From Jason Turner

[exec.envs]

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+ ## Queryable utilities <a id="exec.envs">[[exec.envs]]</a>
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+
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+ ### Class template `prop` <a id="exec.prop">[[exec.prop]]</a>
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+
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+ ``` cpp
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+ namespace std::execution {
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+ template<class QueryTag, class ValueType>
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+ struct prop {
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+ QueryTag query_; // exposition only
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+ ValueType value_; // exposition only
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+
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+ constexpr const ValueType& query(QueryTag) const noexcept {
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+ return value_;
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+ }
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+ };
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+
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+ template<class QueryTag, class ValueType>
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+ prop(QueryTag, ValueType) -> prop<QueryTag, unwrap_reference_t<ValueType>>;
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+ }
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+ ```
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+
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+ Class template `prop` is for building a queryable object from a query
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+ object and a value.
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+
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+ *Mandates:* `callable<QueryTag, prop-like<ValueType>>` is modeled, where
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+ *`prop-like`* is the following exposition-only class template:
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+
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+ ``` cpp
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+ template<class ValueType>
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+ struct prop-like { // exposition only
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+ const ValueType& query(auto) const noexcept;
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+ };
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+ ```
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+
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+ [*Example 1*:
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+
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+ ``` cpp
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+ template<sender Sndr>
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+ sender auto parameterize_work(Sndr sndr) {
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+ // Make an environment such that get_allocator(env) returns a reference to a copy of my_alloc{}.
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+ auto e = prop(get_allocator, my_alloc{});
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+
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+ // Parameterize the input sender so that it will use our custom execution environment.
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+ return write_env(sndr, e);
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+ }
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+ ```
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+
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+ — *end example*]
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+
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+ Specializations of `prop` are not assignable.
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+
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+ ### Class template `env` <a id="exec.env">[[exec.env]]</a>
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+
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+ ``` cpp
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+ namespace std::execution {
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+ template<queryable... Envs>
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+ struct env {
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+ Envs_0 envs_0; // exposition only
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+ Envs_1 envs_1; // exposition only
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+ ⋮
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+ Envs_{n-1} envs_{n-1}; // exposition only
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+
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+ template<class QueryTag>
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+ constexpr decltype(auto) query(QueryTag q) const noexcept(see below);
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+ };
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+
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+ template<class... Envs>
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+ env(Envs...) -> env<unwrap_reference_t<Envs>...>;
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+ }
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+ ```
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+
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+ The class template `env` is used to construct a queryable object from
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+ several queryable objects. Query invocations on the resulting object are
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+ resolved by attempting to query each subobject in lexical order.
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+
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+ Specializations of `env` are not assignable.
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+
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+ It is unspecified whether `env` supports initialization using a
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+ parenthesized *expression-list* [[dcl.init]], unless the
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+ *expression-list* consist of a single element of type (possibly const)
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+ `env`.
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+
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+ [*Example 1*:
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+
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+ ``` cpp
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+ template<sender Sndr>
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+ sender auto parameterize_work(Sndr sndr) {
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+ // Make an environment such that:
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+ // get_allocator(env) returns a reference to a copy of my_alloc{}
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+ // get_scheduler(env) returns a reference to a copy of my_sched{}
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+ auto e = env{prop(get_allocator, my_alloc{}),
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+ prop(get_scheduler, my_sched{})};
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+
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+ // Parameterize the input sender so that it will use our custom execution environment.
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+ return write_env(sndr, e);
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+ }
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+ ```
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+
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+ — *end example*]
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+
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+ ``` cpp
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+ template<class QueryTag>
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+ constexpr decltype(auto) query(QueryTag q) const noexcept(see below);
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+ ```
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+
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+ Let `has-query` be the following exposition-only concept:
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+
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+ ``` cpp
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+ template<class Env, class QueryTag>
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+ concept has-query = // exposition only
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+ requires (const Env& env) {
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+ env.query(QueryTag());
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+ };
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+ ```
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+
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+ Let *fe* be the first element of envs₀, envs₁, …, envsₙ₋₁ such that the
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+ expression *`fe`*`.query(q)` is well-formed.
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+
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+ *Constraints:* `(has-query<Envs, QueryTag> || ...)` is `true`.
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+
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+ *Effects:* Equivalent to: `return `*`fe`*`.query(q);`
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+
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+ *Remarks:* The expression in the `noexcept` clause is equivalent to
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+ `noexcept(`*`fe`*`.query(q))`.
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+