1   
//
1   
//
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// Copyright (c) 2026 Michael Vandeberg
2   
// Copyright (c) 2026 Michael Vandeberg
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//
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/concept/executor.hpp>
14   
#include <boost/capy/concept/executor.hpp>
15   
#include <boost/capy/concept/io_awaitable.hpp>
15   
#include <boost/capy/concept/io_awaitable.hpp>
16   
#include <coroutine>
16   
#include <coroutine>
17   
#include <boost/capy/ex/executor_ref.hpp>
17   
#include <boost/capy/ex/executor_ref.hpp>
18   
#include <boost/capy/ex/frame_allocator.hpp>
18   
#include <boost/capy/ex/frame_allocator.hpp>
19   
#include <boost/capy/ex/io_env.hpp>
19   
#include <boost/capy/ex/io_env.hpp>
20   
#include <boost/capy/task.hpp>
20   
#include <boost/capy/task.hpp>
21  
21  
22   
#include <array>
22   
#include <array>
23   
#include <atomic>
23   
#include <atomic>
24   
#include <exception>
24   
#include <exception>
25   
#include <optional>
25   
#include <optional>
26   
#include <ranges>
26   
#include <ranges>
27   
#include <stdexcept>
27   
#include <stdexcept>
28   
#include <stop_token>
28   
#include <stop_token>
29   
#include <tuple>
29   
#include <tuple>
30   
#include <type_traits>
30   
#include <type_traits>
31   
#include <utility>
31   
#include <utility>
32   
#include <variant>
32   
#include <variant>
33   
#include <vector>
33   
#include <vector>
34  
34  
35   
/*
35   
/*
36   
   when_any - Race multiple tasks, return first completion
36   
   when_any - Race multiple tasks, return first completion
37   
   ========================================================
37   
   ========================================================
38  
38  
39   
   OVERVIEW:
39   
   OVERVIEW:
40   
   ---------
40   
   ---------
41   
   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
42   
   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
43   
   waits for them to acknowledge before returning.
43   
   waits for them to acknowledge before returning.
44  
44  
45   
   ARCHITECTURE:
45   
   ARCHITECTURE:
46   
   -------------
46   
   -------------
47   
   The design mirrors when_all but with inverted completion semantics:
47   
   The design mirrors when_all but with inverted completion semantics:
48  
48  
49   
     when_all:  complete when remaining_count reaches 0 (all done)
49   
     when_all:  complete when remaining_count reaches 0 (all done)
50   
     when_any:  complete when has_winner becomes true (first done)
50   
     when_any:  complete when has_winner becomes true (first done)
51   
                BUT still wait for remaining_count to reach 0 for cleanup
51   
                BUT still wait for remaining_count to reach 0 for cleanup
52  
52  
53   
   Key components:
53   
   Key components:
54   
     - when_any_state:    Shared state tracking winner and completion
54   
     - when_any_state:    Shared state tracking winner and completion
55   
     - when_any_runner:   Wrapper coroutine for each child task
55   
     - when_any_runner:   Wrapper coroutine for each child task
56   
     - when_any_launcher: Awaitable that starts all runners concurrently
56   
     - when_any_launcher: Awaitable that starts all runners concurrently
57  
57  
58   
   CRITICAL INVARIANTS:
58   
   CRITICAL INVARIANTS:
59   
   --------------------
59   
   --------------------
60   
   1. Exactly one task becomes the winner (via atomic compare_exchange)
60   
   1. Exactly one task becomes the winner (via atomic compare_exchange)
61   
   2. All tasks must complete before parent resumes (cleanup safety)
61   
   2. All tasks must complete before parent resumes (cleanup safety)
62   
   3. Stop is requested immediately when winner is determined
62   
   3. Stop is requested immediately when winner is determined
63   
   4. Only the winner's result/exception is stored
63   
   4. Only the winner's result/exception is stored
64  
64  
65   
   POSITIONAL VARIANT:
65   
   POSITIONAL VARIANT:
66   
   -------------------
66   
   -------------------
67   
   The variadic overload returns a std::variant with one alternative per
67   
   The variadic overload returns a std::variant with one alternative per
68   
   input task, preserving positional correspondence. Use .index() on
68   
   input task, preserving positional correspondence. Use .index() on
69   
   the variant to identify which task won.
69   
   the variant to identify which task won.
70  
70  
71   
   Example: when_any(task<int>, task<string>, task<int>)
71   
   Example: when_any(task<int>, task<string>, task<int>)
72   
     - Raw types after void->monostate: int, string, int
72   
     - Raw types after void->monostate: int, string, int
73   
     - Result variant: std::variant<int, string, int>
73   
     - Result variant: std::variant<int, string, int>
74   
     - variant.index() tells you which task won (0, 1, or 2)
74   
     - variant.index() tells you which task won (0, 1, or 2)
75  
75  
76   
   VOID HANDLING:
76   
   VOID HANDLING:
77   
   --------------
77   
   --------------
78   
   void tasks contribute std::monostate to the variant.
78   
   void tasks contribute std::monostate to the variant.
79   
   All-void tasks result in: variant<monostate, monostate, monostate>
79   
   All-void tasks result in: variant<monostate, monostate, monostate>
80  
80  
81   
   MEMORY MODEL:
81   
   MEMORY MODEL:
82   
   -------------
82   
   -------------
83   
   Synchronization chain from winner's write to parent's read:
83   
   Synchronization chain from winner's write to parent's read:
84  
84  
85   
   1. Winner thread writes result_/winner_exception_ (non-atomic)
85   
   1. Winner thread writes result_/winner_exception_ (non-atomic)
86   
   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_
87   
   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()
88   
      → fetch_sub(acq_rel) on remaining_count_, observing count becomes 0
88   
      → fetch_sub(acq_rel) on remaining_count_, observing count becomes 0
89   
   4. Last task returns caller_ex_.dispatch(continuation_) via symmetric transfer
89   
   4. Last task returns caller_ex_.dispatch(continuation_) via symmetric transfer
90   
   5. Parent coroutine resumes and reads result_/winner_exception_
90   
   5. Parent coroutine resumes and reads result_/winner_exception_
91  
91  
92   
   Synchronization analysis:
92   
   Synchronization analysis:
93   
   - All fetch_sub operations on remaining_count_ form a release sequence
93   
   - All fetch_sub operations on remaining_count_ form a release sequence
94   
   - Winner's fetch_sub releases; subsequent fetch_sub operations participate
94   
   - Winner's fetch_sub releases; subsequent fetch_sub operations participate
95   
     in the modification order of remaining_count_
95   
     in the modification order of remaining_count_
96   
   - 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
97   
     modification order, establishing happens-before from winner's writes
97   
     modification order, establishing happens-before from winner's writes
98   
   - Executor dispatch() is expected to provide queue-based synchronization
98   
   - Executor dispatch() is expected to provide queue-based synchronization
99   
     (release-on-post, acquire-on-execute) completing the chain to parent
99   
     (release-on-post, acquire-on-execute) completing the chain to parent
100   
   - Even inline executors work (same thread = sequenced-before)
100   
   - Even inline executors work (same thread = sequenced-before)
101  
101  
102   
   Alternative considered: Adding winner_ready_ atomic (set with release after
102   
   Alternative considered: Adding winner_ready_ atomic (set with release after
103   
   storing winner data, acquired before reading) would make synchronization
103   
   storing winner data, acquired before reading) would make synchronization
104   
   self-contained and not rely on executor implementation details. Current
104   
   self-contained and not rely on executor implementation details. Current
105   
   approach is correct but requires careful reasoning about release sequences
105   
   approach is correct but requires careful reasoning about release sequences
106   
   and executor behavior.
106   
   and executor behavior.
