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4209. default_domain::transform_env should be returning FWD-ENV(env)

Section: 33.9.5 [exec.domain.default] Status: WP Submitter: Eric Niebler Opened: 2025-02-07 Last modified: 2025-06-23

Priority: Not Prioritized

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Discussion:

Imported from cplusplus/sender-receiver #168.

When writing a generic recursive sender transform, you need to ability to unpack an unknown sender S and recursively transform the children.

For that, it can be useful to know the type of the environment that S will use when connecting its child senders, which is why transform_env exists.

For an environment E and a sender S with tag T child C, the expression default_domain().transform_env(S, E) should return an environment E2 that is identical to the environment of the receiver that S uses to connect C.

default_domain().transform_env(S, E) will first check whether T().transform_env(S, E) is well-formed. If so, it will return that (e.g. when_all_t has a transform_env that adds a stop token to the environment).

If T().transform_env(S, E) is not well-formed, what should default_domain::transform_env do? At present, it returns E unmodified.

But 33.9.12.1 [exec.adapt.general] has this:

[unless otherwise specified, when] a parent sender is connected to a receiver rcvr, any receiver used to connect a child sender has an associated environment equal to FWD-ENV(get_env(rcvr)).

So the correct thing for default_domain::transform_env to do is to return FWD-ENV(get_env(rcvr)).

[Hagenberg 2025-02-11; move to Ready]

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.9.5 [exec.domain.default] as indicated:

   template<sender Sndr, queryable Env>
     constexpr queryable decltype(auto) transform_env(Sndr&& sndr, Env&& env) noexcept;
   

   -5- Let e be the expression

   tag_of_t<Sndr>().transform_env(std::forward<Sndr>(sndr), std::forward<Env>(env))
   

   if that expression is well-formed; otherwise, static_cast<Env>FWD-ENV(std::forward<Env>(env)).

   -6- Mandates: noexcept(e) is true.

   -7- Returns: e.