[execution.syn]

# 33 Execution control library [[exec]](./#exec)

## 33.4 Header <execution> synopsis [execution.syn]

[🔗](#header:%3cexecution%3e)

namespace std {// [[execpol.type]](execpol.type "26.3.6.2 Execution policy type trait"), execution policy type traittemplate<class T> struct is_execution_policy; // freestandingtemplate<class T> constexpr bool [is_execution_policy_v](#lib:is_execution_policy_v "33.4 Header <execution> synopsis [execution.syn]") = // freestanding is_execution_policy<T>::value;}namespace std::execution {// [[execpol.seq]](execpol.seq "26.3.6.3 Sequenced execution policy"), sequenced execution policyclass sequenced_policy; // [[execpol.par]](execpol.par "26.3.6.4 Parallel execution policy"), parallel execution policyclass parallel_policy; // [[execpol.parunseq]](execpol.parunseq "26.3.6.5 Parallel and unsequenced execution policy"), parallel and unsequenced execution policyclass parallel_unsequenced_policy; // [[execpol.unseq]](execpol.unseq "26.3.6.6 Unsequenced execution policy"), unsequenced execution policyclass unsequenced_policy; // [[execpol.objects]](execpol.objects "26.3.6.7 Execution policy objects"), execution policy objectsinline constexpr sequenced_policy seq{ *unspecified* }; inline constexpr parallel_policy par{ *unspecified* }; inline constexpr parallel_unsequenced_policy par_unseq{ *unspecified* }; inline constexpr unsequenced_policy unseq{ *unspecified* };}namespace std {// [[exec.general]](exec.general "33.1 General"), helper conceptstemplate<class T>concept [*movable-value*](exec.general#concept:movable-value "33.1 General [exec.general]") = *see below*; // *exposition only*template<class From, class To>concept [*decays-to*](#concept:decays-to "33.4 Header <execution> synopsis [execution.syn]") = [same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<decay_t<From>, To>; // *exposition only*template<class T>concept [*class-type*](#concept:class-type "33.4 Header <execution> synopsis [execution.syn]") = [*decays-to*](#concept:decays-to "33.4 Header <execution> synopsis [execution.syn]")<T, T> && is_class_v<T>; // *exposition only*// [[exec.queryable]](exec.queryable "33.2 Queries and queryables"), queryable objectstemplate<class T>concept [*queryable*](exec.queryable.concept#concept:queryable "33.2.2 queryable concept [exec.queryable.concept]") = *see below*; // *exposition only*// [[exec.queries]](exec.queries "33.5 Queries"), queriesstruct [forwarding_query_t](#lib:forwarding_query_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [get_allocator_t](#lib:get_allocator_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [get_stop_token_t](#lib:get_stop_token_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; inline constexpr forwarding_query_t [forwarding_query](#lib:forwarding_query "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr get_allocator_t [get_allocator](#lib:get_allocator "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr get_stop_token_t [get_stop_token](#lib:get_stop_token "33.4 Header <execution> synopsis [execution.syn]"){}; template<class T>using stop_token_of_t = remove_cvref_t<decltype(get_stop_token(declval<T>()))>; template<class T>concept [*forwarding-query*](#concept:forwarding-query "33.4 Header <execution> synopsis [execution.syn]") = forwarding_query(T{}); // *exposition only*}namespace std::execution {// [[exec.queries]](exec.queries "33.5 Queries"), queriesstruct [get_domain_t](#lib:get_domain_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [get_scheduler_t](#lib:get_scheduler_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [get_delegation_scheduler_t](#lib:get_delegation_scheduler_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [get_forward_progress_guarantee_t](#lib:get_forward_progress_guarantee_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; template<class CPO>struct [get_completion_scheduler_t](#lib:get_completion_scheduler_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct get_await_completion_adaptor_t { *unspecified* }; inline constexpr get_domain_t [get_domain](#lib:get_domain "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr get_scheduler_t [get_scheduler](#lib:get_scheduler "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr get_delegation_scheduler_t [get_delegation_scheduler](#lib:get_delegation_scheduler "33.4 Header <execution> synopsis [execution.syn]"){}; enum class forward_progress_guarantee; inline constexpr get_forward_progress_guarantee_t [get_forward_progress_guarantee](#lib:get_forward_progress_guarantee "33.4 Header <execution> synopsis [execution.syn]"){}; template<class CPO>constexpr get_completion_scheduler_t<CPO> [get_completion_scheduler](#lib:get_completion_scheduler "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr get_await_completion_adaptor_t get_await_completion_adaptor{}; struct [get_env_t](#lib:get_env_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; inline constexpr get_env_t [get_env](#lib:get_env "33.4 Header <execution> synopsis [execution.syn]"){}; template<class T>using [env_of_t](#lib:env_of_t "33.4 Header <execution> synopsis [execution.syn]") = decltype(get_env(declval<T>())); // [[exec.prop]](exec.prop "33.11.1 Class template prop"), class template proptemplate<class QueryTag, class ValueType>struct prop; // [[exec.env]](exec.env "33.11.2 Class template env"), class template envtemplate<[*queryable*](exec.queryable.concept#concept:queryable "33.2.2 queryable concept [exec.queryable.concept]")... Envs>struct env; // [[exec.domain.default]](exec.domain.default "33.9.5 execution​::​default_­domain"), execution domainsstruct default_domain; // [[exec.sched]](exec.sched "33.6 Schedulers"), schedulersstruct [scheduler_t](#lib:scheduler_t "33.4 Header <execution> synopsis [execution.syn]") {}; template<class Sch>concept [scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]") = *see below*; // [[exec.recv]](exec.recv "33.7 Receivers"), receiversstruct [receiver_t](#lib:receiver_t "33.4 Header <execution> synopsis [execution.syn]") {}; template<class Rcvr>concept [receiver](exec.recv.concepts#concept:receiver "33.7.1 Receiver concepts [exec.recv.concepts]") = *see below*; template<class Rcvr, class Completions>concept [receiver_of](exec.recv.concepts#concept:receiver_of "33.7.1 Receiver concepts [exec.recv.concepts]") = *see below*; struct [set_value_t](#lib:set_value_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [set_error_t](#lib:set_error_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [set_stopped_t](#lib:set_stopped_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; inline constexpr set_value_t [set_value](#lib:set_value "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr set_error_t [set_error](#lib:set_error "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr set_stopped_t [set_stopped](#lib:set_stopped "33.4 Header <execution> synopsis [execution.syn]"){}; // [[exec.opstate]](exec.opstate "33.8 Operation states"), operation statesstruct [operation_state_t](#lib:operation_state_t "33.4 Header <execution> synopsis [execution.syn]") {}; template<class O>concept [operation_state](exec.opstate.general#concept:operation_state "33.8.1 General [exec.opstate.general]") = *see below*; struct [start_t](#lib:start_t "33.4 Header <execution> synopsis [execution.syn]"); inline constexpr start_t [start](#lib:start "33.4 Header <execution> synopsis [execution.syn]"){}; // [[exec.snd]](exec.snd "33.9 Senders"), sendersstruct [sender_t](#lib:sender_t "33.4 Header <execution> synopsis [execution.syn]") {}; template<class Sndr>inline constexpr bool enable_sender = *see below*; template<class Sndr>concept [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") = *see below*; template<class Sndr, class... Env>concept [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]") = *see below*; template<class Sndr>concept [dependent_sender](exec.snd.concepts#concept:dependent_sender "33.9.3 Sender concepts [exec.snd.concepts]") = *see below*; template<class Sndr, class Rcvr>concept [sender_to](exec.snd.concepts#concept:sender_to "33.9.3 Sender concepts [exec.snd.concepts]") = *see below*; template<class... Ts>struct *type-list*; // *exposition only*template<class... Ts>using *decayed-tuple* = tuple<decay_t<Ts>...>; // *exposition only*template<class... Ts>using *variant-or-empty* = *see below*; // *exposition only*template<class Sndr, class Env = env<>, template<class...> class Tuple = *decayed-tuple*, template<class...> class Variant = *variant-or-empty*>requires [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<Sndr, Env>using value_types_of_t = *see below*; template<class Sndr, class Env = env<>, template<class...> class Variant = *variant-or-empty*>requires [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<Sndr, Env>using error_types_of_t = *see below*; template<class Sndr, class Env = env<>>requires [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<Sndr, Env>constexpr bool sends_stopped = *see below*; template<class Sndr, class... Env>using *single-sender-value-type* = *see below*; // *exposition only*template<class Sndr, class... Env>concept [*single-sender*](#concept:single-sender "33.4 Header <execution> synopsis [execution.syn]") = *see below*; // *exposition only*template<[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sndr>using tag_of_t = *see below*; // [[exec.snd.transform]](exec.snd.transform "33.9.6 execution​::​transform_­sender"), sender transformationstemplate<class Domain, [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sndr, [*queryable*](exec.queryable.concept#concept:queryable "33.2.2 queryable concept [exec.queryable.concept]")... Env>requires (sizeof...(Env) <= 1)constexpr [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") decltype(auto) transform_sender( Domain dom, Sndr&& sndr, const Env&... env) noexcept(*see below*); // [[exec.snd.transform.env]](exec.snd.transform.env "33.9.7 execution​::​transform_­env"), environment transformationstemplate<class Domain, [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sndr, [*queryable*](exec.queryable.concept#concept:queryable "33.2.2 queryable concept [exec.queryable.concept]") Env>constexpr [*queryable*](exec.queryable.concept#concept:queryable "33.2.2 queryable concept [exec.queryable.concept]") decltype(auto) transform_env( Domain dom, Sndr&& sndr, Env&& env) noexcept; // [[exec.snd.apply]](exec.snd.apply "33.9.8 execution​::​apply_­sender"), sender algorithm applicationtemplate<class Domain, class Tag, [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sndr, class... Args>constexpr decltype(auto) apply_sender( Domain dom, Tag, Sndr&& sndr, Args&&... args) noexcept(*see below*); // [[exec.connect]](exec.connect "33.9.10 execution​::​connect"), the connect sender algorithmstruct [connect_t](#lib:connect_t "33.4 Header <execution> synopsis [execution.syn]"); inline constexpr connect_t [connect](#lib:connect "33.4 Header <execution> synopsis [execution.syn]"){}; template<class Sndr, class Rcvr>using [connect_result_t](#lib:connect_result_t "33.4 Header <execution> synopsis [execution.syn]") =decltype(connect(declval<Sndr>(), declval<Rcvr>())); // [[exec.factories]](exec.factories "33.9.11 Sender factories"), sender factoriesstruct [just_t](#lib:just_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [just_error_t](#lib:just_error_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [just_stopped_t](#lib:just_stopped_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [schedule_t](#lib:schedule_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; inline constexpr just_t [just](#lib:just "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr just_error_t [just_error](#lib:just_error "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr just_stopped_t [just_stopped](#lib:just_stopped "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr schedule_t [schedule](#lib:schedule "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr *unspecified* [read_env](#lib:read_env "33.4 Header <execution> synopsis [execution.syn]"){}; template<[scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]") Sch>using [schedule_result_t](#lib:schedule_result_t "33.4 Header <execution> synopsis [execution.syn]") = decltype(schedule(declval<Sch>())); // [[exec.adapt]](exec.adapt "33.9.12 Sender adaptors"), sender adaptorstemplate<[*class-type*](#concept:class-type "33.4 Header <execution> synopsis [execution.syn]") D>struct [sender_adaptor_closure](#lib:sender_adaptor_closure "33.4 Header <execution> synopsis [execution.syn]") { }; struct [starts_on_t](#lib:starts_on_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [continues_on_t](#lib:continues_on_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [on_t](#lib:on_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [schedule_from_t](#lib:schedule_from_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [then_t](#lib:then_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [upon_error_t](#lib:upon_error_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [upon_stopped_t](#lib:upon_stopped_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [let_value_t](#lib:let_value_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [let_error_t](#lib:let_error_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [let_stopped_t](#lib:let_stopped_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [bulk_t](#lib:bulk_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [bulk_chunked_t](#lib:bulk_chunked_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [bulk_unchunked_t](#lib:bulk_unchunked_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [when_all_t](#lib:when_all_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [when_all_with_variant_t](#lib:when_all_with_variant_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [into_variant_t](#lib:into_variant_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [stopped_as_optional_t](#lib:stopped_as_optional_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [stopped_as_error_t](#lib:stopped_as_error_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [associate_t](#lib:associate_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [spawn_future_t](#lib:spawn_future_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; inline