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Fix run_loop sender's zero-environment completion signatures #321

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Merged
dietmarkuehl merged 1 commit into from
Sep 22, 2026
+25 −3
Merged

Fix run_loop sender's zero-environment completion signatures #321

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17 changes: 14 additions & 3 deletions include/beman/execution/detail/run_loop.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,7 @@ import beman.execution.detail.set_value;
import beman.execution.detail.unstoppable_token;
#else
#include <beman/execution/detail/completion_signatures.hpp>
#include <beman/execution/detail/env.hpp>
#include <beman/execution/detail/get_completion_scheduler.hpp>
#include <beman/execution/detail/get_env.hpp>
#include <beman/execution/detail/get_forward_progress_guarantee.hpp>
Expand Down Expand Up @@ -90,10 +91,20 @@ class run_loop {
};
struct sender {
using sender_concept = ::beman::execution::sender_tag;
template <typename, typename... Env>
template <typename Self, typename... Env>
static consteval auto get_completion_signatures() noexcept {
if constexpr (::beman::execution::unstoppable_token<decltype(::beman::execution::get_stop_token(
std::declval<Env>()...))>)
// [exec.getcomplsigs] permits sizeof...(Env) == 0. Answering that

@steve-downey steve-downey Sep 21, 2026

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This amount of comment is probably not actually warranted.

// query as if for env<> matches [exec.run.loop]'s formula, whose
// E = env<> has an unstoppable stop token, and matches how the
// other environment-sensitive senders here (then, into_variant)
// handle the empty pack. Without this the member's body is
// ill-formed for an empty pack -- get_stop_token() with no
// argument -- and because the return type is deduced, that is a
// hard error rather than a substitution failure.
if constexpr (0u == sizeof...(Env))
return get_completion_signatures<Self, ::beman::execution::env<>>();
else if constexpr (::beman::execution::unstoppable_token<decltype(::beman::execution::get_stop_token(
std::declval<Env>()...))>)
return ::beman::execution::completion_signatures<::beman::execution::set_value_t()>{};
else
return ::beman::execution::completion_signatures<::beman::execution::set_value_t(),
Expand Down
11 changes: 11 additions & 0 deletions tests/beman/execution/exec-run-loop-types.test.cpp
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Expand Up @@ -16,6 +16,7 @@ import beman.execution;
#include <beman/execution/detail/inplace_stop_source.hpp>
#include <beman/execution/detail/completion_signatures.hpp>
#include <beman/execution/detail/connect.hpp>
#include <beman/execution/detail/dependent_sender.hpp>
#include <beman/execution/detail/start.hpp>
#include <beman/execution/detail/get_completion_signatures.hpp>
#include <beman/execution/detail/get_env.hpp>
Expand Down Expand Up @@ -98,6 +99,16 @@ TEST(exec_run_loop_types) {
::std::same_as<test_std::completion_signatures<test_std::set_value_t(), test_std::set_stopped_t()>,
decltype(test_std::get_completion_signatures<decltype(sender),
decltype(token_env{source.get_token()})>())>);
// [exec.getcomplsigs] constrains sizeof...(Env) <= 1, so the
// zero-environment query is well-formed and must be answered. It is
// answered as for env<>, whose stop token is unstoppable.
static_assert(::std::same_as<test_std::completion_signatures<test_std::set_value_t()>,
decltype(test_std::get_completion_signatures<decltype(sender)>())>);
// ... and because detail::non_dependent_sender is itself spelled with that
// query inside a requires-expression, a sender that cannot answer it makes
// the concept ill-formed rather than false. Asserting the concept is what
// pins that down.
static_assert(not test_std::dependent_sender<decltype(sender)>);
// p7:
static_assert(
test_std::receiver_of<receiver, decltype(test_std::get_completion_signatures<decltype(sender), env>())>);
Expand Down
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