107  
107  
108   
   EXCEPTION SEMANTICS:
108   
   EXCEPTION SEMANTICS:
109   
   --------------------
109   
   --------------------
110   
   Unlike when_all (which captures first exception, discards others), when_any
110   
   Unlike when_all (which captures first exception, discards others), when_any
111   
   treats exceptions as valid completions. If the winning task threw, that
111   
   treats exceptions as valid completions. If the winning task threw, that
112   
   exception is rethrown. Exceptions from non-winners are silently discarded.
112   
   exception is rethrown. Exceptions from non-winners are silently discarded.
113   
*/
113   
*/
114  
114  
115   
namespace boost {
115   
namespace boost {
116   
namespace capy {
116   
namespace capy {
117  
117  
118   
/** Convert void to monostate for variant storage.
118   
/** Convert void to monostate for variant storage.
119  
119  
120   
    std::variant<void, ...> is ill-formed, so void tasks contribute
120   
    std::variant<void, ...> is ill-formed, so void tasks contribute
121   
    std::monostate to the result variant instead. Non-void types
121   
    std::monostate to the result variant instead. Non-void types
122   
    pass through unchanged.
122   
    pass through unchanged.
123  
123  
124   
    @tparam T The type to potentially convert (void becomes monostate).
124   
    @tparam T The type to potentially convert (void becomes monostate).
125   
*/
125   
*/
126   
template<typename T>
126   
template<typename T>
127   
using void_to_monostate_t = std::conditional_t<std::is_void_v<T>, std::monostate, T>;
127   
using void_to_monostate_t = std::conditional_t<std::is_void_v<T>, std::monostate, T>;
128  
128  
129   
namespace detail {
129   
namespace detail {
130  
130  
131   
/** Core shared state for when_any operations.
131   
/** Core shared state for when_any operations.
132  
132  
133   
    Contains all members and methods common to both heterogeneous (variadic)
133   
    Contains all members and methods common to both heterogeneous (variadic)
134   
    and homogeneous (range) when_any implementations. State classes embed
134   
    and homogeneous (range) when_any implementations. State classes embed
135   
    this via composition to avoid CRTP destructor ordering issues.
135   
    this via composition to avoid CRTP destructor ordering issues.
136  
136  
137   
    @par Thread Safety
137   
    @par Thread Safety
138   
    Atomic operations protect winner selection and completion count.
138   
    Atomic operations protect winner selection and completion count.
139   
*/
139   
*/
140   
struct when_any_core
140   
struct when_any_core
141   
{
141   
{
142   
    std::atomic<std::size_t> remaining_count_;
142   
    std::atomic<std::size_t> remaining_count_;
143   
    std::size_t winner_index_{0};
143   
    std::size_t winner_index_{0};
144   
    std::exception_ptr winner_exception_;
144   
    std::exception_ptr winner_exception_;
145   
    std::stop_source stop_source_;
145   
    std::stop_source stop_source_;
146  
146  
147   
    // Bridges parent's stop token to our stop_source
147   
    // Bridges parent's stop token to our stop_source
148   
    struct stop_callback_fn
148   
    struct stop_callback_fn
149   
    {
149   
    {
150   
        std::stop_source* source_;
150   
        std::stop_source* source_;
151   
        void operator()() const noexcept { source_->request_stop(); }
151   
        void operator()() const noexcept { source_->request_stop(); }
152   
    };
152   
    };
153   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
153   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
154   
    std::optional<stop_callback_t> parent_stop_callback_;
154   
    std::optional<stop_callback_t> parent_stop_callback_;
155  
155  
156   
    std::coroutine_handle<> continuation_;
156   
    std::coroutine_handle<> continuation_;
157   
    io_env const* caller_env_ = nullptr;
157   
    io_env const* caller_env_ = nullptr;
158  
158  
159   
    // 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)
160   
    std::atomic<bool> has_winner_{false};
160   
    std::atomic<bool> has_winner_{false};
161  
161  
162   
    explicit when_any_core(std::size_t count) noexcept
162   
    explicit when_any_core(std::size_t count) noexcept
163   
        : remaining_count_(count)
163   
        : remaining_count_(count)
164   
    {
164   
    {
165   
    }
165   
    }
166  
166  
167   
    /** Atomically claim winner status; exactly one task succeeds. */
167   
    /** Atomically claim winner status; exactly one task succeeds. */
168   
    bool try_win(std::size_t index) noexcept
168   
    bool try_win(std::size_t index) noexcept
169   
    {
169   
    {
170   
        bool expected = false;
170   
        bool expected = false;
171   
        if(has_winner_.compare_exchange_strong(
171   
        if(has_winner_.compare_exchange_strong(
172   
            expected, true, std::memory_order_acq_rel))
172   
            expected, true, std::memory_order_acq_rel))
173   
        {
173   
        {
174   
            winner_index_ = index;
174   
            winner_index_ = index;
175   
            stop_source_.request_stop();
175   
            stop_source_.request_stop();
176   
            return true;
176   
            return true;
177   
        }
177   
        }
178   
        return false;
178   
        return false;
179   
    }
179   
    }
180  
180  
181   
    /** @pre try_win() returned true. */
181   
    /** @pre try_win() returned true. */
182   
    void set_winner_exception(std::exception_ptr ep) noexcept
182   
    void set_winner_exception(std::exception_ptr ep) noexcept
183   
    {
183   
    {
184   
        winner_exception_ = ep;
184   
        winner_exception_ = ep;
185   
    }
185   
    }
186  
186  
187   
    // Runners signal completion directly via final_suspend; no member function needed.
187   
    // Runners signal completion directly via final_suspend; no member function needed.
188   
};
188   
};
189  
189  
190   
/** Shared state for heterogeneous when_any operation.
190   
/** Shared state for heterogeneous when_any operation.
191  
191  
192   
    Coordinates winner selection, result storage, and completion tracking
192   
    Coordinates winner selection, result storage, and completion tracking
193   
    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
194   
    when_any_core for shared functionality.
194   
    when_any_core for shared functionality.
195  
195  
196   
    @par Lifetime
196   
    @par Lifetime
197   
    Allocated on the parent coroutine's frame, outlives all runners.
197   
    Allocated on the parent coroutine's frame, outlives all runners.
198  
198  
199   
    @tparam Ts Task result types.
199   
    @tparam Ts Task result types.
200   
*/
200   
*/
201   
template<typename... Ts>
201   
template<typename... Ts>
202   
struct when_any_state
202   
struct when_any_state
203   
{
203   
{
204   
    static constexpr std::size_t task_count = sizeof...(Ts);
204   
    static constexpr std::size_t task_count = sizeof...(Ts);
205   
    using variant_type = std::variant<void_to_monostate_t<Ts>...>;
205   
    using variant_type = std::variant<void_to_monostate_t<Ts>...>;
206  
206  
207   
    when_any_core core_;
207   
    when_any_core core_;
208   
    std::optional<variant_type> result_;
208   
    std::optional<variant_type> result_;
209   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
209   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
210  
210  
211   
    when_any_state()
211   
    when_any_state()
212   
        : core_(task_count)
212   
        : core_(task_count)
213   
    {
213   
    {
214   
    }
214   
    }
215  
215  
216   
    // Runners self-destruct in final_suspend. No destruction needed here.
216   
    // Runners self-destruct in final_suspend. No destruction needed here.
217  
217  
218   
    /** @pre core_.try_win() returned true.
218   
    /** @pre core_.try_win() returned true.
219   
        @note Uses in_place_index (not type) for positional variant access.
219   
        @note Uses in_place_index (not type) for positional variant access.
220   
    */
220   
    */
221   
    template<std::size_t I, typename T>
221   
    template<std::size_t I, typename T>
222   
    void set_winner_result(T value)
222   
    void set_winner_result(T value)
223   
        noexcept(std::is_nothrow_move_constructible_v<T>)
223   
        noexcept(std::is_nothrow_move_constructible_v<T>)
224   
    {
224   
    {
225   
        result_.emplace(std::in_place_index<I>, std::move(value));
225   
        result_.emplace(std::in_place_index<I>, std::move(value));
226   
    }
226   
    }
227  
227  
228   
    /** @pre core_.try_win() returned true. */
228   
    /** @pre core_.try_win() returned true. */
229   
    template<std::size_t I>
229   
    template<std::size_t I>
230   
    void set_winner_void() noexcept
230   
    void set_winner_void() noexcept
231   
    {
231   
    {
232   
        result_.emplace(std::in_place_index<I>, std::monostate{});
232   
        result_.emplace(std::in_place_index<I>, std::monostate{});
233   
    }
233   
    }
234   
};
234   
};
235  
235  
236   
/** Wrapper coroutine that runs a single child task for when_any.
