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)
5   
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6   
//
6   
//
7   
// Official repository: https://github.com/cppalliance/capy
7   
// Official repository: https://github.com/cppalliance/capy
8   
//
8   
//
9  
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/void_to_monostate.hpp>
 
14   
#include <boost/capy/detail/config.hpp>
13   
#include <boost/capy/detail/config.hpp>
15   
#include <boost/capy/concept/executor.hpp>
14   
#include <boost/capy/concept/executor.hpp>
16   
#include <boost/capy/concept/io_awaitable.hpp>
15   
#include <boost/capy/concept/io_awaitable.hpp>
17   
#include <coroutine>
16   
#include <coroutine>
18   
#include <boost/capy/ex/executor_ref.hpp>
17   
#include <boost/capy/ex/executor_ref.hpp>
19   
#include <boost/capy/ex/frame_allocator.hpp>
18   
#include <boost/capy/ex/frame_allocator.hpp>
20   
#include <boost/capy/ex/io_env.hpp>
19   
#include <boost/capy/ex/io_env.hpp>
21   
#include <boost/capy/task.hpp>
20   
#include <boost/capy/task.hpp>
22  
21  
23   
#include <array>
22   
#include <array>
24   
#include <atomic>
23   
#include <atomic>
25   
#include <exception>
24   
#include <exception>
26   
#include <optional>
25   
#include <optional>
27   
#include <ranges>
26   
#include <ranges>
28   
#include <stdexcept>
27   
#include <stdexcept>
29   
#include <stop_token>
28   
#include <stop_token>
30   
#include <tuple>
29   
#include <tuple>
31   
#include <type_traits>
30   
#include <type_traits>
32   
#include <utility>
31   
#include <utility>
33   
#include <variant>
32   
#include <variant>
34   
#include <vector>
33   
#include <vector>
35  
34  
36   
/*
35   
/*
37   
   when_any - Race multiple tasks, return first completion
36   
   when_any - Race multiple tasks, return first completion
38   
   ========================================================
37   
   ========================================================
39  
38  
40   
   OVERVIEW:
39   
   OVERVIEW:
41   
   ---------
40   
   ---------
42   
   when_any launches N tasks concurrently and completes when the FIRST task
41   
   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
42   
   finishes (success or failure). It then requests stop for all siblings and
44   
   waits for them to acknowledge before returning.
43   
   waits for them to acknowledge before returning.
45  
44  
46   
   ARCHITECTURE:
45   
   ARCHITECTURE:
47   
   -------------
46   
   -------------
48   
   The design mirrors when_all but with inverted completion semantics:
47   
   The design mirrors when_all but with inverted completion semantics:
49  
48  
50   
     when_all:  complete when remaining_count reaches 0 (all done)
49   
     when_all:  complete when remaining_count reaches 0 (all done)
51   
     when_any:  complete when has_winner becomes true (first done)
50   
     when_any:  complete when has_winner becomes true (first done)
52   
                BUT still wait for remaining_count to reach 0 for cleanup
51   
                BUT still wait for remaining_count to reach 0 for cleanup
53  
52  
54   
   Key components:
53   
   Key components:
55   
     - when_any_state:    Shared state tracking winner and completion
54   
     - when_any_state:    Shared state tracking winner and completion
56   
     - when_any_runner:   Wrapper coroutine for each child task
55   
     - when_any_runner:   Wrapper coroutine for each child task
57   
     - when_any_launcher: Awaitable that starts all runners concurrently
56   
     - when_any_launcher: Awaitable that starts all runners concurrently
58  
57  
59   
   CRITICAL INVARIANTS:
58   
   CRITICAL INVARIANTS:
60   
   --------------------
59   
   --------------------
61   
   1. Exactly one task becomes the winner (via atomic compare_exchange)
60   
   1. Exactly one task becomes the winner (via atomic compare_exchange)
62   
   2. All tasks must complete before parent resumes (cleanup safety)
61   
   2. All tasks must complete before parent resumes (cleanup safety)
63   
   3. Stop is requested immediately when winner is determined
62   
   3. Stop is requested immediately when winner is determined
64   
   4. Only the winner's result/exception is stored
63   
   4. Only the winner's result/exception is stored
65  
64  
66   
   POSITIONAL VARIANT:
65   
   POSITIONAL VARIANT:
67   
   -------------------
66   
   -------------------
68   
   The variadic overload returns a std::variant with one alternative per
67   
   The variadic overload returns a std::variant with one alternative per
69   
   input task, preserving positional correspondence. Use .index() on
68   
   input task, preserving positional correspondence. Use .index() on
70   
   the variant to identify which task won.
69   
   the variant to identify which task won.
71  
70  
72   
   Example: when_any(task<int>, task<string>, task<int>)
71   
   Example: when_any(task<int>, task<string>, task<int>)
73   
     - Raw types after void->monostate: int, string, int
72   
     - Raw types after void->monostate: int, string, int
74   
     - Result variant: std::variant<int, string, int>
73   
     - Result variant: std::variant<int, string, int>
75   
     - variant.index() tells you which task won (0, 1, or 2)
74   
     - variant.index() tells you which task won (0, 1, or 2)
76  
75  
77   
   VOID HANDLING:
76   
   VOID HANDLING:
78   
   --------------
77   
   --------------
79   
   void tasks contribute std::monostate to the variant.
78   
   void tasks contribute std::monostate to the variant.
80   
   All-void tasks result in: variant<monostate, monostate, monostate>
79   
   All-void tasks result in: variant<monostate, monostate, monostate>
81  
80  
82   
   MEMORY MODEL:
81   
   MEMORY MODEL:
83   
   -------------
82   
   -------------
84   
   Synchronization chain from winner's write to parent's read:
83   
   Synchronization chain from winner's write to parent's read:
85  
84  
86   
   1. Winner thread writes result_/winner_exception_ (non-atomic)
85   
   1. Winner thread writes result_/winner_exception_ (non-atomic)
87   
   2. Winner thread calls signal_completion() → fetch_sub(acq_rel) on remaining_count_
86   
   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()
87   
   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
88   
      → fetch_sub(acq_rel) on remaining_count_, observing count becomes 0
90   
   4. Last task returns caller_ex_.dispatch(continuation_) via symmetric transfer
89   
   4. Last task returns caller_ex_.dispatch(continuation_) via symmetric transfer
91   
   5. Parent coroutine resumes and reads result_/winner_exception_
90   
   5. Parent coroutine resumes and reads result_/winner_exception_
92  
91  
93   
   Synchronization analysis:
92   
   Synchronization analysis:
94   
   - All fetch_sub operations on remaining_count_ form a release sequence
93   
   - All fetch_sub operations on remaining_count_ form a release sequence
95   
   - Winner's fetch_sub releases; subsequent fetch_sub operations participate
94   
   - Winner's fetch_sub releases; subsequent fetch_sub operations participate
96   
     in the modification order of remaining_count_
95   
     in the modification order of remaining_count_
97   
   - Last task's fetch_sub(acq_rel) synchronizes-with prior releases in the
96   
   - Last task's fetch_sub(acq_rel) synchronizes-with prior releases in the
98   
     modification order, establishing happens-before from winner's writes
97   
     modification order, establishing happens-before from winner's writes
99   
   - Executor dispatch() is expected to provide queue-based synchronization
98   
   - Executor dispatch() is expected to provide queue-based synchronization
100   
     (release-on-post, acquire-on-execute) completing the chain to parent
99   
     (release-on-post, acquire-on-execute) completing the chain to parent
101   
   - Even inline executors work (same thread = sequenced-before)
100   
   - Even inline executors work (same thread = sequenced-before)
102  
101  
103   
   Alternative considered: Adding winner_ready_ atomic (set with release after
102   
   Alternative considered: Adding winner_ready_ atomic (set with release after
104   
   storing winner data, acquired before reading) would make synchronization
103   
   storing winner data, acquired before reading) would make synchronization
105   
   self-contained and not rely on executor implementation details. Current
104   
   self-contained and not rely on executor implementation details. Current
106   
   approach is correct but requires careful reasoning about release sequences
105   
   approach is correct but requires careful reasoning about release sequences
107   
   and executor behavior.
106   
   and executor behavior.
108  
107  
109   
   EXCEPTION SEMANTICS:
108   
   EXCEPTION SEMANTICS:
110   
   --------------------
109   
   --------------------
111   
   Unlike when_all (which captures first exception, discards others), when_any
110   
   Unlike when_all (which captures first exception, discards others), when_any
112   
   treats exceptions as valid completions. If the winning task threw, that
111   
   treats exceptions as valid completions. If the winning task threw, that
113   
   exception is rethrown. Exceptions from non-winners are silently discarded.