constexpr *unspecified* [write_env](#lib:write_env "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr *unspecified* [unstoppable](#lib:unstoppable "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr starts_on_t [starts_on](#lib:starts_on "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr continues_on_t [continues_on](#lib:continues_on "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr on_t [on](#lib:on "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr schedule_from_t [schedule_from](#lib:schedule_from "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr then_t [then](#lib:then "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr upon_error_t [upon_error](#lib:upon_error "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr upon_stopped_t [upon_stopped](#lib:upon_stopped "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr let_value_t [let_value](#lib:let_value "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr let_error_t [let_error](#lib:let_error "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr let_stopped_t [let_stopped](#lib:let_stopped "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr bulk_t [bulk](#lib:bulk "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr bulk_chunked_t [bulk_chunked](#lib:bulk_chunked "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr bulk_unchunked_t [bulk_unchunked](#lib:bulk_unchunked "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr when_all_t [when_all](#lib:when_all "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr when_all_with_variant_t [when_all_with_variant](#lib:when_all_with_variant "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr into_variant_t [into_variant](#lib:into_variant "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr stopped_as_optional_t [stopped_as_optional](#lib:stopped_as_optional "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr stopped_as_error_t [stopped_as_error](#lib:stopped_as_error "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr associate_t [associate](#lib:associate "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr spawn_future_t [spawn_future](#lib:spawn_future "33.4 Header <execution> synopsis [execution.syn]"){}; // [[exec.cmplsig]](exec.cmplsig "33.10 Completion signatures"), completion signaturestemplate<class Fn>concept [*completion-signature*](exec.cmplsig#concept:completion-signature "33.10 Completion signatures [exec.cmplsig]") = *see below*; // *exposition only*template<[*completion-signature*](exec.cmplsig#concept:completion-signature "33.10 Completion signatures [exec.cmplsig]")... Fns>struct [completion_signatures](#lib:completion_signatures "33.4 Header <execution> synopsis [execution.syn]"); template<class Sigs>concept [*valid-completion-signatures*](#concept:valid-completion-signatures "33.4 Header <execution> synopsis [execution.syn]") = *see below*; // *exposition only*struct dependent_sender_error : exception {}; // [[exec.getcomplsigs]](exec.getcomplsigs "33.9.9 execution​::​get_­completion_­signatures"), get completion signaturestemplate<class Sndr, class... Env>consteval auto get_completion_signatures() -> [*valid-completion-signatures*](#concept:valid-completion-signatures "33.4 Header <execution> synopsis [execution.syn]") auto; template<class Sndr, class... Env>requires [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<Sndr, Env...>using completion_signatures_of_t = decltype(get_completion_signatures<Sndr, Env...>()); // [[exec.run.loop]](exec.run.loop "33.12.1 execution​::​run_­loop"), run_loopclass run_loop;}namespace std::this_thread {// [[exec.consumers]](exec.consumers "33.9.13 Sender consumers"), consumersstruct [sync_wait_t](#lib:sync_wait_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; struct [sync_wait_with_variant_t](#lib:sync_wait_with_variant_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; inline constexpr sync_wait_t [sync_wait](#lib:sync_wait "33.4 Header <execution> synopsis [execution.syn]"){}; inline constexpr sync_wait_with_variant_t [sync_wait_with_variant](#lib:sync_wait_with_variant "33.4 Header <execution> synopsis [execution.syn]"){};}namespace std::execution {// [[exec.consumers]](exec.consumers "33.9.13 Sender consumers"), consumersstruct [spawn_t](#lib:spawn_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; inline constexpr spawn_t spawn{}; // [[exec.as.awaitable]](exec.as.awaitable "33.13.1 execution​::​as_­awaitable"), coroutine utility as_awaitablestruct [as_awaitable_t](#lib:as_awaitable_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; inline constexpr as_awaitable_t [as_awaitable](#lib:as_awaitable "33.4 Header <execution> synopsis [execution.syn]"){}; // [[exec.with.awaitable.senders]](exec.with.awaitable.senders "33.13.2 execution​::​with_­awaitable_­senders"), coroutine utility with_awaitable_senderstemplate<[*class-type*](#concept:class-type "33.4 Header <execution> synopsis [execution.syn]") Promise>struct with_awaitable_senders; // [[exec.scope.concepts]](exec.scope.concepts "33.14.1 Execution scope concepts"), scope conceptstemplate<class Token>concept [scope_token](exec.scope.concepts#concept:scope_token "33.14.1 Execution scope concepts [exec.scope.concepts]") = *see below*; // [[exec.scope.simple.counting]](exec.scope.simple.counting "33.14.2.2 Simple Counting Scope"), simple counting scopeclass simple_counting_scope; // [[exec.scope.counting]](exec.scope.counting "33.14.2.3 Counting Scope"), counting scopeclass counting_scope;}namespace std::execution {// [[exec.par.scheduler]](exec.par.scheduler "33.15 Parallel scheduler"), parallel schedulerclass [parallel_scheduler](#lib:parallel_scheduler "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* };
 parallel_scheduler get_parallel_scheduler();}// [[exec.sysctxrepl]](exec.sysctxrepl "33.16 Namespace system_­context_­replaceability"), namespace system_context_replaceabilitynamespace std::execution::[system_context_replaceability](#lib:system_context_replaceability "33.4 Header <execution> synopsis [execution.syn]") {struct receiver_proxy; struct bulk_item_receiver_proxy; struct parallel_scheduler_backend;

