4198. schedule_from isn't starting the schedule sender if decay-copying results throws

Section: 33.9.12.7 [exec.schedule.from] Status: WP Submitter: Eric Niebler Opened: 2025-02-03 Last modified: 2025-06-23

Priority: 1

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Discussion:

Imported from cplusplus/sender-receiver #304.

33.9.12.7 [exec.schedule.from]/p11 specifies schedule_from's completion operation as follows:

[]<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept
    -> void {
  using result_t = decayed-tuple<Tag, Args...>;
  constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>;

  try {
    state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
  } catch (...) {
    if constexpr (!nothrow) {
      set_error(std::move(rcvr), current_exception());
      return;
    }
  }
  start(state.op-state);
};

Suggested resolution

The right thing, i think, is to catch the exception, emplace the exception_ptr into the async-result variant, and then start the schedule operation, as shown below:

[]<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept
    -> void {
  using result_t = decayed-tuple<Tag, Args...>;
  constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>;

  try {
    state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
  } catch(...) {
    if constexpr (!nothrow)
      state.async-result.template emplace<tuple<set_error_t, exception_ptr>>(set_error, current_exception());
  }

  start(state.op-state);
}

Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>. Let as-tuple be an alias template that transforms a completion signature Tag(Args...) into the tuple specialization decayed-tuple<Tag, Args...>. such that as-tuple<Tag(Args...)> denotes the tuple specialization decayed-tuple<Tag, Args...>, and let is-nothrow-decay-copy-sig be a variable template such that is-nothrow-decay-copy-sig<Tag(Args...)> is a core constant expression of type bool const and whose value is true if the types Args... are all nothrow decay-copyable, and false otherwise. Let error-completion be a pack consisting of the type set_error_t(exception_ptr) if (is-nothrow-decay-copy-sig<Sigs> &&...) is false, and an empty pack otherwise. Then variant_t denotes the type variant<monostate, as-tuple<Sigs>..., error-completion...>, except with duplicate types removed.

[This touches the same text as LWG 4203.]

[2025-02-07; Reflector poll]

Set priority to 1 after reflector poll.

[Hagenberg 2025-02-11; LWG]

Direction seems right. Decay-copyable is not a defined term.

[2025-02-12 Tim adds PR]

This also corrects the issue that nothrow is currently relying on the unspecified exception specification of tuple's constructor.

[Hagenberg 2025-02-12; move to Ready]

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

Proposed resolution:

This wording is relative to N5001.

1. Modify 33.9.12.7 [exec.schedule.from] p8 as indicated:

   -8- Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>. Let as-tuple be an alias template that transforms a completion signature Tag(Args...) into the tuple specialization decayed-tuple<Tag, Args...>. such that as-tuple<Tag(Args...)> denotes the type decayed-tuple<Tag, Args...>, and let is-nothrow-decay-copy-sig be a variable template such that auto(is-nothrow-decay-copy-sig<Tag(Args...)>) is a constant expression of type bool and equal to (is_nothrow_constructible_v<decay_t<Args>, Args> && ...). Let error-completion be a pack consisting of the type set_error_t(exception_ptr) if (is-nothrow-decay-copy-sig<Sigs> &&...) is false, and an empty pack otherwise. Then variant_t denotes the type variant<monostate, as-tuple<Sigs>..., error-completion...>, except with duplicate types removed.

2. Modify 33.9.12.7 [exec.schedule.from] p11 as indicated:

   -11- The member impls-for<schedule_from_t>::complete is initialized with a callable object equivalent to the following lambda:

   
   []<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept
       -> void {
     using result_t = decayed-tuple<Tag, Args...>;
     constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>
                              (is_nothrow_constructible_v<decay_t<Args>, Args> && ...);
   
     try {
       state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
     } catch (...) {
       if constexpr (!nothrow) {
         state.async-result.template emplace<tuple<set_error_t, exception_ptr>>(set_error, current_exception());
         set_error(std::move(rcvr), current_exception());
         return;
       }
     }
     start(state.op-state);
   };