236   
/** Wrapper coroutine that runs a single child task for when_any.
237  
237  
238   
    Propagates executor/stop_token to the child, attempts to claim winner
238   
    Propagates executor/stop_token to the child, attempts to claim winner
239   
    status on completion, and signals completion for cleanup coordination.
239   
    status on completion, and signals completion for cleanup coordination.
240  
240  
241   
    @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).
242   
*/
242   
*/
243   
template<typename StateType>
243   
template<typename StateType>
244   
struct when_any_runner
244   
struct when_any_runner
245   
{
245   
{
246   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
246   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
247   
    {
247   
    {
248   
        StateType* state_ = nullptr;
248   
        StateType* state_ = nullptr;
249   
        std::size_t index_ = 0;
249   
        std::size_t index_ = 0;
250   
        io_env env_;
250   
        io_env env_;
251  
251  
252   
        when_any_runner get_return_object() noexcept
252   
        when_any_runner get_return_object() noexcept
253   
        {
253   
        {
254   
            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));
255   
        }
255   
        }
256  
256  
257   
        // Starts suspended; launcher sets up state/ex/token then resumes
257   
        // Starts suspended; launcher sets up state/ex/token then resumes
258   
        std::suspend_always initial_suspend() noexcept
258   
        std::suspend_always initial_suspend() noexcept
259   
        {
259   
        {
260   
            return {};
260   
            return {};
261   
        }
261   
        }
262  
262  
263   
        auto final_suspend() noexcept
263   
        auto final_suspend() noexcept
264   
        {
264   
        {
265   
            struct awaiter
265   
            struct awaiter
266   
            {
266   
            {
267   
                promise_type* p_;
267   
                promise_type* p_;
268   
                bool await_ready() const noexcept { return false; }
268   
                bool await_ready() const noexcept { return false; }
269   
                auto await_suspend(std::coroutine_handle<> h) noexcept
269   
                auto await_suspend(std::coroutine_handle<> h) noexcept
270   
                {
270   
                {
271   
                    // Extract everything needed before self-destruction.
271   
                    // Extract everything needed before self-destruction.
272   
                    auto& core = p_->state_->core_;
272   
                    auto& core = p_->state_->core_;
273   
                    auto* counter = &core.remaining_count_;
273   
                    auto* counter = &core.remaining_count_;
274   
                    auto* caller_env = core.caller_env_;
274   
                    auto* caller_env = core.caller_env_;
275   
                    auto cont = core.continuation_;
275   
                    auto cont = core.continuation_;
276  
276  
277   
                    h.destroy();
277   
                    h.destroy();
278  
278  
279   
                    // If last runner, dispatch parent for symmetric transfer.
279   
                    // If last runner, dispatch parent for symmetric transfer.
280   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
280   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
281   
                    if(remaining == 1)
281   
                    if(remaining == 1)
282   
                        return detail::symmetric_transfer(caller_env->executor.dispatch(cont));
282   
                        return detail::symmetric_transfer(caller_env->executor.dispatch(cont));
283   
                    return detail::symmetric_transfer(std::noop_coroutine());
283   
                    return detail::symmetric_transfer(std::noop_coroutine());
284   
                }
284   
                }
285   
                void await_resume() const noexcept {}
285   
                void await_resume() const noexcept {}
286   
            };
286   
            };
287   
            return awaiter{this};
287   
            return awaiter{this};
288   
        }
288   
        }
289  
289  
290   
        void return_void() noexcept {}
290   
        void return_void() noexcept {}
291  
291  
292   
        // Exceptions are valid completions in when_any (unlike when_all)
292   
        // Exceptions are valid completions in when_any (unlike when_all)
293   
        void unhandled_exception()
293   
        void unhandled_exception()
294   
        {
294   
        {
295   
            if(state_->core_.try_win(index_))
295   
            if(state_->core_.try_win(index_))
296   
                state_->core_.set_winner_exception(std::current_exception());
296   
                state_->core_.set_winner_exception(std::current_exception());
297   
        }
297   
        }
298  
298  
299   
        /** Injects executor and stop token into child awaitables. */
299   
        /** Injects executor and stop token into child awaitables. */
300   
        template<class Awaitable>
300   
        template<class Awaitable>
301   
        struct transform_awaiter
301   
        struct transform_awaiter
302   
        {
302   
        {
303   
            std::decay_t<Awaitable> a_;
303   
            std::decay_t<Awaitable> a_;
304   
            promise_type* p_;
304   
            promise_type* p_;
305  
305  
306   
            bool await_ready() { return a_.await_ready(); }
306   
            bool await_ready() { return a_.await_ready(); }
307   
            auto await_resume() { return a_.await_resume(); }
307   
            auto await_resume() { return a_.await_resume(); }
308  
308  
309   
            template<class Promise>
309   
            template<class Promise>
310   
            auto await_suspend(std::coroutine_handle<Promise> h)
310   
            auto await_suspend(std::coroutine_handle<Promise> h)
311   
            {
311   
            {
312   
                using R = decltype(a_.await_suspend(h, &p_->env_));
312   
                using R = decltype(a_.await_suspend(h, &p_->env_));
313   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
313   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
314   
                    return detail::symmetric_transfer(a_.await_suspend(h, &p_->env_));
314   
                    return detail::symmetric_transfer(a_.await_suspend(h, &p_->env_));
315   
                else
315   
                else
316   
                    return a_.await_suspend(h, &p_->env_);
316   
                    return a_.await_suspend(h, &p_->env_);
317   
            }
317   
            }
318   
        };
318   
        };
319  
319  
320   
        template<class Awaitable>
320   
        template<class Awaitable>
321   
        auto await_transform(Awaitable&& a)
321   
        auto await_transform(Awaitable&& a)
322   
        {
322   
        {
323   
            using A = std::decay_t<Awaitable>;
323   
            using A = std::decay_t<Awaitable>;
324   
            if constexpr (IoAwaitable<A>)
324   
            if constexpr (IoAwaitable<A>)
325   
            {
325   
            {
326   
                return transform_awaiter<Awaitable>{
326   
                return transform_awaiter<Awaitable>{
327   
                    std::forward<Awaitable>(a), this};
327   
                    std::forward<Awaitable>(a), this};
328   
            }
328   
            }
329   
            else
329   
            else
330   
            {
330   
            {
331   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
331   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
332   
            }
332   
            }
333   
        }
333   
        }
334   
    };
334   
    };
335  
335  
336   
    std::coroutine_handle<promise_type> h_;
336   
    std::coroutine_handle<promise_type> h_;
337  
337  
338   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
338   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
339   
        : h_(h)
339   
        : h_(h)
340   
    {
340   
    {
341   
    }
341   
    }
342  
342  
343   
    // Enable move for all clang versions - some versions need it
343   
    // Enable move for all clang versions - some versions need it
344   
    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)) {}
345  
345  
346   
    // Non-copyable
346   
    // Non-copyable
347   
    when_any_runner(when_any_runner const&) = delete;
347   
    when_any_runner(when_any_runner const&) = delete;
348   
    when_any_runner& operator=(when_any_runner const&) = delete;
348   
    when_any_runner& operator=(when_any_runner const&) = delete;
349   
    when_any_runner& operator=(when_any_runner&&) = delete;
349   
    when_any_runner& operator=(when_any_runner&&) = delete;
350  
350  
351   
    auto release() noexcept
351   
    auto release() noexcept
352   
    {
352   
    {
353   
        return std::exchange(h_, nullptr);
353   
        return std::exchange(h_, nullptr);
354   
    }
354   
    }
355   
};
355   
};
356  
356  
357   
/** Indexed overload for heterogeneous when_any (compile-time index).
