From Jason Turner

[exec.queries]

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+ ## Queries <a id="exec.queries">[[exec.queries]]</a>
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+
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+ ### `forwarding_query` <a id="exec.fwd.env">[[exec.fwd.env]]</a>
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+
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+ `forwarding_query` asks a query object whether it should be forwarded
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+ through queryable adaptors.
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+
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+ The name `forwarding_query` denotes a query object. For some query
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+ object `q` of type `Q`, `forwarding_query(q)` is expression-equivalent
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+ to:
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+
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+ - `MANDATE-NOTHROW(q.query(forwarding_query))` if that expression is
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+ well-formed. *Mandates:* The expression above has type `bool` and is a
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+ core constant expression if `q` is a core constant expression.
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+ - Otherwise, `true` if `derived_from<Q, forwarding_query_t>` is `true`.
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+ - Otherwise, `false`.
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+
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+ ### `get_allocator` <a id="exec.get.allocator">[[exec.get.allocator]]</a>
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+
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+ `get_allocator` asks a queryable object for its associated allocator.
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+
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+ The name `get_allocator` denotes a query object. For a subexpression
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+ `env`, `get_allocator(env)` is expression-equivalent to
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+ `MANDATE-NOTHROW(AS-CONST(env).query(get_allocator))`.
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+
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+ *Mandates:* If the expression above is well-formed, its type satisfies
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+ `simple-allocator` [[allocator.requirements.general]].
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+
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+ `forwarding_query(get_allocator)` is a core constant expression and has
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+ value `true`.
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+
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+ ### `get_stop_token` <a id="exec.get.stop.token">[[exec.get.stop.token]]</a>
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+
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+ `get_stop_token` asks a queryable object for an associated stop token.
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+
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+ The name `get_stop_token` denotes a query object. For a subexpression
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+ `env`, `get_stop_token(env)` is expression-equivalent to:
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+
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+ - `MANDATE-NOTHROW(AS-CONST(env).query(get_stop_token))` if that
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+ expression is well-formed. *Mandates:* The type of the expression
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+ above satisfies `stoppable_token`.
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+ - Otherwise, `never_stop_token{}`.
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+
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+ `forwarding_query(get_stop_token)` is a core constant expression and has
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+ value `true`.
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+
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+ ### `execution::get_env` <a id="exec.get.env">[[exec.get.env]]</a>
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+
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+ `execution::get_env` is a customization point object. For a
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+ subexpression `o`, `execution::get_env(o)` is expression-equivalent to:
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+
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+ - `MANDATE-NOTHROW(AS-CONST(o).get_env())` if that expression is
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+ well-formed. *Mandates:* The type of the expression above satisfies
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+ `queryable` [[exec.queryable]].
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+ - Otherwise, `env<>{}`.
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+
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+ The value of `get_env(o)` shall be valid while `o` is valid.
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+
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+ [*Note 1*: When passed a sender object, `get_env` returns the sender’s
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+ associated attributes. When passed a receiver, `get_env` returns the
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+ receiver’s associated execution environment. — *end note*]
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+
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+ ### `execution::get_domain` <a id="exec.get.domain">[[exec.get.domain]]</a>
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+
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+ `get_domain` asks a queryable object for its associated execution domain
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+ tag.
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+
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+ The name `get_domain` denotes a query object. For a subexpression `env`,
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+ `get_domain(env)` is expression-equivalent to
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+ `MANDATE-NOTHROW(AS-CONST(env).query(get_domain))`.
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+
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+ `forwarding_query(execution::get_domain)` is a core constant expression
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+ and has value `true`.
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+
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+ ### `execution::get_scheduler` <a id="exec.get.scheduler">[[exec.get.scheduler]]</a>
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+
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+ `get_scheduler` asks a queryable object for its associated scheduler.
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+
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+ The name `get_scheduler` denotes a query object. For a subexpression
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+ `env`, `get_scheduler(env)` is expression-equivalent to
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+ `MANDATE-NOTHROW(AS-CONST(env).query(get_scheduler))`.
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+
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+ *Mandates:* If the expression above is well-formed, its type satisfies
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+ `scheduler`.
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+
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+ `forwarding_query(execution::get_scheduler)` is a core constant
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+ expression and has value `true`.
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+
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+ ### `execution::get_delegation_scheduler` <a id="exec.get.delegation.scheduler">[[exec.get.delegation.scheduler]]</a>
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+
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+ `get_delegation_scheduler` asks a queryable object for a scheduler that
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+ can be used to delegate work to for the purpose of forward progress
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+ delegation [[intro.progress]].
