1   
//
1   
//
2   
// Copyright (c) 2026 Michael Vandeberg
2   
// Copyright (c) 2026 Michael Vandeberg
3   
//
3   
//
4   
// Distributed under the Boost Software License, Version 1.0. (See accompanying
4   
// Distributed under the Boost Software License, Version 1.0. (See accompanying
5   
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//
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//
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// Official repository: https://github.com/cppalliance/capy
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// Official repository: https://github.com/cppalliance/capy
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//
8   
//
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9  
10   
#ifndef BOOST_CAPY_WHEN_ANY_HPP
10   
#ifndef BOOST_CAPY_WHEN_ANY_HPP
11   
#define BOOST_CAPY_WHEN_ANY_HPP
11   
#define BOOST_CAPY_WHEN_ANY_HPP
12  
12  
13   
#include <boost/capy/detail/config.hpp>
13   
#include <boost/capy/detail/config.hpp>
14   
#include <boost/capy/detail/void_to_monostate.hpp>
14   
#include <boost/capy/detail/void_to_monostate.hpp>
15   
#include <boost/capy/concept/executor.hpp>
15   
#include <boost/capy/concept/executor.hpp>
16   
#include <boost/capy/concept/io_awaitable.hpp>
16   
#include <boost/capy/concept/io_awaitable.hpp>
17   
#include <coroutine>
17   
#include <coroutine>
18   
#include <boost/capy/ex/executor_ref.hpp>
18   
#include <boost/capy/ex/executor_ref.hpp>
19   
#include <boost/capy/ex/frame_allocator.hpp>
19   
#include <boost/capy/ex/frame_allocator.hpp>
20   
#include <boost/capy/ex/io_env.hpp>
20   
#include <boost/capy/ex/io_env.hpp>
21   
#include <boost/capy/task.hpp>
21   
#include <boost/capy/task.hpp>
22  
22  
23   
#include <array>
23   
#include <array>
24   
#include <atomic>
24   
#include <atomic>
25   
#include <exception>
25   
#include <exception>
26   
#include <optional>
26   
#include <optional>
27   
#include <ranges>
27   
#include <ranges>
28   
#include <stdexcept>
28   
#include <stdexcept>
29   
#include <stop_token>
29   
#include <stop_token>
30   
#include <tuple>
30   
#include <tuple>
31   
#include <type_traits>
31   
#include <type_traits>
32   
#include <utility>
32   
#include <utility>
33   
#include <variant>
33   
#include <variant>
34   
#include <vector>
34   
#include <vector>
35  
35  
36   
/*
36   
/*
37   
   when_any - Race multiple tasks, return first completion
37   
   when_any - Race multiple tasks, return first completion
38   
   ========================================================
38   
   ========================================================
39  
39  
40   
   OVERVIEW:
40   
   OVERVIEW:
41   
   ---------
41   
   ---------
42   
   when_any launches N tasks concurrently and completes when the FIRST task
42   
   when_any launches N tasks concurrently and completes when the FIRST task
43   
   finishes (success or failure). It then requests stop for all siblings and
43   
   finishes (success or failure). It then requests stop for all siblings and
44   
   waits for them to acknowledge before returning.
44   
   waits for them to acknowledge before returning.
45  
45  
46   
   ARCHITECTURE:
46   
   ARCHITECTURE:
47   
   -------------
47   
   -------------
48   
   The design mirrors when_all but with inverted completion semantics:
48   
   The design mirrors when_all but with inverted completion semantics:
49  
49  
50   
     when_all:  complete when remaining_count reaches 0 (all done)
50   
     when_all:  complete when remaining_count reaches 0 (all done)
51   
     when_any:  complete when has_winner becomes true (first done)
51   
     when_any:  complete when has_winner becomes true (first done)
52   
                BUT still wait for remaining_count to reach 0 for cleanup
52   
                BUT still wait for remaining_count to reach 0 for cleanup
53  
53  
54   
   Key components:
54   
   Key components:
55   
     - when_any_state:    Shared state tracking winner and completion
55   
     - when_any_state:    Shared state tracking winner and completion
56   
     - when_any_runner:   Wrapper coroutine for each child task
56   
     - when_any_runner:   Wrapper coroutine for each child task
57   
     - when_any_launcher: Awaitable that starts all runners concurrently
57   
     - when_any_launcher: Awaitable that starts all runners concurrently
58  
58  
59   
   CRITICAL INVARIANTS:
59   
   CRITICAL INVARIANTS:
60   
   --------------------
60   
   --------------------
61   
   1. Exactly one task becomes the winner (via atomic compare_exchange)
61   
   1. Exactly one task becomes the winner (via atomic compare_exchange)
62   
   2. All tasks must complete before parent resumes (cleanup safety)
62   
   2. All tasks must complete before parent resumes (cleanup safety)
63   
   3. Stop is requested immediately when winner is determined
63   
   3. Stop is requested immediately when winner is determined
64   
   4. Only the winner's result/exception is stored
64   
   4. Only the winner's result/exception is stored
65  
65  
66   
   POSITIONAL VARIANT:
66   
   POSITIONAL VARIANT:
67   
   -------------------
67   
   -------------------
68   
   The variadic overload returns a std::variant with one alternative per
68   
   The variadic overload returns a std::variant with one alternative per
69   
   input task, preserving positional correspondence. Use .index() on
69   
   input task, preserving positional correspondence. Use .index() on
70   
   the variant to identify which task won.
70   
   the variant to identify which task won.
71  
71  
72   
   Example: when_any(task<int>, task<string>, task<int>)
72   
   Example: when_any(task<int>, task<string>, task<int>)
73   
     - Raw types after void->monostate: int, string, int
73   
     - Raw types after void->monostate: int, string, int
74   
     - Result variant: std::variant<int, string, int>
74   
     - Result variant: std::variant<int, string, int>
75   
     - variant.index() tells you which task won (0, 1, or 2)
75   
     - variant.index() tells you which task won (0, 1, or 2)
76  
76  
77   
   VOID HANDLING:
77   
   VOID HANDLING:
78   
   --------------
78   
   --------------
79   
   void tasks contribute std::monostate to the variant.
79   
   void tasks contribute std::monostate to the variant.
80   
   All-void tasks result in: variant<monostate, monostate, monostate>
80   
   All-void tasks result in: variant<monostate, monostate, monostate>
81  
81  
82   
   MEMORY MODEL:
82   
   MEMORY MODEL:
83   
   -------------
83   
   -------------
84   
   Synchronization chain from winner's write to parent's read:
84   
   Synchronization chain from winner's write to parent's read:
85  
85  
86   
   1. Winner thread writes result_/winner_exception_ (non-atomic)
86   
   1. Winner thread writes result_/winner_exception_ (non-atomic)
87   
   2. Winner thread calls signal_completion() → fetch_sub(acq_rel) on remaining_count_
87   
   2. Winner thread calls signal_completion() → fetch_sub(acq_rel) on remaining_count_
88   
   3. Last task thread (may be winner or non-winner) calls signal_completion()
88   
   3. Last task thread (may be winner or non-winner) calls signal_completion()
89   
      → fetch_sub(acq_rel) on remaining_count_, observing count becomes 0
89   
      → fetch_sub(acq_rel) on remaining_count_, observing count becomes 0
90   
   4. Last task returns caller_ex_.dispatch(continuation_) via symmetric transfer
90   
   4. Last task returns caller_ex_.dispatch(continuation_) via symmetric transfer
91   
   5. Parent coroutine resumes and reads result_/winner_exception_
91   
   5. Parent coroutine resumes and reads result_/winner_exception_
92  
92  
93   
   Synchronization analysis:
93   
   Synchronization analysis:
94   
   - All fetch_sub operations on remaining_count_ form a release sequence
94   
   - All fetch_sub operations on remaining_count_ form a release sequence
95   
   - Winner's fetch_sub releases; subsequent fetch_sub operations participate
95   
   - Winner's fetch_sub releases; subsequent fetch_sub operations participate
96   
     in the modification order of remaining_count_
96   
     in the modification order of remaining_count_
97   
   - Last task's fetch_sub(acq_rel) synchronizes-with prior releases in the
97   
   - Last task's fetch_sub(acq_rel) synchronizes-with prior releases in the
98   
     modification order, establishing happens-before from winner's writes
98   
     modification order, establishing happens-before from winner's writes
99   
   - Executor dispatch() is expected to provide queue-based synchronization
99   
   - Executor dispatch() is expected to provide queue-based synchronization
100   
     (release-on-post, acquire-on-execute) completing the chain to parent
100   
     (release-on-post, acquire-on-execute) completing the chain to parent
101   
   - Even inline executors work (same thread = sequenced-before)
101   
   - Even inline executors work (same thread = sequenced-before)
102  
102  
103   
   Alternative considered: Adding winner_ready_ atomic (set with release after
103   
   Alternative considered: Adding winner_ready_ atomic (set with release after
104   
   storing winner data, acquired before reading) would make synchronization
104   
   storing winner data, acquired before reading) would make synchronization