112   
   exception is rethrown. Exceptions from non-winners are silently discarded.
114   
*/
113   
*/
115  
114  
116   
namespace boost {
115   
namespace boost {
117   
namespace capy {
116   
namespace capy {
 
117 +
 
118 + 
/** Convert void to monostate for variant storage.
 
119 +
 
120 + 
    std::variant<void, ...> is ill-formed, so void tasks contribute
 
121 + 
    std::monostate to the result variant instead. Non-void types
 
122 + 
    pass through unchanged.
 
123 +
 
124 + 
    @tparam T The type to potentially convert (void becomes monostate).
 
125 + 
*/
 
126 + 
template<typename T>
 
127 + 
using void_to_monostate_t = std::conditional_t<std::is_void_v<T>, std::monostate, T>;
118  
128  
119   
namespace detail {
129   
namespace detail {
120  
130  
121   
/** Core shared state for when_any operations.
131   
/** Core shared state for when_any operations.
122  
132  
123   
    Contains all members and methods common to both heterogeneous (variadic)
133   
    Contains all members and methods common to both heterogeneous (variadic)
124   
    and homogeneous (range) when_any implementations. State classes embed
134   
    and homogeneous (range) when_any implementations. State classes embed
125   
    this via composition to avoid CRTP destructor ordering issues.
135   
    this via composition to avoid CRTP destructor ordering issues.
126  
136  
127   
    @par Thread Safety
137   
    @par Thread Safety
128   
    Atomic operations protect winner selection and completion count.
138   
    Atomic operations protect winner selection and completion count.
129   
*/
139   
*/
130   
struct when_any_core
140   
struct when_any_core
131   
{
141   
{
132   
    std::atomic<std::size_t> remaining_count_;
142   
    std::atomic<std::size_t> remaining_count_;
133   
    std::size_t winner_index_{0};
143   
    std::size_t winner_index_{0};
134   
    std::exception_ptr winner_exception_;
144   
    std::exception_ptr winner_exception_;
135   
    std::stop_source stop_source_;
145   
    std::stop_source stop_source_;
136  
146  
137   
    // Bridges parent's stop token to our stop_source
147   
    // Bridges parent's stop token to our stop_source
138   
    struct stop_callback_fn
148   
    struct stop_callback_fn
139   
    {
149   
    {
140   
        std::stop_source* source_;
150   
        std::stop_source* source_;
141   
        void operator()() const noexcept { source_->request_stop(); }
151   
        void operator()() const noexcept { source_->request_stop(); }
142   
    };
152   
    };
143   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
153   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
144   
    std::optional<stop_callback_t> parent_stop_callback_;
154   
    std::optional<stop_callback_t> parent_stop_callback_;
145  
155  
146   
    std::coroutine_handle<> continuation_;
156   
    std::coroutine_handle<> continuation_;
147   
    io_env const* caller_env_ = nullptr;
157   
    io_env const* caller_env_ = nullptr;
148  
158  
149   
    // Placed last to avoid padding (1-byte atomic followed by 8-byte aligned members)
159   
    // Placed last to avoid padding (1-byte atomic followed by 8-byte aligned members)
150   
    std::atomic<bool> has_winner_{false};
160   
    std::atomic<bool> has_winner_{false};
151  
161  
152   
    explicit when_any_core(std::size_t count) noexcept
162   
    explicit when_any_core(std::size_t count) noexcept
153   
        : remaining_count_(count)
163   
        : remaining_count_(count)
154   
    {
164   
    {
155   
    }
165   
    }
156  
166  
157   
    /** Atomically claim winner status; exactly one task succeeds. */
167   
    /** Atomically claim winner status; exactly one task succeeds. */
158   
    bool try_win(std::size_t index) noexcept
168   
    bool try_win(std::size_t index) noexcept
159   
    {
169   
    {
160   
        bool expected = false;
170   
        bool expected = false;
161   
        if(has_winner_.compare_exchange_strong(
171   
        if(has_winner_.compare_exchange_strong(
162   
            expected, true, std::memory_order_acq_rel))
172   
            expected, true, std::memory_order_acq_rel))
163   
        {
173   
        {
164   
            winner_index_ = index;
174   
            winner_index_ = index;
165   
            stop_source_.request_stop();
175   
            stop_source_.request_stop();
166   
            return true;
176   
            return true;
167   
        }
177   
        }
168   
        return false;
178   
        return false;
169   
    }
179   
    }
170  
180  
171   
    /** @pre try_win() returned true. */
181   
    /** @pre try_win() returned true. */
172   
    void set_winner_exception(std::exception_ptr ep) noexcept
182   
    void set_winner_exception(std::exception_ptr ep) noexcept
173   
    {
183   
    {
174   
        winner_exception_ = ep;
184   
        winner_exception_ = ep;
175   
    }
185   
    }
176  
186  
177   
    // Runners signal completion directly via final_suspend; no member function needed.
187   
    // Runners signal completion directly via final_suspend; no member function needed.
178   
};
188   
};
179  
189  
180   
/** Shared state for heterogeneous when_any operation.
190   
/** Shared state for heterogeneous when_any operation.
181  
191  
182   
    Coordinates winner selection, result storage, and completion tracking
192   
    Coordinates winner selection, result storage, and completion tracking
183   
    for all child tasks in a when_any operation. Uses composition with
193   
    for all child tasks in a when_any operation. Uses composition with
184   
    when_any_core for shared functionality.
194   
    when_any_core for shared functionality.
185  
195  
186   
    @par Lifetime
196   
    @par Lifetime
187   
    Allocated on the parent coroutine's frame, outlives all runners.
197   
    Allocated on the parent coroutine's frame, outlives all runners.
188  
198  
189   
    @tparam Ts Task result types.
199   
    @tparam Ts Task result types.
190   
*/
200   
*/
191   
template<typename... Ts>
201   
template<typename... Ts>
192   
struct when_any_state
202   
struct when_any_state
193   
{
203   
{
194   
    static constexpr std::size_t task_count = sizeof...(Ts);
204   
    static constexpr std::size_t task_count = sizeof...(Ts);
195   
    using variant_type = std::variant<void_to_monostate_t<Ts>...>;
205   
    using variant_type = std::variant<void_to_monostate_t<Ts>...>;
196  
206  
197   
    when_any_core core_;
207   
    when_any_core core_;
198   
    std::optional<variant_type> result_;
208   
    std::optional<variant_type> result_;
199   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
209   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
200  
210  
201   
    when_any_state()
211   
    when_any_state()
202   
        : core_(task_count)
212   
        : core_(task_count)
203   
    {
213   
    {
204   
    }
214   
    }
205  
215  
206   
    // Runners self-destruct in final_suspend. No destruction needed here.
216   
    // Runners self-destruct in final_suspend. No destruction needed here.
207  
217  
208   
    /** @pre core_.try_win() returned true.
218   
    /** @pre core_.try_win() returned true.
209   
        @note Uses in_place_index (not type) for positional variant access.
219   
        @note Uses in_place_index (not type) for positional variant access.
210   
    */
220   
    */
211   
    template<std::size_t I, typename T>
221   
    template<std::size_t I, typename T>
212   
    void set_winner_result(T value)
222   
    void set_winner_result(T value)
213   
        noexcept(std::is_nothrow_move_constructible_v<T>)
223   
        noexcept(std::is_nothrow_move_constructible_v<T>)
214   
    {
224   
    {
215   
        result_.emplace(std::in_place_index<I>, std::move(value));
225   
        result_.emplace(std::in_place_index<I>, std::move(value));
216   
    }
226   
    }
217  
227  
218   
    /** @pre core_.try_win() returned true. */
228   
    /** @pre core_.try_win() returned true. */
219   
    template<std::size_t I>
229   
    template<std::size_t I>
220   
    void set_winner_void() noexcept
230   
    void set_winner_void() noexcept
221   
    {
231   
    {
222   
        result_.emplace(std::in_place_index<I>, std::monostate{});
232   
        result_.emplace(std::in_place_index<I>, std::monostate{});
223   
    }
233   
    }
224   
};
234   
};
225  
235  
226   
/** Wrapper coroutine that runs a single child task for when_any.
236   
/** Wrapper coroutine that runs a single child task for when_any.
227  
237  
228   
    Propagates executor/stop_token to the child, attempts to claim winner
238   
    Propagates executor/stop_token to the child, attempts to claim winner
229   
    status on completion, and signals completion for cleanup coordination.
239   
    status on completion, and signals completion for cleanup coordination.
230  
240  
231   
    @tparam StateType The state type (when_any_state or when_any_homogeneous_state).