357   
/** Indexed overload for heterogeneous when_any (compile-time index).
358  
358  
359   
    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.
360   
    Called from when_any_launcher::launch_one<I>().
360   
    Called from when_any_launcher::launch_one<I>().
361   
*/
361   
*/
362   
template<std::size_t I, IoAwaitable Awaitable, typename StateType>
362   
template<std::size_t I, IoAwaitable Awaitable, typename StateType>
363   
when_any_runner<StateType>
363   
when_any_runner<StateType>
364   
make_when_any_runner(Awaitable inner, StateType* state)
364   
make_when_any_runner(Awaitable inner, StateType* state)
365   
{
365   
{
366   
    using T = awaitable_result_t<Awaitable>;
366   
    using T = awaitable_result_t<Awaitable>;
367   
    if constexpr (std::is_void_v<T>)
367   
    if constexpr (std::is_void_v<T>)
368   
    {
368   
    {
369   
        co_await std::move(inner);
369   
        co_await std::move(inner);
370   
        if(state->core_.try_win(I))
370   
        if(state->core_.try_win(I))
371   
            state->template set_winner_void<I>();
371   
            state->template set_winner_void<I>();
372   
    }
372   
    }
373   
    else
373   
    else
374   
    {
374   
    {
375   
        auto result = co_await std::move(inner);
375   
        auto result = co_await std::move(inner);
376   
        if(state->core_.try_win(I))
376   
        if(state->core_.try_win(I))
377   
        {
377   
        {
378   
            try
378   
            try
379   
            {
379   
            {
380   
                state->template set_winner_result<I>(std::move(result));
380   
                state->template set_winner_result<I>(std::move(result));
381   
            }
381   
            }
382   
            catch(...)
382   
            catch(...)
383   
            {
383   
            {
384   
                state->core_.set_winner_exception(std::current_exception());
384   
                state->core_.set_winner_exception(std::current_exception());
385   
            }
385   
            }
386   
        }
386   
        }
387   
    }
387   
    }
388   
}
388   
}
389  
389  
390   
/** Runtime-index overload for homogeneous when_any (range path).
390   
/** Runtime-index overload for homogeneous when_any (range path).
391  
391  
392   
    Uses requires-expressions to detect state capabilities:
392   
    Uses requires-expressions to detect state capabilities:
393   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
393   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
394   
    - set_winner_result(): for non-void tasks
394   
    - set_winner_result(): for non-void tasks
395   
    - Neither: for homogeneous void tasks (no result storage)
395   
    - Neither: for homogeneous void tasks (no result storage)
396   
*/
396   
*/
397   
template<IoAwaitable Awaitable, typename StateType>
397   
template<IoAwaitable Awaitable, typename StateType>
398   
when_any_runner<StateType>
398   
when_any_runner<StateType>
399   
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)
400   
{
400   
{
401   
    using T = awaitable_result_t<Awaitable>;
401   
    using T = awaitable_result_t<Awaitable>;
402   
    if constexpr (std::is_void_v<T>)
402   
    if constexpr (std::is_void_v<T>)
403   
    {
403   
    {
404   
        co_await std::move(inner);
404   
        co_await std::move(inner);
405   
        if(state->core_.try_win(index))
405   
        if(state->core_.try_win(index))
406   
        {
406   
        {
407   
            if constexpr (requires { state->set_winner_void(); })
407   
            if constexpr (requires { state->set_winner_void(); })
408   
                state->set_winner_void();
408   
                state->set_winner_void();
409   
        }
409   
        }
410   
    }
410   
    }
411   
    else
411   
    else
412   
    {
412   
    {
413   
        auto result = co_await std::move(inner);
413   
        auto result = co_await std::move(inner);
414   
        if(state->core_.try_win(index))
414   
        if(state->core_.try_win(index))
415   
        {
415   
        {
416   
            try
416   
            try
417   
            {
417   
            {
418   
                state->set_winner_result(std::move(result));
418   
                state->set_winner_result(std::move(result));
419   
            }
419   
            }
420   
            catch(...)
420   
            catch(...)
421   
            {
421   
            {
422   
                state->core_.set_winner_exception(std::current_exception());
422   
                state->core_.set_winner_exception(std::current_exception());
423   
            }
423   
            }
424   
        }
424   
        }
425   
    }
425   
    }
426   
}
426   
}
427  
427  
428   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
428   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
429   
template<IoAwaitable... Awaitables>
429   
template<IoAwaitable... Awaitables>
430   
class when_any_launcher
430   
class when_any_launcher
431   
{
431   
{
432   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
432   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
433  
433  
434   
    std::tuple<Awaitables...>* tasks_;
434   
    std::tuple<Awaitables...>* tasks_;
435   
    state_type* state_;
435   
    state_type* state_;
436  
436  
437   
public:
437   
public:
438   
    when_any_launcher(
438   
    when_any_launcher(
439   
        std::tuple<Awaitables...>* tasks,
439   
        std::tuple<Awaitables...>* tasks,
440   
        state_type* state)
440   
        state_type* state)
441   
        : tasks_(tasks)
441   
        : tasks_(tasks)
442   
        , state_(state)
442   
        , state_(state)
443   
    {
443   
    {
444   
    }
444   
    }
445  
445  
446   
    bool await_ready() const noexcept
446   
    bool await_ready() const noexcept
447   
    {
447   
    {
448   
        return sizeof...(Awaitables) == 0;
448   
        return sizeof...(Awaitables) == 0;
449   
    }
449   
    }
450  
450  
451   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
451   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
452   
        destroys this object before await_suspend returns. Must not reference
452   
        destroys this object before await_suspend returns. Must not reference
453   
        `this` after the final launch_one call.
453   
        `this` after the final launch_one call.
454   
    */
454   
    */
455   
    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)
456   
    {
456   
    {
457   
        state_->core_.continuation_ = continuation;
457   
        state_->core_.continuation_ = continuation;
458   
        state_->core_.caller_env_ = caller_env;
458   
        state_->core_.caller_env_ = caller_env;
459  
459  
460   
        if(caller_env->stop_token.stop_possible())
460   
        if(caller_env->stop_token.stop_possible())
461   
        {
461   
        {
462   
            state_->core_.parent_stop_callback_.emplace(
462   
            state_->core_.parent_stop_callback_.emplace(
463   
                caller_env->stop_token,
463   
                caller_env->stop_token,
464   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
464   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
465  
465  
466   
            if(caller_env->stop_token.stop_requested())
466   
            if(caller_env->stop_token.stop_requested())
467   
                state_->core_.stop_source_.request_stop();
467   
                state_->core_.stop_source_.request_stop();
468   
        }
468   
        }
469  
469  
470   
        auto token = state_->core_.stop_source_.get_token();
470   
        auto token = state_->core_.stop_source_.get_token();
471   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
471   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
472   
            (..., launch_one<Is>(caller_env->executor, token));
472   
            (..., launch_one<Is>(caller_env->executor, token));
473   
        }(std::index_sequence_for<Awaitables...>{});
473   
        }(std::index_sequence_for<Awaitables...>{});
474  
474  
475   
        return std::noop_coroutine();
475   
        return std::noop_coroutine();
476   
    }
476   
    }
477  
477  
478   
    void await_resume() const noexcept
478   
    void await_resume() const noexcept
479   
    {
479   
    {
480   
    }
480   
    }
481  
481  
482   
private:
482   
private:
483   
    /** @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). */
484   
    template<std::size_t I>
484   
    template<std::size_t I>
485   
    void launch_one(executor_ref caller_ex, std::stop_token token)
485   
    void launch_one(executor_ref caller_ex, std::stop_token token)
486   
    {
486   
    {
487   
        auto runner = make_when_any_runner<I>(
487   
        auto runner = make_when_any_runner<I>(
488   
            std::move(std::get<I>(*tasks_)), state_);
488   
            std::move(std::get<I>(*tasks_)), state_);
489  
489  
490   
        auto h = runner.release();
490   
        auto h = runner.release();
491   
        h.promise().state_ = state_;
491   
        h.promise().state_ = state_;
492   
        h.promise().index_ = I;
492   
        h.promise().index_ = I;
493   
        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};
494  
494  
495   
        std::coroutine_handle<> ch{h};
495   
        std::coroutine_handle<> ch{h};
496   
        state_->runner_handles_[I] = ch;
496   
        state_->runner_handles_[I] = ch;
497   
        caller_ex.post(ch);
497   
        caller_ex.post(ch);
498   
    }
498   
    }
499   
};
499   
};
500  
500  
501   
} // namespace detail
501   
} // namespace detail
502  
502  
503   
/** Wait for the first awaitable to complete.