105   
   self-contained and not rely on executor implementation details. Current
105   
   self-contained and not rely on executor implementation details. Current
106   
   approach is correct but requires careful reasoning about release sequences
106   
   approach is correct but requires careful reasoning about release sequences
107   
   and executor behavior.
107   
   and executor behavior.
108  
108  
109   
   EXCEPTION SEMANTICS:
109   
   EXCEPTION SEMANTICS:
110   
   --------------------
110   
   --------------------
111   
   Unlike when_all (which captures first exception, discards others), when_any
111   
   Unlike when_all (which captures first exception, discards others), when_any
112   
   treats exceptions as valid completions. If the winning task threw, that
112   
   treats exceptions as valid completions. If the winning task threw, that
113   
   exception is rethrown. Exceptions from non-winners are silently discarded.
113   
   exception is rethrown. Exceptions from non-winners are silently discarded.
114   
*/
114   
*/
115  
115  
116   
namespace boost {
116   
namespace boost {
117   
namespace capy {
117   
namespace capy {
118  
118  
119   
namespace detail {
119   
namespace detail {
120  
120  
121   
/** Core shared state for when_any operations.
121   
/** Core shared state for when_any operations.
122  
122  
123   
    Contains all members and methods common to both heterogeneous (variadic)
123   
    Contains all members and methods common to both heterogeneous (variadic)
124   
    and homogeneous (range) when_any implementations. State classes embed
124   
    and homogeneous (range) when_any implementations. State classes embed
125   
    this via composition to avoid CRTP destructor ordering issues.
125   
    this via composition to avoid CRTP destructor ordering issues.
126  
126  
127   
    @par Thread Safety
127   
    @par Thread Safety
128   
    Atomic operations protect winner selection and completion count.
128   
    Atomic operations protect winner selection and completion count.
129   
*/
129   
*/
130   
struct when_any_core
130   
struct when_any_core
131   
{
131   
{
132   
    std::atomic<std::size_t> remaining_count_;
132   
    std::atomic<std::size_t> remaining_count_;
133   
    std::size_t winner_index_{0};
133   
    std::size_t winner_index_{0};
134   
    std::exception_ptr winner_exception_;
134   
    std::exception_ptr winner_exception_;
135   
    std::stop_source stop_source_;
135   
    std::stop_source stop_source_;
136  
136  
137   
    // Bridges parent's stop token to our stop_source
137   
    // Bridges parent's stop token to our stop_source
138   
    struct stop_callback_fn
138   
    struct stop_callback_fn
139   
    {
139   
    {
140   
        std::stop_source* source_;
140   
        std::stop_source* source_;
141   
        void operator()() const noexcept { source_->request_stop(); }
141   
        void operator()() const noexcept { source_->request_stop(); }
142   
    };
142   
    };
143   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
143   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
144   
    std::optional<stop_callback_t> parent_stop_callback_;
144   
    std::optional<stop_callback_t> parent_stop_callback_;
145  
145  
146   
    std::coroutine_handle<> continuation_;
146   
    std::coroutine_handle<> continuation_;
147   
    io_env const* caller_env_ = nullptr;
147   
    io_env const* caller_env_ = nullptr;
148  
148  
149   
    // Placed last to avoid padding (1-byte atomic followed by 8-byte aligned members)
149   
    // Placed last to avoid padding (1-byte atomic followed by 8-byte aligned members)
150   
    std::atomic<bool> has_winner_{false};
150   
    std::atomic<bool> has_winner_{false};
151  
151  
152   
    explicit when_any_core(std::size_t count) noexcept
152   
    explicit when_any_core(std::size_t count) noexcept
153   
        : remaining_count_(count)
153   
        : remaining_count_(count)
154   
    {
154   
    {
155   
    }
155   
    }
156  
156  
157   
    /** Atomically claim winner status; exactly one task succeeds. */
157   
    /** Atomically claim winner status; exactly one task succeeds. */
158   
    bool try_win(std::size_t index) noexcept
158   
    bool try_win(std::size_t index) noexcept
159   
    {
159   
    {
160   
        bool expected = false;
160   
        bool expected = false;
161   
        if(has_winner_.compare_exchange_strong(
161   
        if(has_winner_.compare_exchange_strong(
162   
            expected, true, std::memory_order_acq_rel))
162   
            expected, true, std::memory_order_acq_rel))
163   
        {
163   
        {
164   
            winner_index_ = index;
164   
            winner_index_ = index;
165   
            stop_source_.request_stop();
165   
            stop_source_.request_stop();
166   
            return true;
166   
            return true;
167   
        }
167   
        }
168   
        return false;
168   
        return false;
169   
    }
169   
    }
170  
170  
171   
    /** @pre try_win() returned true. */
171   
    /** @pre try_win() returned true. */
172   
    void set_winner_exception(std::exception_ptr ep) noexcept
172   
    void set_winner_exception(std::exception_ptr ep) noexcept
173   
    {
173   
    {
174   
        winner_exception_ = ep;
174   
        winner_exception_ = ep;
175   
    }
175   
    }
176  
176  
177   
    // Runners signal completion directly via final_suspend; no member function needed.
177   
    // Runners signal completion directly via final_suspend; no member function needed.
178   
};
178   
};
179  
179  
180   
/** Shared state for heterogeneous when_any operation.
180   
/** Shared state for heterogeneous when_any operation.
181  
181  
182   
    Coordinates winner selection, result storage, and completion tracking
182   
    Coordinates winner selection, result storage, and completion tracking
183   
    for all child tasks in a when_any operation. Uses composition with
183   
    for all child tasks in a when_any operation. Uses composition with
184   
    when_any_core for shared functionality.
184   
    when_any_core for shared functionality.
185  
185  
186   
    @par Lifetime
186   
    @par Lifetime
187   
    Allocated on the parent coroutine's frame, outlives all runners.
187   
    Allocated on the parent coroutine's frame, outlives all runners.
188  
188  
189   
    @tparam Ts Task result types.
189   
    @tparam Ts Task result types.
190   
*/
190   
*/
191   
template<typename... Ts>
191   
template<typename... Ts>
192   
struct when_any_state
192   
struct when_any_state
193   
{
193   
{
194   
    static constexpr std::size_t task_count = sizeof...(Ts);
194   
    static constexpr std::size_t task_count = sizeof...(Ts);
195   
    using variant_type = std::variant<void_to_monostate_t<Ts>...>;
195   
    using variant_type = std::variant<void_to_monostate_t<Ts>...>;
196  
196  
197   
    when_any_core core_;
197   
    when_any_core core_;
198   
    std::optional<variant_type> result_;
198   
    std::optional<variant_type> result_;
199   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
199   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
200  
200  
201   
    when_any_state()
201   
    when_any_state()
202   
        : core_(task_count)
202   
        : core_(task_count)
203   
    {
203   
    {
204   
    }
204   
    }
205  
205  
206   
    // Runners self-destruct in final_suspend. No destruction needed here.
206   
    // Runners self-destruct in final_suspend. No destruction needed here.
207  
207  
208   
    /** @pre core_.try_win() returned true.
208   
    /** @pre core_.try_win() returned true.
209   
        @note Uses in_place_index (not type) for positional variant access.
209   
        @note Uses in_place_index (not type) for positional variant access.
210   
    */
210   
    */
211   
    template<std::size_t I, typename T>
211   
    template<std::size_t I, typename T>
212   
    void set_winner_result(T value)
212   
    void set_winner_result(T value)
213   
        noexcept(std::is_nothrow_move_constructible_v<T>)
213   
        noexcept(std::is_nothrow_move_constructible_v<T>)
214   
    {
214   
    {
215   
        result_.emplace(std::in_place_index<I>, std::move(value));
215   
        result_.emplace(std::in_place_index<I>, std::move(value));
216   
    }
216   
    }
217  
217  
218   
    /** @pre core_.try_win() returned true. */
218   
    /** @pre core_.try_win() returned true. */
219   
    template<std::size_t I>
219   
    template<std::size_t I>
220   
    void set_winner_void() noexcept
220   
    void set_winner_void() noexcept
221   
    {
221   
    {
222   
        result_.emplace(std::in_place_index<I>, std::monostate{});
222   
        result_.emplace(std::in_place_index<I>, std::monostate{});
223   
    }
223   
    }
224   
};
224   
};
225  
225  
226   
/** Wrapper coroutine that runs a single child task for when_any.
226   
/** Wrapper coroutine that runs a single child task for when_any.
227  
227  
228   
    Propagates executor/stop_token to the child, attempts to claim winner
228   
    Propagates executor/stop_token to the child, attempts to claim winner
229   
    status on completion, and signals completion for cleanup coordination.