241   
    @tparam StateType The state type (when_any_state or when_any_homogeneous_state).
232   
*/
242   
*/
233   
template<typename StateType>
243   
template<typename StateType>
234   
struct when_any_runner
244   
struct when_any_runner
235   
{
245   
{
236   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
246   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
237   
    {
247   
    {
238   
        StateType* state_ = nullptr;
248   
        StateType* state_ = nullptr;
239   
        std::size_t index_ = 0;
249   
        std::size_t index_ = 0;
240   
        io_env env_;
250   
        io_env env_;
241  
251  
242   
        when_any_runner get_return_object() noexcept
252   
        when_any_runner get_return_object() noexcept
243   
        {
253   
        {
244   
            return when_any_runner(std::coroutine_handle<promise_type>::from_promise(*this));
254   
            return when_any_runner(std::coroutine_handle<promise_type>::from_promise(*this));
245   
        }
255   
        }
246  
256  
247   
        // Starts suspended; launcher sets up state/ex/token then resumes
257   
        // Starts suspended; launcher sets up state/ex/token then resumes
248   
        std::suspend_always initial_suspend() noexcept
258   
        std::suspend_always initial_suspend() noexcept
249   
        {
259   
        {
250   
            return {};
260   
            return {};
251   
        }
261   
        }
252  
262  
253   
        auto final_suspend() noexcept
263   
        auto final_suspend() noexcept
254   
        {
264   
        {
255   
            struct awaiter
265   
            struct awaiter
256   
            {
266   
            {
257   
                promise_type* p_;
267   
                promise_type* p_;
258   
                bool await_ready() const noexcept { return false; }
268   
                bool await_ready() const noexcept { return false; }
259   
                auto await_suspend(std::coroutine_handle<> h) noexcept
269   
                auto await_suspend(std::coroutine_handle<> h) noexcept
260   
                {
270   
                {
261   
                    // Extract everything needed before self-destruction.
271   
                    // Extract everything needed before self-destruction.
262   
                    auto& core = p_->state_->core_;
272   
                    auto& core = p_->state_->core_;
263   
                    auto* counter = &core.remaining_count_;
273   
                    auto* counter = &core.remaining_count_;
264   
                    auto* caller_env = core.caller_env_;
274   
                    auto* caller_env = core.caller_env_;
265   
                    auto cont = core.continuation_;
275   
                    auto cont = core.continuation_;
266  
276  
267   
                    h.destroy();
277   
                    h.destroy();
268  
278  
269   
                    // If last runner, dispatch parent for symmetric transfer.
279   
                    // If last runner, dispatch parent for symmetric transfer.
270   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
280   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
271   
                    if(remaining == 1)
281   
                    if(remaining == 1)
272   
                        return detail::symmetric_transfer(caller_env->executor.dispatch(cont));
282   
                        return detail::symmetric_transfer(caller_env->executor.dispatch(cont));
273   
                    return detail::symmetric_transfer(std::noop_coroutine());
283   
                    return detail::symmetric_transfer(std::noop_coroutine());
274   
                }
284   
                }
275   
                void await_resume() const noexcept {}
285   
                void await_resume() const noexcept {}
276   
            };
286   
            };
277   
            return awaiter{this};
287   
            return awaiter{this};
278   
        }
288   
        }
279  
289  
280   
        void return_void() noexcept {}
290   
        void return_void() noexcept {}
281  
291  
282   
        // Exceptions are valid completions in when_any (unlike when_all)
292   
        // Exceptions are valid completions in when_any (unlike when_all)
283   
        void unhandled_exception()
293   
        void unhandled_exception()
284   
        {
294   
        {
285   
            if(state_->core_.try_win(index_))
295   
            if(state_->core_.try_win(index_))
286   
                state_->core_.set_winner_exception(std::current_exception());
296   
                state_->core_.set_winner_exception(std::current_exception());
287   
        }
297   
        }
288  
298  
289   
        /** Injects executor and stop token into child awaitables. */
299   
        /** Injects executor and stop token into child awaitables. */
290   
        template<class Awaitable>
300   
        template<class Awaitable>
291   
        struct transform_awaiter
301   
        struct transform_awaiter
292   
        {
302   
        {
293   
            std::decay_t<Awaitable> a_;
303   
            std::decay_t<Awaitable> a_;
294   
            promise_type* p_;
304   
            promise_type* p_;
295  
305  
296   
            bool await_ready() { return a_.await_ready(); }
306   
            bool await_ready() { return a_.await_ready(); }
297   
            auto await_resume() { return a_.await_resume(); }
307   
            auto await_resume() { return a_.await_resume(); }
298  
308  
299   
            template<class Promise>
309   
            template<class Promise>
300   
            auto await_suspend(std::coroutine_handle<Promise> h)
310   
            auto await_suspend(std::coroutine_handle<Promise> h)
301   
            {
311   
            {
302   
                using R = decltype(a_.await_suspend(h, &p_->env_));
312   
                using R = decltype(a_.await_suspend(h, &p_->env_));
303   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
313   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
304   
                    return detail::symmetric_transfer(a_.await_suspend(h, &p_->env_));
314   
                    return detail::symmetric_transfer(a_.await_suspend(h, &p_->env_));
305   
                else
315   
                else
306   
                    return a_.await_suspend(h, &p_->env_);
316   
                    return a_.await_suspend(h, &p_->env_);
307   
            }
317   
            }
308   
        };
318   
        };
309  
319  
310   
        template<class Awaitable>
320   
        template<class Awaitable>
311   
        auto await_transform(Awaitable&& a)
321   
        auto await_transform(Awaitable&& a)
312   
        {
322   
        {
313   
            using A = std::decay_t<Awaitable>;
323   
            using A = std::decay_t<Awaitable>;
314   
            if constexpr (IoAwaitable<A>)
324   
            if constexpr (IoAwaitable<A>)
315   
            {
325   
            {
316   
                return transform_awaiter<Awaitable>{
326   
                return transform_awaiter<Awaitable>{
317   
                    std::forward<Awaitable>(a), this};
327   
                    std::forward<Awaitable>(a), this};
318   
            }
328   
            }
319   
            else
329   
            else
320   
            {
330   
            {
321   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
331   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
322   
            }
332   
            }
323   
        }
333   
        }
324   
    };
334   
    };
325  
335  
326   
    std::coroutine_handle<promise_type> h_;
336   
    std::coroutine_handle<promise_type> h_;
327  
337  
328   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
338   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
329   
        : h_(h)
339   
        : h_(h)
330   
    {
340   
    {
331   
    }
341   
    }
332  
342  
333   
    // Enable move for all clang versions - some versions need it
343   
    // 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)) {}
344   
    when_any_runner(when_any_runner&& other) noexcept : h_(std::exchange(other.h_, nullptr)) {}
335  
345  
336   
    // Non-copyable
346   
    // Non-copyable
337   
    when_any_runner(when_any_runner const&) = delete;
347   
    when_any_runner(when_any_runner const&) = delete;
338   
    when_any_runner& operator=(when_any_runner const&) = delete;
348   
    when_any_runner& operator=(when_any_runner const&) = delete;
339   
    when_any_runner& operator=(when_any_runner&&) = delete;
349   
    when_any_runner& operator=(when_any_runner&&) = delete;
340  
350  
341   
    auto release() noexcept
351   
    auto release() noexcept
342   
    {
352   
    {
343   
        return std::exchange(h_, nullptr);
353   
        return std::exchange(h_, nullptr);
344   
    }
354   
    }
345   
};
355   
};
346  
356  
347   
/** Indexed overload for heterogeneous when_any (compile-time index).
357   
/** Indexed overload for heterogeneous when_any (compile-time index).
348  
358  
349   
    Uses compile-time index I for variant construction via in_place_index.
359   
    Uses compile-time index I for variant construction via in_place_index.
350   
    Called from when_any_launcher::launch_one<I>().
360   
    Called from when_any_launcher::launch_one<I>().