 shared_ptr<parallel_scheduler_backend> query_parallel_scheduler_backend();}namespace std::execution {// [[exec.affine.on]](exec.affine.on "33.13.3 execution​::​affine_­on"), coroutine utility affine_onstruct [affine_on_t](#lib:affine_on_t "33.4 Header <execution> synopsis [execution.syn]") { *unspecified* }; inline constexpr affine_on_t [affine_on](#lib:affine_on "33.4 Header <execution> synopsis [execution.syn]"){}; // [[exec.inline.scheduler]](exec.inline.scheduler "33.13.4 execution​::​inline_­scheduler"), inline schedulerclass [inline_scheduler](exec.inline.scheduler#lib:inline_scheduler "33.13.4 execution​::​inline_­scheduler [exec.inline.scheduler]"); // [[exec.task.scheduler]](exec.task.scheduler "33.13.5 execution​::​task_­scheduler"), task schedulerclass [task_scheduler](exec.task.scheduler#lib:task_scheduler "33.13.5 execution​::​task_­scheduler [exec.task.scheduler]"); template<class E>struct [with_error](#lib:with_error "33.4 Header <execution> synopsis [execution.syn]") {using type = remove_cvref_t<E>;
 type error; }; template<class E> with_error(E) -> with_error<E>; template<[scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]") Sch>struct change_coroutine_scheduler {using type = remove_cvref_t<Sch>;
 type scheduler; }; template<[scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]") Sch> change_coroutine_scheduler(Sch) -> change_coroutine_scheduler<Sch>; // [[exec.task]](exec.task "33.13.6 execution​::​task"), class template tasktemplate<class T, class Environment>class [task](task.class#lib:task "33.13.6.2 Class template task [task.class]");}