229   
    status on completion, and signals completion for cleanup coordination.
230  
230  
231   
    @tparam StateType The state type (when_any_state or when_any_homogeneous_state).
231   
    @tparam StateType The state type (when_any_state or when_any_homogeneous_state).
232   
*/
232   
*/
233   
template<typename StateType>
233   
template<typename StateType>
234   
struct when_any_runner
234   
struct when_any_runner
235   
{
235   
{
236   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
236   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
237   
    {
237   
    {
238   
        StateType* state_ = nullptr;
238   
        StateType* state_ = nullptr;
239   
        std::size_t index_ = 0;
239   
        std::size_t index_ = 0;
240   
        io_env env_;
240   
        io_env env_;
241  
241  
242   
        when_any_runner get_return_object() noexcept
242   
        when_any_runner get_return_object() noexcept
243   
        {
243   
        {
244   
            return when_any_runner(std::coroutine_handle<promise_type>::from_promise(*this));
244   
            return when_any_runner(std::coroutine_handle<promise_type>::from_promise(*this));
245   
        }
245   
        }
246  
246  
247   
        // Starts suspended; launcher sets up state/ex/token then resumes
247   
        // Starts suspended; launcher sets up state/ex/token then resumes
248   
        std::suspend_always initial_suspend() noexcept
248   
        std::suspend_always initial_suspend() noexcept
249   
        {
249   
        {
250   
            return {};
250   
            return {};
251   
        }
251   
        }
252  
252  
253   
        auto final_suspend() noexcept
253   
        auto final_suspend() noexcept
254   
        {
254   
        {
255   
            struct awaiter
255   
            struct awaiter
256   
            {
256   
            {
257   
                promise_type* p_;
257   
                promise_type* p_;
258   
                bool await_ready() const noexcept { return false; }
258   
                bool await_ready() const noexcept { return false; }
259   
                auto await_suspend(std::coroutine_handle<> h) noexcept
259   
                auto await_suspend(std::coroutine_handle<> h) noexcept
260   
                {
260   
                {
261   
                    // Extract everything needed before self-destruction.
261   
                    // Extract everything needed before self-destruction.
262   
                    auto& core = p_->state_->core_;
262   
                    auto& core = p_->state_->core_;
263   
                    auto* counter = &core.remaining_count_;
263   
                    auto* counter = &core.remaining_count_;
264   
                    auto* caller_env = core.caller_env_;
264   
                    auto* caller_env = core.caller_env_;
265   
                    auto cont = core.continuation_;
265   
                    auto cont = core.continuation_;
266  
266  
267   
                    h.destroy();
267   
                    h.destroy();
268  
268  
269   
                    // If last runner, dispatch parent for symmetric transfer.
269   
                    // If last runner, dispatch parent for symmetric transfer.
270   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
270   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
271   
                    if(remaining == 1)
271   
                    if(remaining == 1)
272   
                        return detail::symmetric_transfer(caller_env->executor.dispatch(cont));
272   
                        return detail::symmetric_transfer(caller_env->executor.dispatch(cont));
273   
                    return detail::symmetric_transfer(std::noop_coroutine());
273   
                    return detail::symmetric_transfer(std::noop_coroutine());
274   
                }
274   
                }
275   
                void await_resume() const noexcept {}
275   
                void await_resume() const noexcept {}
276   
            };
276   
            };
277   
            return awaiter{this};
277   
            return awaiter{this};
278   
        }
278   
        }
279  
279  
280   
        void return_void() noexcept {}
280   
        void return_void() noexcept {}
281  
281  
282   
        // Exceptions are valid completions in when_any (unlike when_all)
282   
        // Exceptions are valid completions in when_any (unlike when_all)
283   
        void unhandled_exception()
283   
        void unhandled_exception()
284   
        {
284   
        {
285   
            if(state_->core_.try_win(index_))
285   
            if(state_->core_.try_win(index_))
286   
                state_->core_.set_winner_exception(std::current_exception());
286   
                state_->core_.set_winner_exception(std::current_exception());
287   
        }
287   
        }
288  
288  
289   
        /** Injects executor and stop token into child awaitables. */
289   
        /** Injects executor and stop token into child awaitables. */
290   
        template<class Awaitable>
290   
        template<class Awaitable>
291   
        struct transform_awaiter
291   
        struct transform_awaiter
292   
        {
292   
        {
293   
            std::decay_t<Awaitable> a_;
293   
            std::decay_t<Awaitable> a_;
294   
            promise_type* p_;
294   
            promise_type* p_;
295  
295  
296   
            bool await_ready() { return a_.await_ready(); }
296   
            bool await_ready() { return a_.await_ready(); }
297   
            auto await_resume() { return a_.await_resume(); }
297   
            auto await_resume() { return a_.await_resume(); }
298  
298  
299   
            template<class Promise>
299   
            template<class Promise>
300   
            auto await_suspend(std::coroutine_handle<Promise> h)
300   
            auto await_suspend(std::coroutine_handle<Promise> h)
301   
            {
301   
            {
302   
                using R = decltype(a_.await_suspend(h, &p_->env_));
302   
                using R = decltype(a_.await_suspend(h, &p_->env_));
303   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
303   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
304   
                    return detail::symmetric_transfer(a_.await_suspend(h, &p_->env_));
304   
                    return detail::symmetric_transfer(a_.await_suspend(h, &p_->env_));
305   
                else
305   
                else
306   
                    return a_.await_suspend(h, &p_->env_);
306   
                    return a_.await_suspend(h, &p_->env_);
307   
            }
307   
            }
308   
        };
308   
        };
309  
309  
310   
        template<class Awaitable>
310   
        template<class Awaitable>
311   
        auto await_transform(Awaitable&& a)
311   
        auto await_transform(Awaitable&& a)
312   
        {
312   
        {
313   
            using A = std::decay_t<Awaitable>;
313   
            using A = std::decay_t<Awaitable>;
314   
            if constexpr (IoAwaitable<A>)
314   
            if constexpr (IoAwaitable<A>)
315   
            {
315   
            {
316   
                return transform_awaiter<Awaitable>{
316   
                return transform_awaiter<Awaitable>{
317   
                    std::forward<Awaitable>(a), this};
317   
                    std::forward<Awaitable>(a), this};
318   
            }
318   
            }
319   
            else
319   
            else
320   
            {
320   
            {
321   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
321   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
322   
            }
322   
            }
323   
        }
323   
        }
324   
    };
324   
    };
325  
325  
326   
    std::coroutine_handle<promise_type> h_;
326   
    std::coroutine_handle<promise_type> h_;
327  
327  
328   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
328   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
329   
        : h_(h)
329   
        : h_(h)
330   
    {
330   
    {
331   
    }
331   
    }
332  
332  
333   
    // Enable move for all clang versions - some versions need it
333   
    // Enable move for all clang versions - some versions need it
334   
    when_any_runner(when_any_runner&& other) noexcept : h_(std::exchange(other.h_, nullptr)) {}
334   
    when_any_runner(when_any_runner&& other) noexcept : h_(std::exchange(other.h_, nullptr)) {}
335  
335  
336   
    // Non-copyable
336   
    // Non-copyable
337   
    when_any_runner(when_any_runner const&) = delete;
337   
    when_any_runner(when_any_runner const&) = delete;
338   
    when_any_runner& operator=(when_any_runner const&) = delete;
338   
    when_any_runner& operator=(when_any_runner const&) = delete;
339   
    when_any_runner& operator=(when_any_runner&&) = delete;
339   
    when_any_runner& operator=(when_any_runner&&) = delete;
340  
340  
341   
    auto release() noexcept
341   
    auto release() noexcept
342   
    {
342   
    {
343   
        return std::exchange(h_, nullptr);
343   
        return std::exchange(h_, nullptr);
344   
    }
344   
    }
345   
};
345   
};
346  
346  
347   
/** Indexed overload for heterogeneous when_any (compile-time index).
347   
/** Indexed overload for heterogeneous when_any (compile-time index).