351   
*/
361   
*/
352   
template<std::size_t I, IoAwaitable Awaitable, typename StateType>
362   
template<std::size_t I, IoAwaitable Awaitable, typename StateType>
353   
when_any_runner<StateType>
363   
when_any_runner<StateType>
354   
make_when_any_runner(Awaitable inner, StateType* state)
364   
make_when_any_runner(Awaitable inner, StateType* state)
355   
{
365   
{
356   
    using T = awaitable_result_t<Awaitable>;
366   
    using T = awaitable_result_t<Awaitable>;
357   
    if constexpr (std::is_void_v<T>)
367   
    if constexpr (std::is_void_v<T>)
358   
    {
368   
    {
359   
        co_await std::move(inner);
369   
        co_await std::move(inner);
360   
        if(state->core_.try_win(I))
370   
        if(state->core_.try_win(I))
361   
            state->template set_winner_void<I>();
371   
            state->template set_winner_void<I>();
362   
    }
372   
    }
363   
    else
373   
    else
364   
    {
374   
    {
365   
        auto result = co_await std::move(inner);
375   
        auto result = co_await std::move(inner);
366   
        if(state->core_.try_win(I))
376   
        if(state->core_.try_win(I))
367   
        {
377   
        {
368   
            try
378   
            try
369   
            {
379   
            {
370   
                state->template set_winner_result<I>(std::move(result));
380   
                state->template set_winner_result<I>(std::move(result));
371   
            }
381   
            }
372   
            catch(...)
382   
            catch(...)
373   
            {
383   
            {
374   
                state->core_.set_winner_exception(std::current_exception());
384   
                state->core_.set_winner_exception(std::current_exception());
375   
            }
385   
            }
376   
        }
386   
        }
377   
    }
387   
    }
378   
}
388   
}
379  
389  
380   
/** Runtime-index overload for homogeneous when_any (range path).
390   
/** Runtime-index overload for homogeneous when_any (range path).
381  
391  
382   
    Uses requires-expressions to detect state capabilities:
392   
    Uses requires-expressions to detect state capabilities:
383   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
393   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
384   
    - set_winner_result(): for non-void tasks
394   
    - set_winner_result(): for non-void tasks
385   
    - Neither: for homogeneous void tasks (no result storage)
395   
    - Neither: for homogeneous void tasks (no result storage)
386   
*/
396   
*/
387   
template<IoAwaitable Awaitable, typename StateType>
397   
template<IoAwaitable Awaitable, typename StateType>
388   
when_any_runner<StateType>
398   
when_any_runner<StateType>
389   
make_when_any_runner(Awaitable inner, StateType* state, std::size_t index)
399   
make_when_any_runner(Awaitable inner, StateType* state, std::size_t index)
390   
{
400   
{
391   
    using T = awaitable_result_t<Awaitable>;
401   
    using T = awaitable_result_t<Awaitable>;
392   
    if constexpr (std::is_void_v<T>)
402   
    if constexpr (std::is_void_v<T>)
393   
    {
403   
    {
394   
        co_await std::move(inner);
404   
        co_await std::move(inner);
395   
        if(state->core_.try_win(index))
405   
        if(state->core_.try_win(index))
396   
        {
406   
        {
397   
            if constexpr (requires { state->set_winner_void(); })
407   
            if constexpr (requires { state->set_winner_void(); })
398   
                state->set_winner_void();
408   
                state->set_winner_void();
399   
        }
409   
        }
400   
    }
410   
    }
401   
    else
411   
    else
402   
    {
412   
    {
403   
        auto result = co_await std::move(inner);
413   
        auto result = co_await std::move(inner);
404   
        if(state->core_.try_win(index))
414   
        if(state->core_.try_win(index))
405   
        {
415   
        {
406   
            try
416   
            try
407   
            {
417   
            {
408   
                state->set_winner_result(std::move(result));
418   
                state->set_winner_result(std::move(result));
409   
            }
419   
            }
410   
            catch(...)
420   
            catch(...)
411   
            {
421   
            {
412   
                state->core_.set_winner_exception(std::current_exception());
422   
                state->core_.set_winner_exception(std::current_exception());
413   
            }
423   
            }
414   
        }
424   
        }
415   
    }
425   
    }
416   
}
426   
}
417  
427  
418   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
428   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
419   
template<IoAwaitable... Awaitables>
429   
template<IoAwaitable... Awaitables>
420   
class when_any_launcher
430   
class when_any_launcher
421   
{
431   
{
422   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
432   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
423  
433  
424   
    std::tuple<Awaitables...>* tasks_;
434   
    std::tuple<Awaitables...>* tasks_;
425   
    state_type* state_;
435   
    state_type* state_;
426  
436  
427   
public:
437   
public:
428   
    when_any_launcher(
438   
    when_any_launcher(
429   
        std::tuple<Awaitables...>* tasks,
439   
        std::tuple<Awaitables...>* tasks,
430   
        state_type* state)
440   
        state_type* state)
431   
        : tasks_(tasks)
441   
        : tasks_(tasks)
432   
        , state_(state)
442   
        , state_(state)
433   
    {
443   
    {
434   
    }
444   
    }
435  
445  
436   
    bool await_ready() const noexcept
446   
    bool await_ready() const noexcept
437   
    {
447   
    {
438   
        return sizeof...(Awaitables) == 0;
448   
        return sizeof...(Awaitables) == 0;
439   
    }
449   
    }
440  
450  
441   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
451   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
442   
        destroys this object before await_suspend returns. Must not reference
452   
        destroys this object before await_suspend returns. Must not reference
443   
        `this` after the final launch_one call.
453   
        `this` after the final launch_one call.
444   
    */
454   
    */
445   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
455   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
446   
    {
456   
    {
447   
        state_->core_.continuation_ = continuation;
457   
        state_->core_.continuation_ = continuation;
448   
        state_->core_.caller_env_ = caller_env;
458   
        state_->core_.caller_env_ = caller_env;
449  
459  
450   
        if(caller_env->stop_token.stop_possible())
460   
        if(caller_env->stop_token.stop_possible())
451   
        {
461   
        {
452   
            state_->core_.parent_stop_callback_.emplace(
462   
            state_->core_.parent_stop_callback_.emplace(
453   
                caller_env->stop_token,
463   
                caller_env->stop_token,
454   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
464   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
455  
465  
456   
            if(caller_env->stop_token.stop_requested())
466   
            if(caller_env->stop_token.stop_requested())
457   
                state_->core_.stop_source_.request_stop();
467   
                state_->core_.stop_source_.request_stop();
458   
        }
468   
        }
459  
469  
460   
        auto token = state_->core_.stop_source_.get_token();
470   
        auto token = state_->core_.stop_source_.get_token();
461   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
471   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
462   
            (..., launch_one<Is>(caller_env->executor, token));
472   
            (..., launch_one<Is>(caller_env->executor, token));
463   
        }(std::index_sequence_for<Awaitables...>{});
473   
        }(std::index_sequence_for<Awaitables...>{});
464  
474  
465   
        return std::noop_coroutine();
475   
        return std::noop_coroutine();
466   
    }
476   
    }
467  
477  
468   
    void await_resume() const noexcept
478   
    void await_resume() const noexcept
469   
    {
479   
    {
470   
    }
480   
    }
471  
481  
472   
private:
482   
private:
473   
    /** @pre Ex::dispatch() and std::coroutine_handle<>::resume() must not throw (handle may leak). */
483   
    /** @pre Ex::dispatch() and std::coroutine_handle<>::resume() must not throw (handle may leak). */
474   
    template<std::size_t I>
484   
    template<std::size_t I>
475   
    void launch_one(executor_ref caller_ex, std::stop_token token)
485   
    void launch_one(executor_ref caller_ex, std::stop_token token)
476   
    {
486   
    {
477   
        auto runner = make_when_any_runner<I>(
487   
        auto runner = make_when_any_runner<I>(
478   
            std::move(std::get<I>(*tasks_)), state_);
488   
            std::move(std::get<I>(*tasks_)), state_);
479  
489  
480   
        auto h = runner.release();
490   
        auto h = runner.release();
481   
        h.promise().state_ = state_;
491   
        h.promise().state_ = state_;
482   
        h.promise().index_ = I;
492   
        h.promise().index_ = I;
483   
        h.promise().env_ = io_env{caller_ex, token, state_->core_.caller_env_->frame_allocator};
493   
        h.promise().env_ = io_env{caller_ex, token, state_->core_.caller_env_->frame_allocator};
484  
494  
485   
        std::coroutine_handle<> ch{h};
495   
        std::coroutine_handle<> ch{h};
486   
        state_->runner_handles_[I] = ch;
496   
        state_->runner_handles_[I] = ch;
487   
        caller_ex.post(ch);
497   
        caller_ex.post(ch);
488   
    }
498   
    }
489   
};
499   
};
490  
500  
491   
} // namespace detail
501   
} // namespace detail
492  
502  
493   
/** Wait for the first awaitable to complete.
503   
/** Wait for the first awaitable to complete.
494  
504  
495   
    Races multiple heterogeneous awaitables concurrently and returns when the
505   
    Races multiple heterogeneous awaitables concurrently and returns when the
496   
    first one completes. The result is a variant with one alternative per
506   
    first one completes. The result is a variant with one alternative per
497   
    input task, preserving positional correspondence.