503   
/** Wait for the first awaitable to complete.
504  
504  
505   
    Races multiple heterogeneous awaitables concurrently and returns when the
505   
    Races multiple heterogeneous awaitables concurrently and returns when the
506   
    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
507   
    input task, preserving positional correspondence.
507   
    input task, preserving positional correspondence.
508  
508  
509   
    @par Suspends
509   
    @par Suspends
510   
    The calling coroutine suspends when co_await is invoked. All awaitables
510   
    The calling coroutine suspends when co_await is invoked. All awaitables
511   
    are launched concurrently and execute in parallel. The coroutine resumes
511   
    are launched concurrently and execute in parallel. The coroutine resumes
512   
    only after all awaitables have completed, even though the winner is
512   
    only after all awaitables have completed, even though the winner is
513   
    determined by the first to finish.
513   
    determined by the first to finish.
514  
514  
515   
    @par Completion Conditions
515   
    @par Completion Conditions
516   
    @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)
517   
    @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
518   
    @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
519   
    @li Parent coroutine resumes only after all siblings acknowledge completion
519   
    @li Parent coroutine resumes only after all siblings acknowledge completion
520   
    @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
521  
521  
522   
    @par Cancellation Semantics
522   
    @par Cancellation Semantics
523   
    Cancellation is supported via stop_token propagated through the
523   
    Cancellation is supported via stop_token propagated through the
524   
    IoAwaitable protocol:
524   
    IoAwaitable protocol:
525   
    @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
526   
    @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
527   
    @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
528   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
528   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
529   
    @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
530  
530  
531   
    @par Concurrency/Overlap
531   
    @par Concurrency/Overlap
532   
    All awaitables are launched concurrently before any can complete.
532   
    All awaitables are launched concurrently before any can complete.
533   
    The launcher iterates through the arguments, starting each task on the
533   
    The launcher iterates through the arguments, starting each task on the
534   
    caller's executor. Tasks may execute in parallel on multi-threaded
534   
    caller's executor. Tasks may execute in parallel on multi-threaded
535   
    executors or interleave on single-threaded executors. There is no
535   
    executors or interleave on single-threaded executors. There is no
536   
    guaranteed ordering of task completion.
536   
    guaranteed ordering of task completion.
537  
537  
538   
    @par Notable Error Conditions
538   
    @par Notable Error Conditions
539   
    @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
540   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
540   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
541   
    @li Cancellation: tasks may complete via cancellation without throwing
541   
    @li Cancellation: tasks may complete via cancellation without throwing
542  
542  
543   
    @par Example
543   
    @par Example
544   
    @code
544   
    @code
545   
    task<void> example() {
545   
    task<void> example() {
546   
        auto result = co_await when_any(
546   
        auto result = co_await when_any(
547   
            fetch_int(),      // task<int>
547   
            fetch_int(),      // task<int>
548   
            fetch_string()    // task<std::string>
548   
            fetch_string()    // task<std::string>
549   
        );
549   
        );
550   
        // result.index() is 0 or 1
550   
        // result.index() is 0 or 1
551   
        if (result.index() == 0)
551   
        if (result.index() == 0)
552   
            std::cout << "Got int: " << std::get<0>(result) << "\n";
552   
            std::cout << "Got int: " << std::get<0>(result) << "\n";
553   
        else
553   
        else
554   
            std::cout << "Got string: " << std::get<1>(result) << "\n";
554   
            std::cout << "Got string: " << std::get<1>(result) << "\n";
555   
    }
555   
    }
556   
    @endcode
556   
    @endcode
557  
557  
558   
    @param as Awaitables to race concurrently (at least one required; each
558   
    @param as Awaitables to race concurrently (at least one required; each
559   
        must satisfy IoAwaitable).
559   
        must satisfy IoAwaitable).
560   
    @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.
561   
        Use .index() to identify the winner. Void awaitables contribute
561   
        Use .index() to identify the winner. Void awaitables contribute
562   
        std::monostate.
562   
        std::monostate.
563  
563  
564   
    @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.
565  
565  
566   
    @par Remarks
566   
    @par Remarks
567   
    Awaitables are moved into the coroutine frame; original objects become
567   
    Awaitables are moved into the coroutine frame; original objects become
568   
    empty after the call. The variant preserves one alternative per input
568   
    empty after the call. The variant preserves one alternative per input
569   
    task. Use .index() to determine which awaitable completed first.
569   
    task. Use .index() to determine which awaitable completed first.
570   
    Void awaitables contribute std::monostate to the variant.
570   
    Void awaitables contribute std::monostate to the variant.
571  
571  
572   
    @see when_all, IoAwaitable
572   
    @see when_all, IoAwaitable
573   
*/
573   
*/
574   
template<IoAwaitable... As>
574   
template<IoAwaitable... As>
575   
    requires (sizeof...(As) > 0)
575   
    requires (sizeof...(As) > 0)
576   
[[nodiscard]] auto when_any(As... as)
576   
[[nodiscard]] auto when_any(As... as)
577   
    -> task<std::variant<void_to_monostate_t<awaitable_result_t<As>>...>>
577   
    -> task<std::variant<void_to_monostate_t<awaitable_result_t<As>>...>>
578   
{
578   
{
579   
    detail::when_any_state<awaitable_result_t<As>...> state;
579   
    detail::when_any_state<awaitable_result_t<As>...> state;
580   
    std::tuple<As...> awaitable_tuple(std::move(as)...);
580   
    std::tuple<As...> awaitable_tuple(std::move(as)...);
581  
581  
582   
    co_await detail::when_any_launcher<As...>(&awaitable_tuple, &state);
582   
    co_await detail::when_any_launcher<As...>(&awaitable_tuple, &state);
583  
583  
584   
    if(state.core_.winner_exception_)
584   
    if(state.core_.winner_exception_)
585   
        std::rethrow_exception(state.core_.winner_exception_);
585   
        std::rethrow_exception(state.core_.winner_exception_);
586  
586  
587   
    co_return std::move(*state.result_);
587   
    co_return std::move(*state.result_);
588   
}
588   
}
589  
589  
590   
/** Concept for ranges of full I/O awaitables.
590   
/** Concept for ranges of full I/O awaitables.
591  
591  
592   
    A range satisfies `IoAwaitableRange` if it is a sized input range
592   
    A range satisfies `IoAwaitableRange` if it is a sized input range
593   
    whose value type satisfies @ref IoAwaitable. This enables when_any
593   
    whose value type satisfies @ref IoAwaitable. This enables when_any
594   
    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.
595  
595  
596   
    @tparam R The range type.
596   
    @tparam R The range type.