348  
348  
349   
    Uses compile-time index I for variant construction via in_place_index.
349   
    Uses compile-time index I for variant construction via in_place_index.
350   
    Called from when_any_launcher::launch_one<I>().
350   
    Called from when_any_launcher::launch_one<I>().
351   
*/
351   
*/
352   
template<std::size_t I, IoAwaitable Awaitable, typename StateType>
352   
template<std::size_t I, IoAwaitable Awaitable, typename StateType>
353   
when_any_runner<StateType>
353   
when_any_runner<StateType>
354   
make_when_any_runner(Awaitable inner, StateType* state)
354   
make_when_any_runner(Awaitable inner, StateType* state)
355   
{
355   
{
356   
    using T = awaitable_result_t<Awaitable>;
356   
    using T = awaitable_result_t<Awaitable>;
357   
    if constexpr (std::is_void_v<T>)
357   
    if constexpr (std::is_void_v<T>)
358   
    {
358   
    {
359   
        co_await std::move(inner);
359   
        co_await std::move(inner);
360   
        if(state->core_.try_win(I))
360   
        if(state->core_.try_win(I))
361   
            state->template set_winner_void<I>();
361   
            state->template set_winner_void<I>();
362   
    }
362   
    }
363   
    else
363   
    else
364   
    {
364   
    {
365   
        auto result = co_await std::move(inner);
365   
        auto result = co_await std::move(inner);
366   
        if(state->core_.try_win(I))
366   
        if(state->core_.try_win(I))
367   
        {
367   
        {
368   
            try
368   
            try
369   
            {
369   
            {
370   
                state->template set_winner_result<I>(std::move(result));
370   
                state->template set_winner_result<I>(std::move(result));
371   
            }
371   
            }
372   
            catch(...)
372   
            catch(...)
373   
            {
373   
            {
374   
                state->core_.set_winner_exception(std::current_exception());
374   
                state->core_.set_winner_exception(std::current_exception());
375   
            }
375   
            }
376   
        }
376   
        }
377   
    }
377   
    }
378   
}
378   
}
379  
379  
380   
/** Runtime-index overload for homogeneous when_any (range path).
380   
/** Runtime-index overload for homogeneous when_any (range path).
381  
381  
382   
    Uses requires-expressions to detect state capabilities:
382   
    Uses requires-expressions to detect state capabilities:
383   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
383   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
384   
    - set_winner_result(): for non-void tasks
384   
    - set_winner_result(): for non-void tasks
385   
    - Neither: for homogeneous void tasks (no result storage)
385   
    - Neither: for homogeneous void tasks (no result storage)
386   
*/
386   
*/
387   
template<IoAwaitable Awaitable, typename StateType>
387   
template<IoAwaitable Awaitable, typename StateType>
388   
when_any_runner<StateType>
388   
when_any_runner<StateType>
389   
make_when_any_runner(Awaitable inner, StateType* state, std::size_t index)
389   
make_when_any_runner(Awaitable inner, StateType* state, std::size_t index)
390   
{
390   
{
391   
    using T = awaitable_result_t<Awaitable>;
391   
    using T = awaitable_result_t<Awaitable>;
392   
    if constexpr (std::is_void_v<T>)
392   
    if constexpr (std::is_void_v<T>)
393   
    {
393   
    {
394   
        co_await std::move(inner);
394   
        co_await std::move(inner);
395   
        if(state->core_.try_win(index))
395   
        if(state->core_.try_win(index))
396   
        {
396   
        {
397   
            if constexpr (requires { state->set_winner_void(); })
397   
            if constexpr (requires { state->set_winner_void(); })
398   
                state->set_winner_void();
398   
                state->set_winner_void();
399   
        }
399   
        }
400   
    }
400   
    }
401   
    else
401   
    else
402   
    {
402   
    {
403   
        auto result = co_await std::move(inner);
403   
        auto result = co_await std::move(inner);
404   
        if(state->core_.try_win(index))
404   
        if(state->core_.try_win(index))
405   
        {
405   
        {
406   
            try
406   
            try
407   
            {
407   
            {
408   
                state->set_winner_result(std::move(result));
408   
                state->set_winner_result(std::move(result));
409   
            }
409   
            }
410   
            catch(...)
410   
            catch(...)
411   
            {
411   
            {
412   
                state->core_.set_winner_exception(std::current_exception());
412   
                state->core_.set_winner_exception(std::current_exception());
413   
            }
413   
            }
414   
        }
414   
        }
415   
    }
415   
    }
416   
}
416   
}
417  
417  
418   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
418   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
419   
template<IoAwaitable... Awaitables>
419   
template<IoAwaitable... Awaitables>
420   
class when_any_launcher
420   
class when_any_launcher
421   
{
421   
{
422   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
422   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
423  
423  
424   
    std::tuple<Awaitables...>* tasks_;
424   
    std::tuple<Awaitables...>* tasks_;
425   
    state_type* state_;
425   
    state_type* state_;
426  
426  
427   
public:
427   
public:
428   
    when_any_launcher(
428   
    when_any_launcher(
429   
        std::tuple<Awaitables...>* tasks,
429   
        std::tuple<Awaitables...>* tasks,
430   
        state_type* state)
430   
        state_type* state)
431   
        : tasks_(tasks)
431   
        : tasks_(tasks)
432   
        , state_(state)
432   
        , state_(state)
433   
    {
433   
    {
434   
    }
434   
    }
435  
435  
436   
    bool await_ready() const noexcept
436   
    bool await_ready() const noexcept
437   
    {
437   
    {
438   
        return sizeof...(Awaitables) == 0;
438   
        return sizeof...(Awaitables) == 0;
439   
    }
439   
    }
440  
440  
441   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
441   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
442   
        destroys this object before await_suspend returns. Must not reference
442   
        destroys this object before await_suspend returns. Must not reference
443   
        `this` after the final launch_one call.
443   
        `this` after the final launch_one call.
444   
    */
444   
    */
445   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
445   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
446   
    {
446   
    {
447   
        state_->core_.continuation_ = continuation;
447   
        state_->core_.continuation_ = continuation;
448   
        state_->core_.caller_env_ = caller_env;
448   
        state_->core_.caller_env_ = caller_env;
449  
449  
450   
        if(caller_env->stop_token.stop_possible())
450   
        if(caller_env->stop_token.stop_possible())
451   
        {
451   
        {
452   
            state_->core_.parent_stop_callback_.emplace(
452   
            state_->core_.parent_stop_callback_.emplace(
453   
                caller_env->stop_token,
453   
                caller_env->stop_token,
454   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
454   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
455  
455  
456   
            if(caller_env->stop_token.stop_requested())
456   
            if(caller_env->stop_token.stop_requested())
457   
                state_->core_.stop_source_.request_stop();
457   
                state_->core_.stop_source_.request_stop();
458   
        }
458   
        }
459  
459  
460   
        auto token = state_->core_.stop_source_.get_token();
460   
        auto token = state_->core_.stop_source_.get_token();
461   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
461   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
462   
            (..., launch_one<Is>(caller_env->executor, token));
462   
            (..., launch_one<Is>(caller_env->executor, token));
463   
        }(std::index_sequence_for<Awaitables...>{});
463   
        }(std::index_sequence_for<Awaitables...>{});
464  
464  
465   
        return std::noop_coroutine();
465   
        return std::noop_coroutine();
466   
    }
466   
    }
467  
467  
468   
    void await_resume() const noexcept
468   
    void await_resume() const noexcept
469   
    {
469   
    {
470   
    }
470   
    }
471  
471  
472   
private:
472   
private:
473   
    /** @pre Ex::dispatch() and std::coroutine_handle<>::resume() must not throw (handle may leak). */
473   
    /** @pre Ex::dispatch() and std::coroutine_handle<>::resume() must not throw (handle may leak). */
474   
    template<std::size_t I>
474   
    template<std::size_t I>
475   
    void launch_one(executor_ref caller_ex, std::stop_token token)
475   
    void launch_one(executor_ref caller_ex, std::stop_token token)
476   
    {
476   
    {
477   
        auto runner = make_when_any_runner<I>(
477   
        auto runner = make_when_any_runner<I>(
478   
            std::move(std::get<I>(*tasks_)), state_);
478   
            std::move(std::get<I>(*tasks_)), state_);
479  
479  