507   
    input task, preserving positional correspondence.
498  
508  
499   
    @par Suspends
509   
    @par Suspends
500   
    The calling coroutine suspends when co_await is invoked. All awaitables
510   
    The calling coroutine suspends when co_await is invoked. All awaitables
501   
    are launched concurrently and execute in parallel. The coroutine resumes
511   
    are launched concurrently and execute in parallel. The coroutine resumes
502   
    only after all awaitables have completed, even though the winner is
512   
    only after all awaitables have completed, even though the winner is
503   
    determined by the first to finish.
513   
    determined by the first to finish.
504  
514  
505   
    @par Completion Conditions
515   
    @par Completion Conditions
506   
    @li Winner is determined when the first awaitable completes (success or exception)
516   
    @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
517   
    @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
518   
    @li Once a winner exists, stop is requested for all remaining siblings
509   
    @li Parent coroutine resumes only after all siblings acknowledge completion
519   
    @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
520   
    @li The winner's result is returned; if the winner threw, the exception is rethrown
511  
521  
512   
    @par Cancellation Semantics
522   
    @par Cancellation Semantics
513   
    Cancellation is supported via stop_token propagated through the
523   
    Cancellation is supported via stop_token propagated through the
514   
    IoAwaitable protocol:
524   
    IoAwaitable protocol:
515   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
525   
    @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
526   
    @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
527   
    @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
528   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
519   
    @li Stop requests are cooperative; tasks must check and respond to them
529   
    @li Stop requests are cooperative; tasks must check and respond to them
520  
530  
521   
    @par Concurrency/Overlap
531   
    @par Concurrency/Overlap
522   
    All awaitables are launched concurrently before any can complete.
532   
    All awaitables are launched concurrently before any can complete.
523   
    The launcher iterates through the arguments, starting each task on the
533   
    The launcher iterates through the arguments, starting each task on the
524   
    caller's executor. Tasks may execute in parallel on multi-threaded
534   
    caller's executor. Tasks may execute in parallel on multi-threaded
525   
    executors or interleave on single-threaded executors. There is no
535   
    executors or interleave on single-threaded executors. There is no
526   
    guaranteed ordering of task completion.
536   
    guaranteed ordering of task completion.
527  
537  
528   
    @par Notable Error Conditions
538   
    @par Notable Error Conditions
529   
    @li Winner exception: if the winning task threw, that exception is rethrown
539   
    @li Winner exception: if the winning task threw, that exception is rethrown
530   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
540   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
531   
    @li Cancellation: tasks may complete via cancellation without throwing
541   
    @li Cancellation: tasks may complete via cancellation without throwing
532  
542  
533   
    @par Example
543   
    @par Example
534   
    @code
544   
    @code
535   
    task<void> example() {
545   
    task<void> example() {
536   
        auto result = co_await when_any(
546   
        auto result = co_await when_any(
537   
            fetch_int(),      // task<int>
547   
            fetch_int(),      // task<int>
538   
            fetch_string()    // task<std::string>
548   
            fetch_string()    // task<std::string>
539   
        );
549   
        );
540   
        // result.index() is 0 or 1
550   
        // result.index() is 0 or 1
541   
        if (result.index() == 0)
551   
        if (result.index() == 0)
542   
            std::cout << "Got int: " << std::get<0>(result) << "\n";
552   
            std::cout << "Got int: " << std::get<0>(result) << "\n";
543   
        else
553   
        else
544   
            std::cout << "Got string: " << std::get<1>(result) << "\n";
554   
            std::cout << "Got string: " << std::get<1>(result) << "\n";
545   
    }
555   
    }
546   
    @endcode
556   
    @endcode
547  
557  
548   
    @param as Awaitables to race concurrently (at least one required; each
558   
    @param as Awaitables to race concurrently (at least one required; each
549   
        must satisfy IoAwaitable).
559   
        must satisfy IoAwaitable).
550   
    @return A task yielding a std::variant with one alternative per awaitable.
560   
    @return A task yielding a std::variant with one alternative per awaitable.
551   
        Use .index() to identify the winner. Void awaitables contribute
561   
        Use .index() to identify the winner. Void awaitables contribute
552   
        std::monostate.
562   
        std::monostate.
553  
563  
554   
    @throws Rethrows the winner's exception if the winning task threw an exception.
564   
    @throws Rethrows the winner's exception if the winning task threw an exception.
555  
565  
556   
    @par Remarks
566   
    @par Remarks
557   
    Awaitables are moved into the coroutine frame; original objects become
567   
    Awaitables are moved into the coroutine frame; original objects become
558   
    empty after the call. The variant preserves one alternative per input
568   
    empty after the call. The variant preserves one alternative per input
559   
    task. Use .index() to determine which awaitable completed first.
569   
    task. Use .index() to determine which awaitable completed first.
560   
    Void awaitables contribute std::monostate to the variant.
570   
    Void awaitables contribute std::monostate to the variant.
561  
571  
562   
    @see when_all, IoAwaitable
572   
    @see when_all, IoAwaitable
563   
*/
573   
*/
564   
template<IoAwaitable... As>
574   
template<IoAwaitable... As>
565   
    requires (sizeof...(As) > 0)
575   
    requires (sizeof...(As) > 0)
566   
[[nodiscard]] auto when_any(As... as)
576   
[[nodiscard]] auto when_any(As... as)
567   
    -> task<std::variant<void_to_monostate_t<awaitable_result_t<As>>...>>
577   
    -> task<std::variant<void_to_monostate_t<awaitable_result_t<As>>...>>
568   
{
578   
{
569   
    detail::when_any_state<awaitable_result_t<As>...> state;
579   
    detail::when_any_state<awaitable_result_t<As>...> state;
570   
    std::tuple<As...> awaitable_tuple(std::move(as)...);
580   
    std::tuple<As...> awaitable_tuple(std::move(as)...);
571  
581  
572   
    co_await detail::when_any_launcher<As...>(&awaitable_tuple, &state);
582   
    co_await detail::when_any_launcher<As...>(&awaitable_tuple, &state);
573  
583  
574   
    if(state.core_.winner_exception_)
584   
    if(state.core_.winner_exception_)
575   
        std::rethrow_exception(state.core_.winner_exception_);
585   
        std::rethrow_exception(state.core_.winner_exception_);
576  
586  
577   
    co_return std::move(*state.result_);
587   
    co_return std::move(*state.result_);
578   
}
588   
}
579  
589  
580   
/** Concept for ranges of full I/O awaitables.
590   
/** Concept for ranges of full I/O awaitables.
581  
591  
582   
    A range satisfies `IoAwaitableRange` if it is a sized input range
592   
    A range satisfies `IoAwaitableRange` if it is a sized input range
583   
    whose value type satisfies @ref IoAwaitable. This enables when_any
593   
    whose value type satisfies @ref IoAwaitable. This enables when_any
584   
    to accept any container or view of awaitables, not just std::vector.
594   
    to accept any container or view of awaitables, not just std::vector.
585  
595  
586   
    @tparam R The range type.
596   
    @tparam R The range type.
587  
597  
588   
    @par Requirements
598   
    @par Requirements
589   
    @li `R` must satisfy `std::ranges::input_range`
599   
    @li `R` must satisfy `std::ranges::input_range`
590   
    @li `R` must satisfy `std::ranges::sized_range`
600   
    @li `R` must satisfy `std::ranges::sized_range`
591   
    @li `std::ranges::range_value_t<R>` must satisfy @ref IoAwaitable
601   
    @li `std::ranges::range_value_t<R>` must satisfy @ref IoAwaitable
592  
602  
593   
    @par Syntactic Requirements
603   
    @par Syntactic Requirements
594   
    Given `r` of type `R`:
604   
    Given `r` of type `R`:
595   
    @li `std::ranges::begin(r)` is valid
605   
    @li `std::ranges::begin(r)` is valid
596   
    @li `std::ranges::end(r)` is valid
606   
    @li `std::ranges::end(r)` is valid
597   
    @li `std::ranges::size(r)` returns `std::ranges::range_size_t<R>`
607   
    @li `std::ranges::size(r)` returns `std::ranges::range_size_t<R>`
598   
    @li `*std::ranges::begin(r)` satisfies @ref IoAwaitable
608   
    @li `*std::ranges::begin(r)` satisfies @ref IoAwaitable
599  
609  
600   
    @par Example
610   
    @par Example
601   
    @code
611   
    @code
602   
    template<IoAwaitableRange R>
612   
    template<IoAwaitableRange R>
603   
    task<void> race_all(R&& awaitables) {
613   
    task<void> race_all(R&& awaitables) {
604   
        auto winner = co_await when_any(std::forward<R>(awaitables));
614   
        auto winner = co_await when_any(std::forward<R>(awaitables));
605   
        // Process winner...