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L773)

The exposition-only type *variant-or-empty*<Ts...> is defined as follows:

- [(1.1)](#1.1)

If sizeof...(Ts) is greater than zero,*variant-or-empty*<Ts...> denotes variant<Us...> where Us... is the pack decay_t<Ts>... with duplicate types removed.

- [(1.2)](#1.2)

Otherwise, *variant-or-empty*<Ts...> denotes
the exposition-only class type:namespace std::execution {struct *empty-variant* { // *exposition only**empty-variant*() = delete; };}

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L794)

For type Sndr and pack of types Env,
let CS be completion_signatures_of_t<Sndr, Env...>[.](#2.sentence-1)

Then *single-sender-value-type*<Sndr, Env...> is ill-formed
if CS is ill-formed or
if sizeof...(Env) > 1 is true;
otherwise, it is an alias for:

- [(2.1)](#2.1)

*gather-signatures*<set_value_t, CS, decay_t, type_identity_t> if that type is well-formed,

- [(2.2)](#2.2)

Otherwise, void if *gather-signatures*<set_value_t, CS, tuple, variant> isvariant<tuple<>> or variant<>,

- [(2.3)](#2.3)

Otherwise, *gather-signatures*<set_value_t, CS, *decayed-tuple*, type_identity_t> if that  
 type is well-formed,

- [(2.4)](#2.4)

Otherwise, *single-sender-value-type*<Sndr, Env...> is ill-formed[.](#2.sentence-2)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L816)

The exposition-only concept [*single-sender*](#concept:single-sender "33.4 Header <execution> synopsis [execution.syn]") is defined as follows:namespace std::execution {template<class Sndr, class... Env>concept [*single-sender*](#concept:single-sender "33.4 Header <execution> synopsis [execution.syn]") = [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<Sndr, Env...> &&requires {typename *single-sender-value-type*<Sndr, Env...>; };}

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L828)

A type satisfies and models the exposition-only concept[*valid-completion-signatures*](#concept:valid-completion-signatures "33.4 Header <execution> synopsis [execution.syn]") if
it is a specialization of the completion_signatures class template[.](#4.sentence-1)