480   
        auto h = runner.release();
480   
        auto h = runner.release();
481   
        h.promise().state_ = state_;
481   
        h.promise().state_ = state_;
482   
        h.promise().index_ = I;
482   
        h.promise().index_ = I;
483   
        h.promise().env_ = io_env{caller_ex, token, state_->core_.caller_env_->frame_allocator};
483   
        h.promise().env_ = io_env{caller_ex, token, state_->core_.caller_env_->frame_allocator};
484  
484  
485   
        std::coroutine_handle<> ch{h};
485   
        std::coroutine_handle<> ch{h};
486   
        state_->runner_handles_[I] = ch;
486   
        state_->runner_handles_[I] = ch;
487   
        caller_ex.post(ch);
487   
        caller_ex.post(ch);
488   
    }
488   
    }
489   
};
489   
};
490  
490  
491   
} // namespace detail
491   
} // namespace detail
492  
492  
493   
/** Wait for the first awaitable to complete.
493   
/** Wait for the first awaitable to complete.
494  
494  
495   
    Races multiple heterogeneous awaitables concurrently and returns when the
495   
    Races multiple heterogeneous awaitables concurrently and returns when the
496   
    first one completes. The result is a variant with one alternative per
496   
    first one completes. The result is a variant with one alternative per
497   
    input task, preserving positional correspondence.
497   
    input task, preserving positional correspondence.
498  
498  
499   
    @par Suspends
499   
    @par Suspends
500   
    The calling coroutine suspends when co_await is invoked. All awaitables
500   
    The calling coroutine suspends when co_await is invoked. All awaitables
501   
    are launched concurrently and execute in parallel. The coroutine resumes
501   
    are launched concurrently and execute in parallel. The coroutine resumes
502   
    only after all awaitables have completed, even though the winner is
502   
    only after all awaitables have completed, even though the winner is
503   
    determined by the first to finish.
503   
    determined by the first to finish.
504  
504  
505   
    @par Completion Conditions
505   
    @par Completion Conditions
506   
    @li Winner is determined when the first awaitable completes (success or exception)
506   
    @li Winner is determined when the first awaitable completes (success or exception)
507   
    @li Only one task can claim winner status via atomic compare-exchange
507   
    @li Only one task can claim winner status via atomic compare-exchange
508   
    @li Once a winner exists, stop is requested for all remaining siblings
508   
    @li Once a winner exists, stop is requested for all remaining siblings
509   
    @li Parent coroutine resumes only after all siblings acknowledge completion
509   
    @li Parent coroutine resumes only after all siblings acknowledge completion
510   
    @li The winner's result is returned; if the winner threw, the exception is rethrown
510   
    @li The winner's result is returned; if the winner threw, the exception is rethrown
511  
511  
512   
    @par Cancellation Semantics
512   
    @par Cancellation Semantics
513   
    Cancellation is supported via stop_token propagated through the
513   
    Cancellation is supported via stop_token propagated through the
514   
    IoAwaitable protocol:
514   
    IoAwaitable protocol:
515   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
515   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
516   
    @li When the parent's stop token is activated, the stop is forwarded to all children
516   
    @li When the parent's stop token is activated, the stop is forwarded to all children
517   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
517   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
518   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
518   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
519   
    @li Stop requests are cooperative; tasks must check and respond to them
519   
    @li Stop requests are cooperative; tasks must check and respond to them
520  
520  
521   
    @par Concurrency/Overlap
521   
    @par Concurrency/Overlap
522   
    All awaitables are launched concurrently before any can complete.
522   
    All awaitables are launched concurrently before any can complete.
523   
    The launcher iterates through the arguments, starting each task on the
523   
    The launcher iterates through the arguments, starting each task on the
524   
    caller's executor. Tasks may execute in parallel on multi-threaded
524   
    caller's executor. Tasks may execute in parallel on multi-threaded
525   
    executors or interleave on single-threaded executors. There is no
525   
    executors or interleave on single-threaded executors. There is no
526   
    guaranteed ordering of task completion.
526   
    guaranteed ordering of task completion.
527  
527  
528   
    @par Notable Error Conditions
528   
    @par Notable Error Conditions
529   
    @li Winner exception: if the winning task threw, that exception is rethrown
529   
    @li Winner exception: if the winning task threw, that exception is rethrown
530   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
530   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
531   
    @li Cancellation: tasks may complete via cancellation without throwing
531   
    @li Cancellation: tasks may complete via cancellation without throwing
532  
532  
533   
    @par Example
533   
    @par Example
534   
    @code
534   
    @code
535   
    task<void> example() {
535   
    task<void> example() {
536   
        auto result = co_await when_any(
536   
        auto result = co_await when_any(
537   
            fetch_int(),      // task<int>
537   
            fetch_int(),      // task<int>
538   
            fetch_string()    // task<std::string>
538   
            fetch_string()    // task<std::string>
539   
        );
539   
        );
540   
        // result.index() is 0 or 1
540   
        // result.index() is 0 or 1
541   
        if (result.index() == 0)
541   
        if (result.index() == 0)
542   
            std::cout << "Got int: " << std::get<0>(result) << "\n";
542   
            std::cout << "Got int: " << std::get<0>(result) << "\n";
543   
        else
543   
        else
544   
            std::cout << "Got string: " << std::get<1>(result) << "\n";
544   
            std::cout << "Got string: " << std::get<1>(result) << "\n";
545   
    }
545   
    }
546   
    @endcode
546   
    @endcode
547  
547  
548   
    @param as Awaitables to race concurrently (at least one required; each
548   
    @param as Awaitables to race concurrently (at least one required; each
549   
        must satisfy IoAwaitable).
549   
        must satisfy IoAwaitable).
550   
    @return A task yielding a std::variant with one alternative per awaitable.
550   
    @return A task yielding a std::variant with one alternative per awaitable.
551   
        Use .index() to identify the winner. Void awaitables contribute
551   
        Use .index() to identify the winner. Void awaitables contribute
552   
        std::monostate.
552   
        std::monostate.
553  
553  
554   
    @throws Rethrows the winner's exception if the winning task threw an exception.
554   
    @throws Rethrows the winner's exception if the winning task threw an exception.
555  
555  
556   
    @par Remarks
556   
    @par Remarks
557   
    Awaitables are moved into the coroutine frame; original objects become
557   
    Awaitables are moved into the coroutine frame; original objects become
558   
    empty after the call. The variant preserves one alternative per input
558   
    empty after the call. The variant preserves one alternative per input
559   
    task. Use .index() to determine which awaitable completed first.
559   
    task. Use .index() to determine which awaitable completed first.
560   
    Void awaitables contribute std::monostate to the variant.
560   
    Void awaitables contribute std::monostate to the variant.
561  
561  
562   
    @see when_all, IoAwaitable
562   
    @see when_all, IoAwaitable
563   
*/
563   
*/
564   
template<IoAwaitable... As>
564   
template<IoAwaitable... As>
565   
    requires (sizeof...(As) > 0)
565   
    requires (sizeof...(As) > 0)
566   
[[nodiscard]] auto when_any(As... as)
566   
[[nodiscard]] auto when_any(As... as)
567   
    -> task<std::variant<void_to_monostate_t<awaitable_result_t<As>>...>>
567   
    -> task<std::variant<void_to_monostate_t<awaitable_result_t<As>>...>>
568   
{
568   
{
569   
    detail::when_any_state<awaitable_result_t<As>...> state;
569   
    detail::when_any_state<awaitable_result_t<As>...> state;
570   
    std::tuple<As...> awaitable_tuple(std::move(as)...);
570   
    std::tuple<As...> awaitable_tuple(std::move(as)...);
571  
571  
572   
    co_await detail::when_any_launcher<As...>(&awaitable_tuple, &state);
572   
    co_await detail::when_any_launcher<As...>(&awaitable_tuple, &state);
573  
573  
574   
    if(state.core_.winner_exception_)
574   
    if(state.core_.winner_exception_)
575   
        std::rethrow_exception(state.core_.winner_exception_);
575   
        std::rethrow_exception(state.core_.winner_exception_);
576  
576  
577   
    co_return std::move(*state.result_);
577   
    co_return std::move(*state.result_);
578   
}
578   
}
579  
579  
 