597  
597  
598   
    @par Requirements
598   
    @par Requirements
599   
    @li `R` must satisfy `std::ranges::input_range`
599   
    @li `R` must satisfy `std::ranges::input_range`
600   
    @li `R` must satisfy `std::ranges::sized_range`
600   
    @li `R` must satisfy `std::ranges::sized_range`
601   
    @li `std::ranges::range_value_t<R>` must satisfy @ref IoAwaitable
601   
    @li `std::ranges::range_value_t<R>` must satisfy @ref IoAwaitable
602  
602  
603   
    @par Syntactic Requirements
603   
    @par Syntactic Requirements
604   
    Given `r` of type `R`:
604   
    Given `r` of type `R`:
605   
    @li `std::ranges::begin(r)` is valid
605   
    @li `std::ranges::begin(r)` is valid
606   
    @li `std::ranges::end(r)` is valid
606   
    @li `std::ranges::end(r)` is valid
607   
    @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>`
608   
    @li `*std::ranges::begin(r)` satisfies @ref IoAwaitable
608   
    @li `*std::ranges::begin(r)` satisfies @ref IoAwaitable
609  
609  
610   
    @par Example
610   
    @par Example
611   
    @code
611   
    @code
612   
    template<IoAwaitableRange R>
612   
    template<IoAwaitableRange R>
613   
    task<void> race_all(R&& awaitables) {
613   
    task<void> race_all(R&& awaitables) {
614   
        auto winner = co_await when_any(std::forward<R>(awaitables));
614   
        auto winner = co_await when_any(std::forward<R>(awaitables));
615   
        // Process winner...
615   
        // Process winner...
616   
    }
616   
    }
617   
    @endcode
617   
    @endcode
618  
618  
619   
    @see when_any, IoAwaitable
619   
    @see when_any, IoAwaitable
620   
*/
620   
*/
621   
template<typename R>
621   
template<typename R>
622   
concept IoAwaitableRange =
622   
concept IoAwaitableRange =
623   
    std::ranges::input_range<R> &&
623   
    std::ranges::input_range<R> &&
624   
    std::ranges::sized_range<R> &&
624   
    std::ranges::sized_range<R> &&
625   
    IoAwaitable<std::ranges::range_value_t<R>>;
625   
    IoAwaitable<std::ranges::range_value_t<R>>;
626  
626  
627   
namespace detail {
627   
namespace detail {
628  
628  
629   
/** Shared state for homogeneous when_any (range overload).
629   
/** Shared state for homogeneous when_any (range overload).
630  
630  
631   
    Uses composition with when_any_core for shared functionality.
631   
    Uses composition with when_any_core for shared functionality.
632   
    Simpler than heterogeneous: optional<T> instead of variant, vector
632   
    Simpler than heterogeneous: optional<T> instead of variant, vector
633   
    instead of array for runner handles.
633   
    instead of array for runner handles.
634   
*/
634   
*/
635   
template<typename T>
635   
template<typename T>
636   
struct when_any_homogeneous_state
636   
struct when_any_homogeneous_state
637   
{
637   
{
638   
    when_any_core core_;
638   
    when_any_core core_;
639   
    std::optional<T> result_;
639   
    std::optional<T> result_;
640   
    std::vector<std::coroutine_handle<>> runner_handles_;
640   
    std::vector<std::coroutine_handle<>> runner_handles_;
641  
641  
642   
    explicit when_any_homogeneous_state(std::size_t count)
642   
    explicit when_any_homogeneous_state(std::size_t count)
643   
        : core_(count)
643   
        : core_(count)
644   
        , runner_handles_(count)
644   
        , runner_handles_(count)
645   
    {
645   
    {
646   
    }
646   
    }
647  
647  
648   
    // Runners self-destruct in final_suspend. No destruction needed here.
648   
    // Runners self-destruct in final_suspend. No destruction needed here.
649  
649  
650   
    /** @pre core_.try_win() returned true. */
650   
    /** @pre core_.try_win() returned true. */
651   
    void set_winner_result(T value)
651   
    void set_winner_result(T value)
652   
        noexcept(std::is_nothrow_move_constructible_v<T>)
652   
        noexcept(std::is_nothrow_move_constructible_v<T>)
653   
    {
653   
    {
654   
        result_.emplace(std::move(value));
654   
        result_.emplace(std::move(value));
655   
    }
655   
    }
656   
};
656   
};
657  
657  
658   
/** Specialization for void tasks (no result storage needed). */
658   
/** Specialization for void tasks (no result storage needed). */
659   
template<>
659   
template<>
660   
struct when_any_homogeneous_state<void>
660   
struct when_any_homogeneous_state<void>
661   
{
661   
{
662   
    when_any_core core_;
662   
    when_any_core core_;
663   
    std::vector<std::coroutine_handle<>> runner_handles_;
663   
    std::vector<std::coroutine_handle<>> runner_handles_;
664  
664  
665   
    explicit when_any_homogeneous_state(std::size_t count)
665   
    explicit when_any_homogeneous_state(std::size_t count)
666   
        : core_(count)
666   
        : core_(count)
667   
        , runner_handles_(count)
667   
        , runner_handles_(count)
668   
    {
668   
    {
669   
    }
669   
    }
670  
670  
671   
    // Runners self-destruct in final_suspend. No destruction needed here.
671   
    // Runners self-destruct in final_suspend. No destruction needed here.
672  
672  
673   
    // No set_winner_result - void tasks have no result to store
673   
    // No set_winner_result - void tasks have no result to store
674   
};
674   
};
675  
675  
676   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
676   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
677   
template<IoAwaitableRange Range>
677   
template<IoAwaitableRange Range>
678   
class when_any_homogeneous_launcher
678   
class when_any_homogeneous_launcher
679   
{
679   
{
680   
    using Awaitable = std::ranges::range_value_t<Range>;
680   
    using Awaitable = std::ranges::range_value_t<Range>;
681   
    using T = awaitable_result_t<Awaitable>;
681   
    using T = awaitable_result_t<Awaitable>;
682  
682  
683   
    Range* range_;
683   
    Range* range_;
684   
    when_any_homogeneous_state<T>* state_;
684   
    when_any_homogeneous_state<T>* state_;
685  
685  
686   
public:
686   
public:
687   
    when_any_homogeneous_launcher(
687   
    when_any_homogeneous_launcher(
688   
        Range* range,
688   
        Range* range,
689   
        when_any_homogeneous_state<T>* state)
689   
        when_any_homogeneous_state<T>* state)
690   
        : range_(range)
690   
        : range_(range)
691   
        , state_(state)
691   
        , state_(state)
692   
    {
692   
    {
693   
    }
693   
    }
694  
694  
695   
    bool await_ready() const noexcept
695   
    bool await_ready() const noexcept
696   
    {
696   
    {
697   
        return std::ranges::empty(*range_);
697   
        return std::ranges::empty(*range_);
698   
    }
698   
    }
699  
699  
700   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
700   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
701   
        destroys this object before await_suspend returns. Must not reference
701   
        destroys this object before await_suspend returns. Must not reference
702   
        `this` after dispatching begins.
702   
        `this` after dispatching begins.
703  
703  
704   
        Two-phase approach:
704   
        Two-phase approach:
705   
        1. Create all runners (safe - no dispatch yet)
705   
        1. Create all runners (safe - no dispatch yet)
706   
        2. Dispatch all runners (any may complete synchronously)
706   
        2. Dispatch all runners (any may complete synchronously)
707   
    */
707   
    */
708   
    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)
709   
    {
709   
    {
710   
        state_->core_.continuation_ = continuation;
710   
        state_->core_.continuation_ = continuation;
711   
        state_->core_.caller_env_ = caller_env;
711   
        state_->core_.caller_env_ = caller_env;
712  
712  
713   
        if(caller_env->stop_token.stop_possible())
713   
        if(caller_env->stop_token.stop_possible())
714   
        {
714   
        {
715   
            state_->core_.parent_stop_callback_.emplace(
715   
            state_->core_.parent_stop_callback_.emplace(
716   
                caller_env->stop_token,
716   
                caller_env->stop_token,
717   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
717   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
718  
718  
719   
            if(caller_env->stop_token.stop_requested())
719   
            if(caller_env->stop_token.stop_requested())
720   
                state_->core_.stop_source_.request_stop();
720   
                state_->core_.stop_source_.request_stop();
721   
        }
721   
        }
722  
722  
723   
        auto token = state_->core_.stop_source_.get_token();
723   
        auto token = state_->core_.stop_source_.get_token();
724  
724  
725   
        // Phase 1: Create all runners without dispatching.
