33 Execution control library [exec]

write_env is a sender adaptor that accepts a sender and a queryable object, and that returns a sender that, when connected with a receiver rcvr, connects the adapted sender with a receiver whose execution environment is the result of joining the queryable object to the result of get_env(rcvr).

write_env is a customization point object.

For some subexpressions sndr and env, if decltype((sndr)) does not satisfy sender or if decltype((env)) does not satisfy queryable, the expression write_env(sndr, env) is ill-formed.

Otherwise, it is expression-equivalent to make-sender(write_env, env, sndr).

Let write-env-t denote the type decltype(auto(write_env)).

The exposition-only class template impls-for ([exec.snd.general]) is specialized for write-env-t as follows: template<> struct impls-for<write-env-t> : default-impls { static constexpr auto get-env = [](auto, const auto& state, const auto& rcvr) noexcept { return see below; }; };

Invocation of impls-for<write-env-t>​::​get-env returns an object e such that

• (3.1)
  decltype(e) models queryable and
• (3.2)
  given a query object q, the expression e.query(q) is expression-equivalent to state.query(q) if that expression is valid, otherwise, e.query(q) is expression-equivalent to get_env(rcvr).query(q).