580 + 
/** Concept for ranges of full I/O awaitables.
 
581 +
 
582 + 
    A range satisfies `IoAwaitableRange` if it is a sized input range
 
583 + 
    whose value type satisfies @ref IoAwaitable. This enables when_any
 
584 + 
    to accept any container or view of awaitables, not just std::vector.
 
585 +
 
586 + 
    @tparam R The range type.
 
587 +
 
588 + 
    @par Requirements
 
589 + 
    @li `R` must satisfy `std::ranges::input_range`
 
590 + 
    @li `R` must satisfy `std::ranges::sized_range`
 
591 + 
    @li `std::ranges::range_value_t<R>` must satisfy @ref IoAwaitable
 
592 +
 
593 + 
    @par Syntactic Requirements
 
594 + 
    Given `r` of type `R`:
 
595 + 
    @li `std::ranges::begin(r)` is valid
 
596 + 
    @li `std::ranges::end(r)` is valid
 
597 + 
    @li `std::ranges::size(r)` returns `std::ranges::range_size_t<R>`
 
598 + 
    @li `*std::ranges::begin(r)` satisfies @ref IoAwaitable
 
599 +
 
600 + 
    @par Example
 
601 + 
    @code
 
602 + 
    template<IoAwaitableRange R>
 
603 + 
    task<void> race_all(R&& awaitables) {
 
604 + 
        auto winner = co_await when_any(std::forward<R>(awaitables));
 
605 + 
        // Process winner...
 
606 + 
    }
 
607 + 
    @endcode
 
608 +
 
609 + 
    @see when_any, IoAwaitable
 
610 + 
*/
 
611 + 
template<typename R>
 
612 + 
concept IoAwaitableRange =
 
613 + 
    std::ranges::input_range<R> &&
 
614 + 
    std::ranges::sized_range<R> &&
 
615 + 
    IoAwaitable<std::ranges::range_value_t<R>>;
 
