t<Sndr> is an alias for decltype(auto(tag)).
(6.2)
Otherwise, tag_of_t<Sndr> is ill-formed.

Let sender-for be an exposition-only concept defined as follows: namespace std::execution { template<class Sndr, class Tag> concept sender-for = sender<Sndr> && same_as<tag_of_t<Sndr>, Tag>; }

For a type T, SET-VALUE-SIG(T) denotes the type set_value_t() if T is cv void; otherwise, it denotes the type set_value_t(T).

Library-provided sender types

(9.1)
always expose an overload of a member connect that accepts an rvalue sender and
(9.2)
only expose an overload of a member connect that accepts an lvalue sender if they model copy_constructible .