615   
        // Process winner...
606   
    }
616   
    }
607   
    @endcode
617   
    @endcode
608  
618  
609   
    @see when_any, IoAwaitable
619   
    @see when_any, IoAwaitable
610   
*/
620   
*/
611   
template<typename R>
621   
template<typename R>
612   
concept IoAwaitableRange =
622   
concept IoAwaitableRange =
613   
    std::ranges::input_range<R> &&
623   
    std::ranges::input_range<R> &&
614   
    std::ranges::sized_range<R> &&
624   
    std::ranges::sized_range<R> &&
615   
    IoAwaitable<std::ranges::range_value_t<R>>;
625   
    IoAwaitable<std::ranges::range_value_t<R>>;
616  
626  
617   
namespace detail {
627   
namespace detail {
618  
628  
619   
/** Shared state for homogeneous when_any (range overload).
629   
/** Shared state for homogeneous when_any (range overload).
620  
630  
621   
    Uses composition with when_any_core for shared functionality.
631   
    Uses composition with when_any_core for shared functionality.
622   
    Simpler than heterogeneous: optional<T> instead of variant, vector
632   
    Simpler than heterogeneous: optional<T> instead of variant, vector
623   
    instead of array for runner handles.
633   
    instead of array for runner handles.
624   
*/
634   
*/
625   
template<typename T>
635   
template<typename T>
626   
struct when_any_homogeneous_state
636   
struct when_any_homogeneous_state
627   
{
637   
{
628   
    when_any_core core_;
638   
    when_any_core core_;
629   
    std::optional<T> result_;
639   
    std::optional<T> result_;
630   
    std::vector<std::coroutine_handle<>> runner_handles_;
640   
    std::vector<std::coroutine_handle<>> runner_handles_;
631  
641  
632   
    explicit when_any_homogeneous_state(std::size_t count)
642   
    explicit when_any_homogeneous_state(std::size_t count)
633   
        : core_(count)
643   
        : core_(count)
634   
        , runner_handles_(count)
644   
        , runner_handles_(count)
635   
    {
645   
    {
636   
    }
646   
    }
637  
647  
638   
    // Runners self-destruct in final_suspend. No destruction needed here.
648   
    // Runners self-destruct in final_suspend. No destruction needed here.
639  
649  
640   
    /** @pre core_.try_win() returned true. */
650   
    /** @pre core_.try_win() returned true. */
641   
    void set_winner_result(T value)
651   
    void set_winner_result(T value)
642   
        noexcept(std::is_nothrow_move_constructible_v<T>)
652   
        noexcept(std::is_nothrow_move_constructible_v<T>)
643   
    {
653   
    {
644   
        result_.emplace(std::move(value));
654   
        result_.emplace(std::move(value));
645   
    }
655   
    }
646   
};
656   
};
647  
657  
648   
/** Specialization for void tasks (no result storage needed). */
658   
/** Specialization for void tasks (no result storage needed). */
649   
template<>
659   
template<>
650   
struct when_any_homogeneous_state<void>
660   
struct when_any_homogeneous_state<void>
651   
{
661   
{
652   
    when_any_core core_;
662   
    when_any_core core_;
653   
    std::vector<std::coroutine_handle<>> runner_handles_;
663   
    std::vector<std::coroutine_handle<>> runner_handles_;
654  
664  
655   
    explicit when_any_homogeneous_state(std::size_t count)
665   
    explicit when_any_homogeneous_state(std::size_t count)
656   
        : core_(count)
666   
        : core_(count)
657   
        , runner_handles_(count)
667   
        , runner_handles_(count)
658   
    {
668   
    {
659   
    }
669   
    }
660  
670  
661   
    // Runners self-destruct in final_suspend. No destruction needed here.
671   
    // Runners self-destruct in final_suspend. No destruction needed here.
662  
672  
663   
    // No set_winner_result - void tasks have no result to store
673   
    // No set_winner_result - void tasks have no result to store
664   
};
674   
};
665  
675  
666   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
676   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
667   
template<IoAwaitableRange Range>
677   
template<IoAwaitableRange Range>
668   
class when_any_homogeneous_launcher
678   
class when_any_homogeneous_launcher
669   
{
679   
{
670   
    using Awaitable = std::ranges::range_value_t<Range>;
680   
    using Awaitable = std::ranges::range_value_t<Range>;
671   
    using T = awaitable_result_t<Awaitable>;
681   
    using T = awaitable_result_t<Awaitable>;
672  
682  
673   
    Range* range_;
683   
    Range* range_;
674   
    when_any_homogeneous_state<T>* state_;
684   
    when_any_homogeneous_state<T>* state_;
675  
685  
676   
public:
686   
public:
677   
    when_any_homogeneous_launcher(
687   
    when_any_homogeneous_launcher(
678   
        Range* range,
688   
        Range* range,
679   
        when_any_homogeneous_state<T>* state)
689   
        when_any_homogeneous_state<T>* state)
680   
        : range_(range)
690   
        : range_(range)
681   
        , state_(state)
691   
        , state_(state)
682   
    {
692   
    {
683   
    }
693   
    }
684  
694  
685   
    bool await_ready() const noexcept
695   
    bool await_ready() const noexcept
686   
    {
696   
    {
687   
        return std::ranges::empty(*range_);
697   
        return std::ranges::empty(*range_);
688   
    }
698   
    }
689  
699  
690   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
700   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
691   
        destroys this object before await_suspend returns. Must not reference
701   
        destroys this object before await_suspend returns. Must not reference
692   
        `this` after dispatching begins.
702   
        `this` after dispatching begins.
693  
703  
694   
        Two-phase approach:
704   
        Two-phase approach:
695   
        1. Create all runners (safe - no dispatch yet)
705   
        1. Create all runners (safe - no dispatch yet)
696   
        2. Dispatch all runners (any may complete synchronously)
706   
        2. Dispatch all runners (any may complete synchronously)
697   
    */
707   
    */
698   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
708   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
699   
    {
709   
    {
700   
        state_->core_.continuation_ = continuation;
710   
        state_->core_.continuation_ = continuation;
701   
        state_->core_.caller_env_ = caller_env;
711   
        state_->core_.caller_env_ = caller_env;
702  
712  
703   
        if(caller_env->stop_token.stop_possible())
713   
        if(caller_env->stop_token.stop_possible())
704   
        {
714   
        {
705   
            state_->core_.parent_stop_callback_.emplace(
715   
            state_->core_.parent_stop_callback_.emplace(
706   
                caller_env->stop_token,
716   
                caller_env->stop_token,
707   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
717   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
708  
718  
709   
            if(caller_env->stop_token.stop_requested())
719   
            if(caller_env->stop_token.stop_requested())
710   
                state_->core_.stop_source_.request_stop();
720   
                state_->core_.stop_source_.request_stop();
711   
        }
721   
        }
712  
722  
713   
        auto token = state_->core_.stop_source_.get_token();
723   
        auto token = state_->core_.stop_source_.get_token();
714  
724  
715   
        // Phase 1: Create all runners without dispatching.
725   
        // Phase 1: Create all runners without dispatching.
716   
        // This iterates over *range_ safely because no runners execute yet.
726   
        // This iterates over *range_ safely because no runners execute yet.
717   
        std::size_t index = 0;
727   
        std::size_t index = 0;
718   
        for(auto&& a : *range_)
728   
        for(auto&& a : *range_)
719   
        {
729   
        {
720   
            auto runner = make_when_any_runner(
730   
            auto runner = make_when_any_runner(
721   
                std::move(a), state_, index);
731   
                std::move(a), state_, index);
722  
732  
723   
            auto h = runner.release();
733   
            auto h = runner.release();
724   
            h.promise().state_ = state_;
734   
            h.promise().state_ = state_;
725   
            h.promise().index_ = index;
735   
            h.promise().index_ = index;
726   
            h.promise().env_ = io_env{caller_env->executor, token, caller_env->frame_allocator};
736   
            h.promise().env_ = io_env{caller_env->executor, token, caller_env->frame_allocator};
727  
737  
728   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
738   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
729   
            ++index;
739   
            ++index;
730   
        }
740   
        }
731  
741  
732   
        // Phase 2: Post all runners. Any may complete synchronously.
742   
        // Phase 2: Post all runners. Any may complete synchronously.
733   
        // After last post, state_ and this may be destroyed.
743   
        // After last post, state_ and this may be destroyed.
734   
        // Use raw pointer/count captured before posting.