725   
        // Phase 1: Create all runners without dispatching.
726   
        // This iterates over *range_ safely because no runners execute yet.
726   
        // This iterates over *range_ safely because no runners execute yet.
727   
        std::size_t index = 0;
727   
        std::size_t index = 0;
728   
        for(auto&& a : *range_)
728   
        for(auto&& a : *range_)
729   
        {
729   
        {
730   
            auto runner = make_when_any_runner(
730   
            auto runner = make_when_any_runner(
731   
                std::move(a), state_, index);
731   
                std::move(a), state_, index);
732  
732  
733   
            auto h = runner.release();
733   
            auto h = runner.release();
734   
            h.promise().state_ = state_;
734   
            h.promise().state_ = state_;
735   
            h.promise().index_ = index;
735   
            h.promise().index_ = index;
736   
            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};
737  
737  
738   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
738   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
739   
            ++index;
739   
            ++index;
740   
        }
740   
        }
741  
741  
742   
        // Phase 2: Post all runners. Any may complete synchronously.
742   
        // Phase 2: Post all runners. Any may complete synchronously.
743   
        // After last post, state_ and this may be destroyed.
743   
        // After last post, state_ and this may be destroyed.
744   
        // Use raw pointer/count captured before posting.
744   
        // Use raw pointer/count captured before posting.
745   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
745   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
746   
        std::size_t count = state_->runner_handles_.size();
746   
        std::size_t count = state_->runner_handles_.size();
747   
        for(std::size_t i = 0; i < count; ++i)
747   
        for(std::size_t i = 0; i < count; ++i)
748   
            caller_env->executor.post(handles[i]);
748   
            caller_env->executor.post(handles[i]);
749  
749  
750   
        return std::noop_coroutine();
750   
        return std::noop_coroutine();
751   
    }
751   
    }
752  
752  
753   
    void await_resume() const noexcept
753   
    void await_resume() const noexcept
754   
    {
754   
    {
755   
    }
755   
    }
756   
};
756   
};
757  
757  
758   
} // namespace detail
758   
} // namespace detail
759  
759  
760   
/** Wait for the first awaitable to complete (range overload).
760   
/** Wait for the first awaitable to complete (range overload).
761  
761  
762   
    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
763   
    sized input range of IoAwaitable types, enabling use with arrays,
763   
    sized input range of IoAwaitable types, enabling use with arrays,
764   
    spans, or custom containers.
764   
    spans, or custom containers.
765  
765  
766   
    @par Suspends
766   
    @par Suspends
767   
    The calling coroutine suspends when co_await is invoked. All awaitables
767   
    The calling coroutine suspends when co_await is invoked. All awaitables
768   
    in the range are launched concurrently and execute in parallel. The
768   
    in the range are launched concurrently and execute in parallel. The
769   
    coroutine resumes only after all awaitables have completed, even though
769   
    coroutine resumes only after all awaitables have completed, even though
770   
    the winner is determined by the first to finish.
770   
    the winner is determined by the first to finish.
771  
771  
772   
    @par Completion Conditions
772   
    @par Completion Conditions
773   
    @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)
774   
    @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
775   
    @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
776   
    @li Parent coroutine resumes only after all siblings acknowledge completion
776   
    @li Parent coroutine resumes only after all siblings acknowledge completion
777   
    @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
778  
778  
779   
    @par Cancellation Semantics
779   
    @par Cancellation Semantics
780   
    Cancellation is supported via stop_token propagated through the
780   
    Cancellation is supported via stop_token propagated through the
781   
    IoAwaitable protocol:
781   
    IoAwaitable protocol:
782   
    @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
783   
    @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
784   
    @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
785   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
785   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
786   
    @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
787  
787  
788   
    @par Concurrency/Overlap
788   
    @par Concurrency/Overlap
789   
    All awaitables are launched concurrently before any can complete.
789   
    All awaitables are launched concurrently before any can complete.
790   
    The launcher iterates through the range, starting each task on the
790   
    The launcher iterates through the range, starting each task on the
791   
    caller's executor. Tasks may execute in parallel on multi-threaded
791   
    caller's executor. Tasks may execute in parallel on multi-threaded
792   
    executors or interleave on single-threaded executors. There is no
792   
    executors or interleave on single-threaded executors. There is no
793   
    guaranteed ordering of task completion.
793   
    guaranteed ordering of task completion.
794  
794  
795   
    @par Notable Error Conditions
795   
    @par Notable Error Conditions
796   
    @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)
797   
    @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
798   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
798   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
799   
    @li Cancellation: tasks may complete via cancellation without throwing
799   
    @li Cancellation: tasks may complete via cancellation without throwing
800  
800  
801   
    @par Example
801   
    @par Example
802   
    @code
802   
    @code
803   
    task<void> example() {
803   
    task<void> example() {
804   
        std::array<task<Response>, 3> requests = {
804   
        std::array<task<Response>, 3> requests = {
805   
            fetch_from_server(0),
805   
            fetch_from_server(0),
806   
            fetch_from_server(1),
806   
            fetch_from_server(1),
807   
            fetch_from_server(2)
807   
            fetch_from_server(2)
808   
        };
808   
        };
809  
809  
810   
        auto [index, response] = co_await when_any(std::move(requests));
810   
        auto [index, response] = co_await when_any(std::move(requests));
811   
    }
811   
    }
812   
    @endcode
812   
    @endcode
813  
813  
814   
    @par Example with Vector
814   
    @par Example with Vector
815   
    @code
815   
    @code
816   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
816   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
817   
        std::vector<task<Response>> requests;
817   
        std::vector<task<Response>> requests;
818   
        for (auto const& server : servers)
818   
        for (auto const& server : servers)
819   
            requests.push_back(fetch_from(server));
819   
            requests.push_back(fetch_from(server));
820  
820  
821   
        auto [index, response] = co_await when_any(std::move(requests));
821   
        auto [index, response] = co_await when_any(std::move(requests));
822   
        co_return response;
822   
        co_return response;
823   
    }
823   
    }
824   
    @endcode
824   
    @endcode
825  
825  
826   
    @tparam R Range type satisfying IoAwaitableRange.
826   
    @tparam R Range type satisfying IoAwaitableRange.
827   
    @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).
828   
    @return A task yielding a pair of (winner_index, result).
828   
    @return A task yielding a pair of (winner_index, result).
829  
829  
830   
    @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).
831   
    @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.
832  
832  
833   
    @par Remarks
833   
    @par Remarks
834   
    Elements are moved from the range; for lvalue ranges, the original
834   
    Elements are moved from the range; for lvalue ranges, the original
835   
    container will have moved-from elements after this call. The range
835   
    container will have moved-from elements after this call. The range
836   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
836   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
837   
    the variadic overload, no variant wrapper is needed since all tasks
837   
    the variadic overload, no variant wrapper is needed since all tasks
838   
    share the same return type.
838   
    share the same return type.