616 +
580   
namespace detail {
617   
namespace detail {
581  
618  
582   
/** Shared state for homogeneous when_any (range overload).
619   
/** Shared state for homogeneous when_any (range overload).
583  
620  
584   
    Uses composition with when_any_core for shared functionality.
621   
    Uses composition with when_any_core for shared functionality.
585   
    Simpler than heterogeneous: optional<T> instead of variant, vector
622   
    Simpler than heterogeneous: optional<T> instead of variant, vector
586   
    instead of array for runner handles.
623   
    instead of array for runner handles.
587   
*/
624   
*/
588   
template<typename T>
625   
template<typename T>
589   
struct when_any_homogeneous_state
626   
struct when_any_homogeneous_state
590   
{
627   
{
591   
    when_any_core core_;
628   
    when_any_core core_;
592   
    std::optional<T> result_;
629   
    std::optional<T> result_;
593   
    std::vector<std::coroutine_handle<>> runner_handles_;
630   
    std::vector<std::coroutine_handle<>> runner_handles_;
594  
631  
595   
    explicit when_any_homogeneous_state(std::size_t count)
632   
    explicit when_any_homogeneous_state(std::size_t count)
596   
        : core_(count)
633   
        : core_(count)
597   
        , runner_handles_(count)
634   
        , runner_handles_(count)
598   
    {
635   
    {
599   
    }
636   
    }
600  
637  
601   
    // Runners self-destruct in final_suspend. No destruction needed here.
638   
    // Runners self-destruct in final_suspend. No destruction needed here.
602  
639  
603   
    /** @pre core_.try_win() returned true. */
640   
    /** @pre core_.try_win() returned true. */
604   
    void set_winner_result(T value)
641   
    void set_winner_result(T value)
605   
        noexcept(std::is_nothrow_move_constructible_v<T>)
642   
        noexcept(std::is_nothrow_move_constructible_v<T>)
606   
    {
643   
    {
607   
        result_.emplace(std::move(value));
644   
        result_.emplace(std::move(value));
608   
    }
645   
    }
609   
};
646   
};
610  
647  
611   
/** Specialization for void tasks (no result storage needed). */
648   
/** Specialization for void tasks (no result storage needed). */
612   
template<>
649   
template<>
613   
struct when_any_homogeneous_state<void>
650   
struct when_any_homogeneous_state<void>
614   
{
651   
{
615   
    when_any_core core_;
652   
    when_any_core core_;
616   
    std::vector<std::coroutine_handle<>> runner_handles_;
653   
    std::vector<std::coroutine_handle<>> runner_handles_;
617  
654  
618   
    explicit when_any_homogeneous_state(std::size_t count)
655   
    explicit when_any_homogeneous_state(std::size_t count)
619   
        : core_(count)
656   
        : core_(count)
620   
        , runner_handles_(count)
657   
        , runner_handles_(count)
621   
    {
658   
    {
622   
    }
659   
    }
623  
660  
624   
    // Runners self-destruct in final_suspend. No destruction needed here.
661   
    // Runners self-destruct in final_suspend. No destruction needed here.
625  
662  
626   
    // No set_winner_result - void tasks have no result to store
663   
    // No set_winner_result - void tasks have no result to store
627   
};
664   
};
628  
665  
629   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
666   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
630   
template<IoAwaitableRange Range>
667   
template<IoAwaitableRange Range>
631   
class when_any_homogeneous_launcher
668   
class when_any_homogeneous_launcher
632   
{
669   
{
633   
    using Awaitable = std::ranges::range_value_t<Range>;
670   
    using Awaitable = std::ranges::range_value_t<Range>;
634   
    using T = awaitable_result_t<Awaitable>;
671   
    using T = awaitable_result_t<Awaitable>;
635  
672  
636   
    Range* range_;
673   
    Range* range_;
637   
    when_any_homogeneous_state<T>* state_;
674   
    when_any_homogeneous_state<T>* state_;
638  
675  
639   
public:
676   
public:
640   
    when_any_homogeneous_launcher(
677   
    when_any_homogeneous_launcher(
641   
        Range* range,
678   
        Range* range,
642   
        when_any_homogeneous_state<T>* state)
679   
        when_any_homogeneous_state<T>* state)
643   
        : range_(range)
680   
        : range_(range)
644   
        , state_(state)
681   
        , state_(state)
645   
    {
682   
    {
646   
    }
683   
    }
647  
684  
648   
    bool await_ready() const noexcept
685   
    bool await_ready() const noexcept
649   
    {
686   
    {
650   
        return std::ranges::empty(*range_);
687   
        return std::ranges::empty(*range_);
651   
    }
688   
    }
652  
689  
653   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
690   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
654   
        destroys this object before await_suspend returns. Must not reference
691   
        destroys this object before await_suspend returns. Must not reference
655   
        `this` after dispatching begins.
692   
        `this` after dispatching begins.
656  
693  
657   
        Two-phase approach:
694   
        Two-phase approach:
658   
        1. Create all runners (safe - no dispatch yet)
695   
        1. Create all runners (safe - no dispatch yet)
659   
        2. Dispatch all runners (any may complete synchronously)
696   
        2. Dispatch all runners (any may complete synchronously)
660   
    */
697   
    */
661   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
698   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
662   
    {
699   
    {
663   
        state_->core_.continuation_ = continuation;
700   
        state_->core_.continuation_ = continuation;
664   
        state_->core_.caller_env_ = caller_env;
701   
        state_->core_.caller_env_ = caller_env;
665  
702  
666   
        if(caller_env->stop_token.stop_possible())
703   
        if(caller_env->stop_token.stop_possible())
667   
        {
704   
        {
668   
            state_->core_.parent_stop_callback_.emplace(
705   
            state_->core_.parent_stop_callback_.emplace(
669   
                caller_env->stop_token,
706   
                caller_env->stop_token,
670   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
707   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
671  
708  
672   
            if(caller_env->stop_token.stop_requested())
709   
            if(caller_env->stop_token.stop_requested())
673   
                state_->core_.stop_source_.request_stop();
710   
                state_->core_.stop_source_.request_stop();
674   
        }
711   
        }
675  
712  
676   
        auto token = state_->core_.stop_source_.get_token();
713   
        auto token = state_->core_.stop_source_.get_token();
677  
714  
678   
        // Phase 1: Create all runners without dispatching.
715   
        // Phase 1: Create all runners without dispatching.
679   
        // This iterates over *range_ safely because no runners execute yet.
716   
        // This iterates over *range_ safely because no runners execute yet.
680   
        std::size_t index = 0;
717   
        std::size_t index = 0;
681   
        for(auto&& a : *range_)
718   
        for(auto&& a : *range_)
682   
        {
719   
        {
683   
            auto runner = make_when_any_runner(
720   
            auto runner = make_when_any_runner(
684   
                std::move(a), state_, index);
721   
                std::move(a), state_, index);
685  
722  
686   
            auto h = runner.release();
723   
            auto h = runner.release();
687   
            h.promise().state_ = state_;
724   
            h.promise().state_ = state_;
688   
            h.promise().index_ = index;
725   
            h.promise().index_ = index;
689   
            h.promise().env_ = io_env{caller_env->executor, token, caller_env->frame_allocator};
726   
            h.promise().env_ = io_env{caller_env->executor, token, caller_env->frame_allocator};
690  
727  
691   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
728   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
692   
            ++index;
729   
            ++index;
693   
        }
730   
        }
694  
731  
695   
        // Phase 2: Post all runners. Any may complete synchronously.
732   
        // Phase 2: Post all runners. Any may complete synchronously.
696   
        // After last post, state_ and this may be destroyed.
733   
        // After last post, state_ and this may be destroyed.
697   
        // Use raw pointer/count captured before posting.
734   
        // Use raw pointer/count captured before posting.
698   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
735   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
699   
        std::size_t count = state_->runner_handles_.size();
736   
        std::size_t count = state_->runner_handles_.size();
700   
        for(std::size_t i = 0; i < count; ++i)
737   
        for(std::size_t i = 0; i < count; ++i)
701   
            caller_env->executor.post(handles[i]);
738   
            caller_env->executor.post(handles[i]);
702  
739  
703   
        return std::noop_coroutine();
740   
        return std::noop_coroutine();
704   
    }
741   
    }
705  
742  
706   
    void await_resume() const noexcept
743   
    void await_resume() const noexcept
707   
    {
744   
    {
708   
    }
745   
    }
709   
};
746   
};
710  
747  
711   
} // namespace detail
748   
} // namespace detail
712  
749  
713   
/** Wait for the first awaitable to complete (range overload).
750   
/** Wait for the first awaitable to complete (range overload).
714  
751  
715   
    Races a range of awaitables with the same result type. Accepts any
752   
    Races a range of awaitables with the same result type. Accepts any
716   
    sized input range of IoAwaitable types, enabling use with arrays,
753   
    sized input range of IoAwaitable types, enabling use with arrays,
717   
    spans, or custom containers.
754   
    spans, or custom containers.
718  
755  
719   
    @par Suspends
756   
    @par Suspends
720   
    The calling coroutine suspends when co_await is invoked. All awaitables
757   
    The calling coroutine suspends when co_await is invoked. All awaitables
721   
    in the range are launched concurrently and execute in parallel. The
758   
    in the range are launched concurrently and execute in parallel. The
722   
    coroutine resumes only after all awaitables have completed, even though
759   
    coroutine resumes only after all awaitables have completed, even though
723   
    the winner is determined by the first to finish.
760   
    the winner is determined by the first to finish.
724  
761  
725   
    @par Completion Conditions
762   
    @par Completion Conditions
726   
    @li Winner is determined when the first awaitable completes (success or exception)
763   
    @li Winner is determined when the first awaitable completes (success or exception)
727   
    @li Only one task can claim winner status via atomic compare-exchange
764   
    @li Only one task can claim winner status via atomic compare-exchange
728   
    @li Once a winner exists, stop is requested for all remaining siblings
765   
    @li Once a winner exists, stop is requested for all remaining siblings
729   
    @li Parent coroutine resumes only after all siblings acknowledge completion
766   
    @li Parent coroutine resumes only after all siblings acknowledge completion
730   
    @li The winner's index and result are returned; if the winner threw, the exception is rethrown
767   
    @li The winner's index and result are returned; if the winner threw, the exception is rethrown
731  
768  
732   
    @par Cancellation Semantics
769   
    @par Cancellation Semantics
733   
    Cancellation is supported via stop_token propagated through the
770   
    Cancellation is supported via stop_token propagated through the
734   
    IoAwaitable protocol:
771   
    IoAwaitable protocol:
735   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
772   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
736   
    @li When the parent's stop token is activated, the stop is forwarded to all children
773   
    @li When the parent's stop token is activated, the stop is forwarded to all children
737   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
774   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
738   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
775   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
739   
    @li Stop requests are cooperative; tasks must check and respond to them
776   
    @li Stop requests are cooperative; tasks must check and respond to them
740  
777  
741   
    @par Concurrency/Overlap
778   
    @par Concurrency/Overlap
742   
    All awaitables are launched concurrently before any can complete.
779   
    All awaitables are launched concurrently before any can complete.
743   
    The launcher iterates through the range, starting each task on the
780   
    The launcher iterates through the range, starting each task on the
744   
    caller's executor. Tasks may execute in parallel on multi-threaded
781   
    caller's executor. Tasks may execute in parallel on multi-threaded
745   
    executors or interleave on single-threaded executors. There is no
782   
    executors or interleave on single-threaded executors. There is no
746   
    guaranteed ordering of task completion.
783   
    guaranteed ordering of task completion.
747  
784  
748   
    @par Notable Error Conditions
785   
    @par Notable Error Conditions
749   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
786   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
750   
    @li Winner exception: if the winning task threw, that exception is rethrown
787   
    @li Winner exception: if the winning task threw, that exception is rethrown
751   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
788   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
752   
    @li Cancellation: tasks may complete via cancellation without throwing
789   
    @li Cancellation: tasks may complete via cancellation without throwing
753  
790  
754   
    @par Example
791   
    @par Example
755   
    @code
792   
    @code
756   
    task<void> example() {
793   
    task<void> example() {
757   
        std::array<task<Response>, 3> requests = {
794   
        std::array<task<Response>, 3> requests = {
758   
            fetch_from_server(0),
795   
            fetch_from_server(0),
759   
            fetch_from_server(1),
796   
            fetch_from_server(1),
760   
            fetch_from_server(2)
797   
            fetch_from_server(2)
761   
        };
798   
        };
762  
799  
763   
        auto [index, response] = co_await when_any(std::move(requests));