744   
        // Use raw pointer/count captured before posting.
735   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
745   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
736   
        std::size_t count = state_->runner_handles_.size();
746   
        std::size_t count = state_->runner_handles_.size();
737   
        for(std::size_t i = 0; i < count; ++i)
747   
        for(std::size_t i = 0; i < count; ++i)
738   
            caller_env->executor.post(handles[i]);
748   
            caller_env->executor.post(handles[i]);
739  
749  
740   
        return std::noop_coroutine();
750   
        return std::noop_coroutine();
741   
    }
751   
    }
742  
752  
743   
    void await_resume() const noexcept
753   
    void await_resume() const noexcept
744   
    {
754   
    {
745   
    }
755   
    }
746   
};
756   
};
747  
757  
748   
} // namespace detail
758   
} // namespace detail
749  
759  
750   
/** Wait for the first awaitable to complete (range overload).
760   
/** Wait for the first awaitable to complete (range overload).
751  
761  
752   
    Races a range of awaitables with the same result type. Accepts any
762   
    Races a range of awaitables with the same result type. Accepts any
753   
    sized input range of IoAwaitable types, enabling use with arrays,
763   
    sized input range of IoAwaitable types, enabling use with arrays,
754   
    spans, or custom containers.
764   
    spans, or custom containers.
755  
765  
756   
    @par Suspends
766   
    @par Suspends
757   
    The calling coroutine suspends when co_await is invoked. All awaitables
767   
    The calling coroutine suspends when co_await is invoked. All awaitables
758   
    in the range are launched concurrently and execute in parallel. The
768   
    in the range are launched concurrently and execute in parallel. The
759   
    coroutine resumes only after all awaitables have completed, even though
769   
    coroutine resumes only after all awaitables have completed, even though
760   
    the winner is determined by the first to finish.
770   
    the winner is determined by the first to finish.
761  
771  
762   
    @par Completion Conditions
772   
    @par Completion Conditions
763   
    @li Winner is determined when the first awaitable completes (success or exception)
773   
    @li Winner is determined when the first awaitable completes (success or exception)
764   
    @li Only one task can claim winner status via atomic compare-exchange
774   
    @li Only one task can claim winner status via atomic compare-exchange
765   
    @li Once a winner exists, stop is requested for all remaining siblings
775   
    @li Once a winner exists, stop is requested for all remaining siblings
766   
    @li Parent coroutine resumes only after all siblings acknowledge completion
776   
    @li Parent coroutine resumes only after all siblings acknowledge completion
767   
    @li The winner's index and result are returned; if the winner threw, the exception is rethrown
777   
    @li The winner's index and result are returned; if the winner threw, the exception is rethrown
768  
778  
769   
    @par Cancellation Semantics
779   
    @par Cancellation Semantics
770   
    Cancellation is supported via stop_token propagated through the
780   
    Cancellation is supported via stop_token propagated through the
771   
    IoAwaitable protocol:
781   
    IoAwaitable protocol:
772   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
782   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
773   
    @li When the parent's stop token is activated, the stop is forwarded to all children
783   
    @li When the parent's stop token is activated, the stop is forwarded to all children
774   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
784   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
775   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
785   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
776   
    @li Stop requests are cooperative; tasks must check and respond to them
786   
    @li Stop requests are cooperative; tasks must check and respond to them
777  
787  
778   
    @par Concurrency/Overlap
788   
    @par Concurrency/Overlap
779   
    All awaitables are launched concurrently before any can complete.
789   
    All awaitables are launched concurrently before any can complete.
780   
    The launcher iterates through the range, starting each task on the
790   
    The launcher iterates through the range, starting each task on the
781   
    caller's executor. Tasks may execute in parallel on multi-threaded
791   
    caller's executor. Tasks may execute in parallel on multi-threaded
782   
    executors or interleave on single-threaded executors. There is no
792   
    executors or interleave on single-threaded executors. There is no
783   
    guaranteed ordering of task completion.
793   
    guaranteed ordering of task completion.
784  
794  
785   
    @par Notable Error Conditions
795   
    @par Notable Error Conditions
786   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
796   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
787   
    @li Winner exception: if the winning task threw, that exception is rethrown
797   
    @li Winner exception: if the winning task threw, that exception is rethrown
788   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
798   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
789   
    @li Cancellation: tasks may complete via cancellation without throwing
799   
    @li Cancellation: tasks may complete via cancellation without throwing
790  
800  
791   
    @par Example
801   
    @par Example
792   
    @code
802   
    @code
793   
    task<void> example() {
803   
    task<void> example() {
794   
        std::array<task<Response>, 3> requests = {
804   
        std::array<task<Response>, 3> requests = {
795   
            fetch_from_server(0),
805   
            fetch_from_server(0),
796   
            fetch_from_server(1),
806   
            fetch_from_server(1),
797   
            fetch_from_server(2)
807   
            fetch_from_server(2)
798   
        };
808   
        };
799  
809  
800   
        auto [index, response] = co_await when_any(std::move(requests));
810   
        auto [index, response] = co_await when_any(std::move(requests));
801   
    }
811   
    }
802   
    @endcode
812   
    @endcode
803  
813  
804   
    @par Example with Vector
814   
    @par Example with Vector
805   
    @code
815   
    @code
806   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
816   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
807   
        std::vector<task<Response>> requests;
817   
        std::vector<task<Response>> requests;
808   
        for (auto const& server : servers)
818   
        for (auto const& server : servers)
809   
            requests.push_back(fetch_from(server));
819   
            requests.push_back(fetch_from(server));
810  
820  
811   
        auto [index, response] = co_await when_any(std::move(requests));
821   
        auto [index, response] = co_await when_any(std::move(requests));
812   
        co_return response;
822   
        co_return response;
813   
    }
823   
    }
814   
    @endcode
824   
    @endcode
815  
825  
816   
    @tparam R Range type satisfying IoAwaitableRange.
826   
    @tparam R Range type satisfying IoAwaitableRange.
817   
    @param awaitables Range of awaitables to race concurrently (must not be empty).
827   
    @param awaitables Range of awaitables to race concurrently (must not be empty).
818   
    @return A task yielding a pair of (winner_index, result).
828   
    @return A task yielding a pair of (winner_index, result).
819  
829  
820   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
830   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
821   
    @throws Rethrows the winner's exception if the winning task threw an exception.
831   
    @throws Rethrows the winner's exception if the winning task threw an exception.
822  
832  
823   
    @par Remarks
833   
    @par Remarks
824   
    Elements are moved from the range; for lvalue ranges, the original
834   
    Elements are moved from the range; for lvalue ranges, the original
825   
    container will have moved-from elements after this call. The range
835   
    container will have moved-from elements after this call. The range
826   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
836   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
827   
    the variadic overload, no variant wrapper is needed since all tasks
837   
    the variadic overload, no variant wrapper is needed since all tasks
828   
    share the same return type.
838   
    share the same return type.
829  
839  
830   
    @see when_any, IoAwaitableRange
840   
    @see when_any, IoAwaitableRange
831   
*/
841   
*/
832   
template<IoAwaitableRange R>
842   
template<IoAwaitableRange R>
833   
    requires (!std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>)
843   
    requires (!std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>)
834   
[[nodiscard]] auto when_any(R&& awaitables)
844   
[[nodiscard]] auto when_any(R&& awaitables)
835   
    -> task<std::pair<std::size_t, awaitable_result_t<std::ranges::range_value_t<R>>>>
845   
    -> task<std::pair<std::size_t, awaitable_result_t<std::ranges::range_value_t<R>>>>
836   
{
846   
{
837   
    using Awaitable = std::ranges::range_value_t<R>;
847   
    using Awaitable = std::ranges::range_value_t<R>;
838   
    using T = awaitable_result_t<Awaitable>;
848   
    using T = awaitable_result_t<Awaitable>;
839   
    using result_type = std::pair<std::size_t, T>;
849   
    using result_type = std::pair<std::size_t, T>;
840   
    using OwnedRange = std::remove_cvref_t<R>;
850   
    using OwnedRange = std::remove_cvref_t<R>;
841  
851  
842   
    auto count = std::ranges::size(awaitables);
852   
    auto count = std::ranges::size(awaitables);
843   
    if(count == 0)
853   
    if(count == 0)
844   
        throw std::invalid_argument("when_any requires at least one awaitable");
854   
        throw std::invalid_argument("when_any requires at least one awaitable");
845  
855  
846   
    // Move/copy range onto coroutine frame to ensure lifetime
856   
    // Move/copy range onto coroutine frame to ensure lifetime
847   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
857   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
848  
858  
849   
    detail::when_any_homogeneous_state<T> state(count);
859   
    detail::when_any_homogeneous_state<T> state(count);
850  
860  
851   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
861   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
852  
862  
853   
    if(state.core_.winner_exception_)
863   
    if(state.core_.winner_exception_)
854   
        std::rethrow_exception(state.core_.winner_exception_);
864   
        std::rethrow_exception(state.core_.winner_exception_);
855  
865  
856   
    co_return result_type{state.core_.winner_index_, std::move(*state.result_)};
866   
    co_return result_type{state.core_.winner_index_, std::move(*state.result_)};
857   
}
867   
}
858  
868  
859   
/** Wait for the first awaitable to complete (void range overload).