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+
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+ The name `get_delegation_scheduler` denotes a query object. For a
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+ subexpression `env`, `get_delegation_scheduler(env)` is
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+ expression-equivalent to
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+ `MANDATE-NOTHROW(AS-CONST(env).query(get_delegation_scheduler))`.
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+
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+ *Mandates:* If the expression above is well-formed, its type satisfies
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+ `scheduler`.
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+
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+ `forwarding_query(execution::get_delegation_scheduler)` is a core
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+ constant expression and has value `true`.
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+
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+ ### `execution::get_forward_progress_guarantee` <a id="exec.get.fwd.progress">[[exec.get.fwd.progress]]</a>
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+
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+ ``` cpp
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+ namespace std::execution {
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+ enum class forward_progress_guarantee {
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+ concurrent,
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+ parallel,
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+ weakly_parallel
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+ };
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+ }
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+ ```
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+
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+ `get_forward_progress_guarantee` asks a scheduler about the forward
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+ progress guarantee of execution agents created by that scheduler’s
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+ associated execution resource [[intro.progress]].
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+
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+ The name `get_forward_progress_guarantee` denotes a query object. For a
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+ subexpression `sch`, let `Sch` be `decltype((sch))`. If `Sch` does not
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+ satisfy `scheduler`, `get_forward_progress_guarantee` is ill-formed.
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+ Otherwise, `get_forward_progress_guarantee(sch)` is
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+ expression-equivalent to:
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+
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+ - `MANDATE-NOTHROW(AS-CONST(sch).query(get_forward_progress_guarantee))`,
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+ if that expression is well-formed. *Mandates:* The type of the
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+ expression above is `forward_progress_guarantee`.
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+ - Otherwise, `forward_progress_guarantee::weakly_parallel`.
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+
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+ If `get_forward_progress_guarantee(sch)` for some scheduler `sch`
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+ returns `forward_progress_guarantee::concurrent`, all execution agents
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+ created by that scheduler’s associated execution resource shall provide
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+ the concurrent forward progress guarantee. If it returns
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+ `forward_progress_guarantee::parallel`, all such execution agents shall
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+ provide at least the parallel forward progress guarantee.
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+
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+ ### `execution::get_completion_scheduler` <a id="exec.get.compl.sched">[[exec.get.compl.sched]]</a>
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+
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+ `get_completion_scheduler<completion-tag>` obtains the completion
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+ scheduler associated with a completion tag from a sender’s attributes.
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+
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+ The name `get_completion_scheduler` denotes a query object template. For
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+ a subexpression `q`, the expression
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+ `get_completion_scheduler<completion-tag>(q)` is ill-formed if
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+ *`completion-tag`* is not one of `set_value_t`, `set_error_t`, or
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+ `set_stopped_t`. Otherwise,
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+ `get_completion_scheduler<completion-tag>(q)` is expression-equivalent
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+ to
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+
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+ ``` cpp
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+ MANDATE-NOTHROW(AS-CONST(q).query(get_completion_scheduler<completion-tag>))
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+ ```
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+
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+ *Mandates:* If the expression above is well-formed, its type satisfies
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+ `scheduler`.
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+
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+ Let *`completion-fn`* be a completion function [[exec.async.ops]]; let
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+ *`completion-tag`* be the associated completion tag of
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+ *`completion-fn`*; let `args` be a pack of subexpressions; and let
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+ `sndr` be a subexpression such that `sender<decltype((sndr))>` is `true`
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+ and `get_completion_scheduler<completion-tag>(get_env(sndr))` is
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+ well-formed and denotes a scheduler `sch`. If an asynchronous operation
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+ created by connecting `sndr` with a receiver `rcvr` causes the
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+ evaluation of `completion-fn(rcvr, args...)`, the behavior is undefined
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+ unless the evaluation happens on an execution agent that belongs to
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+ `sch`’s associated execution resource.
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+
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+ The expression
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+ `forwarding_query(get_completion_scheduler<completion-tag>)` is a core
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+ constant expression and has value `true`.
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+
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+ ### `execution::get_await_completion_adaptor` <a id="exec.get.await.adapt">[[exec.get.await.adapt]]</a>
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+
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+ `get_await_completion_adaptor` asks a queryable object for its
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+ associated awaitable completion adaptor.
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+
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+ The name `get_await_completion_adaptor` denotes a query object. For a
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+ subexpression `env`,
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+
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+ ``` cpp
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+ get_await_completion_adaptor(env)
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+ ```
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+
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+ is expression-equivalent to
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+
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+ ``` cpp
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+ MANDATE-NOTHROW(AS-CONST(env).query(get_await_completion_adaptor))
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+ ```
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+
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+ `forwarding_query(execution::get_await_completion_adaptor)`
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+
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+ is a core constant expression and has value `true`.
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+