800   
        auto [index, response] = co_await when_any(std::move(requests));
764   
    }
801   
    }
765   
    @endcode
802   
    @endcode
766  
803  
767   
    @par Example with Vector
804   
    @par Example with Vector
768   
    @code
805   
    @code
769   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
806   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
770   
        std::vector<task<Response>> requests;
807   
        std::vector<task<Response>> requests;
771   
        for (auto const& server : servers)
808   
        for (auto const& server : servers)
772   
            requests.push_back(fetch_from(server));
809   
            requests.push_back(fetch_from(server));
773  
810  
774   
        auto [index, response] = co_await when_any(std::move(requests));
811   
        auto [index, response] = co_await when_any(std::move(requests));
775   
        co_return response;
812   
        co_return response;
776   
    }
813   
    }
777   
    @endcode
814   
    @endcode
778  
815  
779   
    @tparam R Range type satisfying IoAwaitableRange.
816   
    @tparam R Range type satisfying IoAwaitableRange.
780   
    @param awaitables Range of awaitables to race concurrently (must not be empty).
817   
    @param awaitables Range of awaitables to race concurrently (must not be empty).
781   
    @return A task yielding a pair of (winner_index, result).
818   
    @return A task yielding a pair of (winner_index, result).
782  
819  
783   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
820   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
784   
    @throws Rethrows the winner's exception if the winning task threw an exception.
821   
    @throws Rethrows the winner's exception if the winning task threw an exception.
785  
822  
786   
    @par Remarks
823   
    @par Remarks
787   
    Elements are moved from the range; for lvalue ranges, the original
824   
    Elements are moved from the range; for lvalue ranges, the original
788   
    container will have moved-from elements after this call. The range
825   
    container will have moved-from elements after this call. The range
789   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
826   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
790   
    the variadic overload, no variant wrapper is needed since all tasks
827   
    the variadic overload, no variant wrapper is needed since all tasks
791   
    share the same return type.
828   
    share the same return type.
792  
829  
793   
    @see when_any, IoAwaitableRange
830   
    @see when_any, IoAwaitableRange
794   
*/
831   
*/
795   
template<IoAwaitableRange R>
832   
template<IoAwaitableRange R>
796   
    requires (!std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>)
833   
    requires (!std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>)
797   
[[nodiscard]] auto when_any(R&& awaitables)
834   
[[nodiscard]] auto when_any(R&& awaitables)
798   
    -> task<std::pair<std::size_t, awaitable_result_t<std::ranges::range_value_t<R>>>>
835   
    -> task<std::pair<std::size_t, awaitable_result_t<std::ranges::range_value_t<R>>>>
799   
{
836   
{
800   
    using Awaitable = std::ranges::range_value_t<R>;
837   
    using Awaitable = std::ranges::range_value_t<R>;
801   
    using T = awaitable_result_t<Awaitable>;
838   
    using T = awaitable_result_t<Awaitable>;
802   
    using result_type = std::pair<std::size_t, T>;
839   
    using result_type = std::pair<std::size_t, T>;
803   
    using OwnedRange = std::remove_cvref_t<R>;
840   
    using OwnedRange = std::remove_cvref_t<R>;
804  
841  
805   
    auto count = std::ranges::size(awaitables);
842   
    auto count = std::ranges::size(awaitables);
806   
    if(count == 0)
843   
    if(count == 0)
807   
        throw std::invalid_argument("when_any requires at least one awaitable");
844   
        throw std::invalid_argument("when_any requires at least one awaitable");
808  
845  
809   
    // Move/copy range onto coroutine frame to ensure lifetime
846   
    // Move/copy range onto coroutine frame to ensure lifetime
810   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
847   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
811  
848  
812   
    detail::when_any_homogeneous_state<T> state(count);
849   
    detail::when_any_homogeneous_state<T> state(count);
813  
850  
814   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
851   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
815  
852  
816   
    if(state.core_.winner_exception_)
853   
    if(state.core_.winner_exception_)
817   
        std::rethrow_exception(state.core_.winner_exception_);
854   
        std::rethrow_exception(state.core_.winner_exception_);
818  
855  
819   
    co_return result_type{state.core_.winner_index_, std::move(*state.result_)};
856   
    co_return result_type{state.core_.winner_index_, std::move(*state.result_)};
820   
}
857   
}
821  
858  
822   
/** Wait for the first awaitable to complete (void range overload).
859   
/** Wait for the first awaitable to complete (void range overload).
823  
860  
824   
    Races a range of void-returning awaitables. Since void awaitables have
861   
    Races a range of void-returning awaitables. Since void awaitables have
825   
    no result value, only the winner's index is returned.
862   
    no result value, only the winner's index is returned.
826  
863  
827   
    @par Suspends
864   
    @par Suspends
828   
    The calling coroutine suspends when co_await is invoked. All awaitables
865   
    The calling coroutine suspends when co_await is invoked. All awaitables
829   
    in the range are launched concurrently and execute in parallel. The
866   
    in the range are launched concurrently and execute in parallel. The
830   
    coroutine resumes only after all awaitables have completed, even though
867   
    coroutine resumes only after all awaitables have completed, even though
831   
    the winner is determined by the first to finish.
868   
    the winner is determined by the first to finish.
832  
869  
833   
    @par Completion Conditions
870   
    @par Completion Conditions
834   
    @li Winner is determined when the first awaitable completes (success or exception)
871   
    @li Winner is determined when the first awaitable completes (success or exception)
835   
    @li Only one task can claim winner status via atomic compare-exchange
872   
    @li Only one task can claim winner status via atomic compare-exchange
836   
    @li Once a winner exists, stop is requested for all remaining siblings
873   
    @li Once a winner exists, stop is requested for all remaining siblings
837   
    @li Parent coroutine resumes only after all siblings acknowledge completion
874   
    @li Parent coroutine resumes only after all siblings acknowledge completion
838   
    @li The winner's index is returned; if the winner threw, the exception is rethrown
875   
    @li The winner's index is returned; if the winner threw, the exception is rethrown
839  
876  
840   
    @par Cancellation Semantics
877   
    @par Cancellation Semantics
841   
    Cancellation is supported via stop_token propagated through the
878   
    Cancellation is supported via stop_token propagated through the
842   
    IoAwaitable protocol:
879   
    IoAwaitable protocol:
843   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
880   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
844   
    @li When the parent's stop token is activated, the stop is forwarded to all children
881   
    @li When the parent's stop token is activated, the stop is forwarded to all children
845   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
882   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
846   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
883   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
847   
    @li Stop requests are cooperative; tasks must check and respond to them
884   
    @li Stop requests are cooperative; tasks must check and respond to them
848  
885  
849   
    @par Concurrency/Overlap
886   
    @par Concurrency/Overlap
850   
    All awaitables are launched concurrently before any can complete.
887   
    All awaitables are launched concurrently before any can complete.
851   
    The launcher iterates through the range, starting each task on the
888   
    The launcher iterates through the range, starting each task on the
852   
    caller's executor. Tasks may execute in parallel on multi-threaded
889   
    caller's executor. Tasks may execute in parallel on multi-threaded
853   
    executors or interleave on single-threaded executors. There is no
890   
    executors or interleave on single-threaded executors. There is no
854   
    guaranteed ordering of task completion.
891   
    guaranteed ordering of task completion.
855  
892  
856   
    @par Notable Error Conditions
893   
    @par Notable Error Conditions
857   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
894   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
858   
    @li Winner exception: if the winning task threw, that exception is rethrown
895   
    @li Winner exception: if the winning task threw, that exception is rethrown
859   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
896   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
860   
    @li Cancellation: tasks may complete via cancellation without throwing
897   
    @li Cancellation: tasks may complete via cancellation without throwing
861  
898  
862   
    @par Example
899   
    @par Example
863   
    @code
900   
    @code
864   
    task<void> example() {
901   
    task<void> example() {
865   
        std::vector<task<void>> tasks;
902   
        std::vector<task<void>> tasks;
866   
        for (int i = 0; i < 5; ++i)
903   
        for (int i = 0; i < 5; ++i)
867   
            tasks.push_back(background_work(i));
904   
            tasks.push_back(background_work(i));
868  
905  
869   
        std::size_t winner = co_await when_any(std::move(tasks));
906   
        std::size_t winner = co_await when_any(std::move(tasks));
870   
        // winner is the index of the first task to complete
907   
        // winner is the index of the first task to complete
871   
    }
908   
    }
872   
    @endcode
909   
    @endcode
873  
910  
874   
    @par Example with Timeout
911   
    @par Example with Timeout
875   
    @code
912   
    @code
876   
    task<void> with_timeout() {
913   
    task<void> with_timeout() {
877   
        std::vector<task<void>> tasks;
914   
        std::vector<task<void>> tasks;
878   
        tasks.push_back(long_running_operation());
915   
        tasks.push_back(long_running_operation());
879   
        tasks.push_back(delay(std::chrono::seconds(5)));
916   
        tasks.push_back(delay(std::chrono::seconds(5)));
880  
917  
881   
        std::size_t winner = co_await when_any(std::move(tasks));
918   
        std::size_t winner = co_await when_any(std::move(tasks));
882   
        if (winner == 1) {
919   
        if (winner == 1) {
883   
            // Timeout occurred
920   
            // Timeout occurred
884   
        }
921   
        }
885   
    }
922   
    }
886   
    @endcode
923   
    @endcode
887  
924  
888   
    @tparam R Range type satisfying IoAwaitableRange with void result.
925   
    @tparam R Range type satisfying IoAwaitableRange with void result.
889   
    @param awaitables Range of void awaitables to race concurrently (must not be empty).
926   
    @param awaitables Range of void awaitables to race concurrently (must not be empty).
890   
    @return A task yielding the winner's index (zero-based).
927   
    @return A task yielding the winner's index (zero-based).
891  
928  
892   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
929   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
893   
    @throws Rethrows the winner's exception if the winning task threw an exception.
930   
    @throws Rethrows the winner's exception if the winning task threw an exception.
894  
931  
895   
    @par Remarks
932   
    @par Remarks
896   
    Elements are moved from the range; for lvalue ranges, the original
933   
    Elements are moved from the range; for lvalue ranges, the original
897   
    container will have moved-from elements after this call. The range
934   
    container will have moved-from elements after this call. The range
898   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
935   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
899   
    the non-void overload, no result storage is needed since void tasks
936   
    the non-void overload, no result storage is needed since void tasks
900   
    produce no value.
937   
    produce no value.
901  
938  
902   
    @see when_any, IoAwaitableRange
939   
    @see when_any, IoAwaitableRange
903   
*/
940   
*/
904   
template<IoAwaitableRange R>
941   
template<IoAwaitableRange R>
905   
    requires std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>
942   
    requires std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>
906   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
943   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
907   
{
944   
{
908   
    using OwnedRange = std::remove_cvref_t<R>;
945   
    using OwnedRange = std::remove_cvref_t<R>;
909  
946  
910   
    auto count = std::ranges::size(awaitables);
947   
    auto count = std::ranges::size(awaitables);
911   
    if(count == 0)
948   
    if(count == 0)
912   
        throw std::invalid_argument("when_any requires at least one awaitable");
949   
        throw std::invalid_argument("when_any requires at least one awaitable");
913  
950  
914   
    // Move/copy range onto coroutine frame to ensure lifetime
951   
    // Move/copy range onto coroutine frame to ensure lifetime
915   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
952   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
916  
953  
917   
    detail::when_any_homogeneous_state<void> state(count);
954   
    detail::when_any_homogeneous_state<void> state(count);
918  
955  
919   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
956   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
920  
957  
921   
    if(state.core_.winner_exception_)
958   
    if(state.core_.winner_exception_)
922   
        std::rethrow_exception(state.core_.winner_exception_);
959   
        std::rethrow_exception(state.core_.winner_exception_);
923  
960  
924   
    co_return state.core_.winner_index_;
961   
    co_return state.core_.winner_index_;
925   
}
962   
}
926  
963  
927   
} // namespace capy
964   
} // namespace capy
928   
} // namespace boost
965   
} // namespace boost
929  
966  
930   
#endif
967   
#endif