839  
839  
840   
    @see when_any, IoAwaitableRange
840   
    @see when_any, IoAwaitableRange
841   
*/
841   
*/
842   
template<IoAwaitableRange R>
842   
template<IoAwaitableRange R>
843   
    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>>>)
844   
[[nodiscard]] auto when_any(R&& awaitables)
844   
[[nodiscard]] auto when_any(R&& awaitables)
845   
    -> 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>>>>
846   
{
846   
{
847   
    using Awaitable = std::ranges::range_value_t<R>;
847   
    using Awaitable = std::ranges::range_value_t<R>;
848   
    using T = awaitable_result_t<Awaitable>;
848   
    using T = awaitable_result_t<Awaitable>;
849   
    using result_type = std::pair<std::size_t, T>;
849   
    using result_type = std::pair<std::size_t, T>;
850   
    using OwnedRange = std::remove_cvref_t<R>;
850   
    using OwnedRange = std::remove_cvref_t<R>;
851  
851  
852   
    auto count = std::ranges::size(awaitables);
852   
    auto count = std::ranges::size(awaitables);
853   
    if(count == 0)
853   
    if(count == 0)
854   
        throw std::invalid_argument("when_any requires at least one awaitable");
854   
        throw std::invalid_argument("when_any requires at least one awaitable");
855  
855  
856   
    // Move/copy range onto coroutine frame to ensure lifetime
856   
    // Move/copy range onto coroutine frame to ensure lifetime
857   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
857   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
858  
858  
859   
    detail::when_any_homogeneous_state<T> state(count);
859   
    detail::when_any_homogeneous_state<T> state(count);
860  
860  
861   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
861   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
862  
862  
863   
    if(state.core_.winner_exception_)
863   
    if(state.core_.winner_exception_)
864   
        std::rethrow_exception(state.core_.winner_exception_);
864   
        std::rethrow_exception(state.core_.winner_exception_);
865  
865  
866   
    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_)};
867   
}
867   
}
868  
868  
869   
/** Wait for the first awaitable to complete (void range overload).
869   
/** Wait for the first awaitable to complete (void range overload).
870  
870  
871   
    Races a range of void-returning awaitables. Since void awaitables have
871   
    Races a range of void-returning awaitables. Since void awaitables have
872   
    no result value, only the winner's index is returned.
872   
    no result value, only the winner's index is returned.
873  
873  
874   
    @par Suspends
874   
    @par Suspends
875   
    The calling coroutine suspends when co_await is invoked. All awaitables
875   
    The calling coroutine suspends when co_await is invoked. All awaitables
876   
    in the range are launched concurrently and execute in parallel. The
876   
    in the range are launched concurrently and execute in parallel. The
877   
    coroutine resumes only after all awaitables have completed, even though
877   
    coroutine resumes only after all awaitables have completed, even though
878   
    the winner is determined by the first to finish.
878   
    the winner is determined by the first to finish.
879  
879  
880   
    @par Completion Conditions
880   
    @par Completion Conditions
881   
    @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)
882   
    @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
883   
    @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
884   
    @li Parent coroutine resumes only after all siblings acknowledge completion
884   
    @li Parent coroutine resumes only after all siblings acknowledge completion
885   
    @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
886  
886  
887   
    @par Cancellation Semantics
887   
    @par Cancellation Semantics
888   
    Cancellation is supported via stop_token propagated through the
888   
    Cancellation is supported via stop_token propagated through the
889   
    IoAwaitable protocol:
889   
    IoAwaitable protocol:
890   
    @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
891   
    @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
892   
    @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
893   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
893   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
894   
    @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
895  
895  
896   
    @par Concurrency/Overlap
896   
    @par Concurrency/Overlap
897   
    All awaitables are launched concurrently before any can complete.
897   
    All awaitables are launched concurrently before any can complete.
898   
    The launcher iterates through the range, starting each task on the
898   
    The launcher iterates through the range, starting each task on the
899   
    caller's executor. Tasks may execute in parallel on multi-threaded
899   
    caller's executor. Tasks may execute in parallel on multi-threaded
900   
    executors or interleave on single-threaded executors. There is no
900   
    executors or interleave on single-threaded executors. There is no
901   
    guaranteed ordering of task completion.
901   
    guaranteed ordering of task completion.
902  
902  
903   
    @par Notable Error Conditions
903   
    @par Notable Error Conditions
904   
    @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)
905   
    @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
906   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
906   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
907   
    @li Cancellation: tasks may complete via cancellation without throwing
907   
    @li Cancellation: tasks may complete via cancellation without throwing
908  
908  
909   
    @par Example
909   
    @par Example
910   
    @code
910   
    @code
911   
    task<void> example() {
911   
    task<void> example() {
912   
        std::vector<task<void>> tasks;
912   
        std::vector<task<void>> tasks;
913   
        for (int i = 0; i < 5; ++i)
913   
        for (int i = 0; i < 5; ++i)
914   
            tasks.push_back(background_work(i));
914   
            tasks.push_back(background_work(i));
915  
915  
916   
        std::size_t winner = co_await when_any(std::move(tasks));
916   
        std::size_t winner = co_await when_any(std::move(tasks));
917   
        // winner is the index of the first task to complete
917   
        // winner is the index of the first task to complete
918   
    }
918   
    }
919   
    @endcode
919   
    @endcode
920  
920  
921   
    @par Example with Timeout
921   
    @par Example with Timeout
922   
    @code
922   
    @code
923   
    task<void> with_timeout() {
923   
    task<void> with_timeout() {
924   
        std::vector<task<void>> tasks;
924   
        std::vector<task<void>> tasks;
925   
        tasks.push_back(long_running_operation());
925   
        tasks.push_back(long_running_operation());
926   
        tasks.push_back(delay(std::chrono::seconds(5)));
926   
        tasks.push_back(delay(std::chrono::seconds(5)));
927  
927  
928   
        std::size_t winner = co_await when_any(std::move(tasks));
928   
        std::size_t winner = co_await when_any(std::move(tasks));
929   
        if (winner == 1) {
929   
        if (winner == 1) {
930   
            // Timeout occurred
930   
            // Timeout occurred
931   
        }
931   
        }
932   
    }
932   
    }
933   
    @endcode
933   
    @endcode
934  
934  
935   
    @tparam R Range type satisfying IoAwaitableRange with void result.
935   
    @tparam R Range type satisfying IoAwaitableRange with void result.
936   
    @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).
937   
    @return A task yielding the winner's index (zero-based).
937   
    @return A task yielding the winner's index (zero-based).
938  
938  
939   
    @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).
940   
    @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.
941  
941  
942   
    @par Remarks
942   
    @par Remarks
943   
    Elements are moved from the range; for lvalue ranges, the original
943   
    Elements are moved from the range; for lvalue ranges, the original
944   
    container will have moved-from elements after this call. The range
944   
    container will have moved-from elements after this call. The range
945   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
945   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
946   
    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
947   
    produce no value.
947   
    produce no value.
948  
948  
949   
    @see when_any, IoAwaitableRange
949   
    @see when_any, IoAwaitableRange
950   
*/
950   
*/
951   
template<IoAwaitableRange R>
951   
template<IoAwaitableRange R>
952   
    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>>>
953   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
953   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
954   
{
954   
{
955   
    using OwnedRange = std::remove_cvref_t<R>;
955   
    using OwnedRange = std::remove_cvref_t<R>;
956  
956  
957   
    auto count = std::ranges::size(awaitables);
957   
    auto count = std::ranges::size(awaitables);
958   
    if(count == 0)
958   
    if(count == 0)
959   
        throw std::invalid_argument("when_any requires at least one awaitable");
959   
        throw std::invalid_argument("when_any requires at least one awaitable");
960  
960  
961   
    // Move/copy range onto coroutine frame to ensure lifetime
961   
    // Move/copy range onto coroutine frame to ensure lifetime
962   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
962   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
963  
963  
964   
    detail::when_any_homogeneous_state<void> state(count);
964   
    detail::when_any_homogeneous_state<void> state(count);
965  
965  
966   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
966   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
967  
967  
968   
    if(state.core_.winner_exception_)
968   
    if(state.core_.winner_exception_)
969   
        std::rethrow_exception(state.core_.winner_exception_);
969   
        std::rethrow_exception(state.core_.winner_exception_);
970  
970  
971   
    co_return state.core_.winner_index_;
971   
    co_return state.core_.winner_index_;
972   
}
972   
}
973  
973  
974   
} // namespace capy
974   
} // namespace capy
975   
} // namespace boost
975   
} // namespace boost
976  
976  
977   
#endif
977   
#endif