869   
/** Wait for the first awaitable to complete (void range overload).
860  
870  
861   
    Races a range of void-returning awaitables. Since void awaitables have
871   
    Races a range of void-returning awaitables. Since void awaitables have
862   
    no result value, only the winner's index is returned.
872   
    no result value, only the winner's index is returned.
863  
873  
864   
    @par Suspends
874   
    @par Suspends
865   
    The calling coroutine suspends when co_await is invoked. All awaitables
875   
    The calling coroutine suspends when co_await is invoked. All awaitables
866   
    in the range are launched concurrently and execute in parallel. The
876   
    in the range are launched concurrently and execute in parallel. The
867   
    coroutine resumes only after all awaitables have completed, even though
877   
    coroutine resumes only after all awaitables have completed, even though
868   
    the winner is determined by the first to finish.
878   
    the winner is determined by the first to finish.
869  
879  
870   
    @par Completion Conditions
880   
    @par Completion Conditions
871   
    @li Winner is determined when the first awaitable completes (success or exception)
881   
    @li Winner is determined when the first awaitable completes (success or exception)
872   
    @li Only one task can claim winner status via atomic compare-exchange
882   
    @li Only one task can claim winner status via atomic compare-exchange
873   
    @li Once a winner exists, stop is requested for all remaining siblings
883   
    @li Once a winner exists, stop is requested for all remaining siblings
874   
    @li Parent coroutine resumes only after all siblings acknowledge completion
884   
    @li Parent coroutine resumes only after all siblings acknowledge completion
875   
    @li The winner's index is returned; if the winner threw, the exception is rethrown
885   
    @li The winner's index is returned; if the winner threw, the exception is rethrown
876  
886  
877   
    @par Cancellation Semantics
887   
    @par Cancellation Semantics
878   
    Cancellation is supported via stop_token propagated through the
888   
    Cancellation is supported via stop_token propagated through the
879   
    IoAwaitable protocol:
889   
    IoAwaitable protocol:
880   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
890   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
881   
    @li When the parent's stop token is activated, the stop is forwarded to all children
891   
    @li When the parent's stop token is activated, the stop is forwarded to all children
882   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
892   
    @li When a winner is determined, stop_source_.request_stop() is called immediately
883   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
893   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
884   
    @li Stop requests are cooperative; tasks must check and respond to them
894   
    @li Stop requests are cooperative; tasks must check and respond to them
885  
895  
886   
    @par Concurrency/Overlap
896   
    @par Concurrency/Overlap
887   
    All awaitables are launched concurrently before any can complete.
897   
    All awaitables are launched concurrently before any can complete.
888   
    The launcher iterates through the range, starting each task on the
898   
    The launcher iterates through the range, starting each task on the
889   
    caller's executor. Tasks may execute in parallel on multi-threaded
899   
    caller's executor. Tasks may execute in parallel on multi-threaded
890   
    executors or interleave on single-threaded executors. There is no
900   
    executors or interleave on single-threaded executors. There is no
891   
    guaranteed ordering of task completion.
901   
    guaranteed ordering of task completion.
892  
902  
893   
    @par Notable Error Conditions
903   
    @par Notable Error Conditions
894   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
904   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
895   
    @li Winner exception: if the winning task threw, that exception is rethrown
905   
    @li Winner exception: if the winning task threw, that exception is rethrown
896   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
906   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
897   
    @li Cancellation: tasks may complete via cancellation without throwing
907   
    @li Cancellation: tasks may complete via cancellation without throwing
898  
908  
899   
    @par Example
909   
    @par Example
900   
    @code
910   
    @code
901   
    task<void> example() {
911   
    task<void> example() {
902   
        std::vector<task<void>> tasks;
912   
        std::vector<task<void>> tasks;
903   
        for (int i = 0; i < 5; ++i)
913   
        for (int i = 0; i < 5; ++i)
904   
            tasks.push_back(background_work(i));
914   
            tasks.push_back(background_work(i));
905  
915  
906   
        std::size_t winner = co_await when_any(std::move(tasks));
916   
        std::size_t winner = co_await when_any(std::move(tasks));
907   
        // winner is the index of the first task to complete
917   
        // winner is the index of the first task to complete
908   
    }
918   
    }
909   
    @endcode
919   
    @endcode
910  
920  
911   
    @par Example with Timeout
921   
    @par Example with Timeout
912   
    @code
922   
    @code
913   
    task<void> with_timeout() {
923   
    task<void> with_timeout() {
914   
        std::vector<task<void>> tasks;
924   
        std::vector<task<void>> tasks;
915   
        tasks.push_back(long_running_operation());
925   
        tasks.push_back(long_running_operation());
916   
        tasks.push_back(delay(std::chrono::seconds(5)));
926   
        tasks.push_back(delay(std::chrono::seconds(5)));
917  
927  
918   
        std::size_t winner = co_await when_any(std::move(tasks));
928   
        std::size_t winner = co_await when_any(std::move(tasks));
919   
        if (winner == 1) {
929   
        if (winner == 1) {
920   
            // Timeout occurred
930   
            // Timeout occurred
921   
        }
931   
        }
922   
    }
932   
    }
923   
    @endcode
933   
    @endcode
924  
934  
925   
    @tparam R Range type satisfying IoAwaitableRange with void result.
935   
    @tparam R Range type satisfying IoAwaitableRange with void result.
926   
    @param awaitables Range of void awaitables to race concurrently (must not be empty).
936   
    @param awaitables Range of void awaitables to race concurrently (must not be empty).
927   
    @return A task yielding the winner's index (zero-based).
937   
    @return A task yielding the winner's index (zero-based).
928  
938  
929   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
939   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
930   
    @throws Rethrows the winner's exception if the winning task threw an exception.
940   
    @throws Rethrows the winner's exception if the winning task threw an exception.
931  
941  
932   
    @par Remarks
942   
    @par Remarks
933   
    Elements are moved from the range; for lvalue ranges, the original
943   
    Elements are moved from the range; for lvalue ranges, the original
934   
    container will have moved-from elements after this call. The range
944   
    container will have moved-from elements after this call. The range
935   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
945   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
936   
    the non-void overload, no result storage is needed since void tasks
946   
    the non-void overload, no result storage is needed since void tasks
937   
    produce no value.
947   
    produce no value.
938  
948  
939   
    @see when_any, IoAwaitableRange
949   
    @see when_any, IoAwaitableRange
940   
*/
950   
*/
941   
template<IoAwaitableRange R>
951   
template<IoAwaitableRange R>
942   
    requires std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>
952   
    requires std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>
943   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
953   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
944   
{
954   
{
945   
    using OwnedRange = std::remove_cvref_t<R>;
955   
    using OwnedRange = std::remove_cvref_t<R>;
946  
956  
947   
    auto count = std::ranges::size(awaitables);
957   
    auto count = std::ranges::size(awaitables);
948   
    if(count == 0)
958   
    if(count == 0)
949   
        throw std::invalid_argument("when_any requires at least one awaitable");
959   
        throw std::invalid_argument("when_any requires at least one awaitable");
950  
960  
951   
    // Move/copy range onto coroutine frame to ensure lifetime
961   
    // Move/copy range onto coroutine frame to ensure lifetime
952   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
962   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
953  
963  
954   
    detail::when_any_homogeneous_state<void> state(count);
964   
    detail::when_any_homogeneous_state<void> state(count);
955  
965  
956   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
966   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
957  
967  
958   
    if(state.core_.winner_exception_)
968   
    if(state.core_.winner_exception_)
959   
        std::rethrow_exception(state.core_.winner_exception_);
969   
        std::rethrow_exception(state.core_.winner_exception_);
960  
970  
961   
    co_return state.core_.winner_index_;
971   
    co_return state.core_.winner_index_;
962   
}
972   
}
963  
973  
964   
} // namespace capy
974   
} // namespace capy
965   
} // namespace boost
975   
} // namespace boost
966  
976  
967   
#endif
977   
#endif