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/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 + 
namespace detail {
 
119 +
118   
/** Convert void to monostate for variant storage.
120   
/** Convert void to monostate for variant storage.
119  
121  
120   
    std::variant<void, ...> is ill-formed, so void tasks contribute
122   
    std::variant<void, ...> is ill-formed, so void tasks contribute
121   
    std::monostate to the result variant instead. Non-void types
123   
    std::monostate to the result variant instead. Non-void types
122   
    pass through unchanged.
124   
    pass through unchanged.
123  
125  
124   
    @tparam T The type to potentially convert (void becomes monostate).
126   
    @tparam T The type to potentially convert (void becomes monostate).
125   
*/
127   
*/
126   
template<typename T>
128   
template<typename T>
127   
using void_to_monostate_t = std::conditional_t<std::is_void_v<T>, std::monostate, T>;
129   
using void_to_monostate_t = std::conditional_t<std::is_void_v<T>, std::monostate, T>;
128  
130  
129 - 
namespace detail {
131 + 
// Result variant: one alternative per task, preserving positional
 
132 + 
// correspondence. Use .index() to identify which task won.
 
133 + 
// void results become monostate.
 
134 + 
template<typename T0, typename... Ts>
 
135 + 
using when_any_variant_t = std::variant<void_to_monostate_t<T0>, void_to_monostate_t<Ts>...>;
130  
136  
131   
/** Core shared state for when_any operations.
137   
/** Core shared state for when_any operations.
132  
138  
133   
    Contains all members and methods common to both heterogeneous (variadic)
139   
    Contains all members and methods common to both heterogeneous (variadic)
134   
    and homogeneous (range) when_any implementations. State classes embed
140   
    and homogeneous (range) when_any implementations. State classes embed
135   
    this via composition to avoid CRTP destructor ordering issues.
141   
    this via composition to avoid CRTP destructor ordering issues.
136  
142  
137   
    @par Thread Safety
143   
    @par Thread Safety
138   
    Atomic operations protect winner selection and completion count.
144   
    Atomic operations protect winner selection and completion count.
139   
*/
145   
*/
140   
struct when_any_core
146   
struct when_any_core
141   
{
147   
{
142   
    std::atomic<std::size_t> remaining_count_;
148   
    std::atomic<std::size_t> remaining_count_;
143   
    std::size_t winner_index_{0};
149   
    std::size_t winner_index_{0};
144   
    std::exception_ptr winner_exception_;
150   
    std::exception_ptr winner_exception_;
145   
    std::stop_source stop_source_;
151   
    std::stop_source stop_source_;
146  
152  
147   
    // Bridges parent's stop token to our stop_source
153   
    // Bridges parent's stop token to our stop_source
148   
    struct stop_callback_fn
154   
    struct stop_callback_fn
149   
    {
155   
    {
150   
        std::stop_source* source_;
156   
        std::stop_source* source_;
151   
        void operator()() const noexcept { source_->request_stop(); }
157   
        void operator()() const noexcept { source_->request_stop(); }
152   
    };
158   
    };
153   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
159   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
154   
    std::optional<stop_callback_t> parent_stop_callback_;
160   
    std::optional<stop_callback_t> parent_stop_callback_;
155  
161  
156   
    std::coroutine_handle<> continuation_;
162   
    std::coroutine_handle<> continuation_;
157   
    io_env const* caller_env_ = nullptr;
163   
    io_env const* caller_env_ = nullptr;
158  
164  
159   
    // Placed last to avoid padding (1-byte atomic followed by 8-byte aligned members)
165   
    // Placed last to avoid padding (1-byte atomic followed by 8-byte aligned members)
160   
    std::atomic<bool> has_winner_{false};
166   
    std::atomic<bool> has_winner_{false};
161  
167  
162   
    explicit when_any_core(std::size_t count) noexcept
168   
    explicit when_any_core(std::size_t count) noexcept
163   
        : remaining_count_(count)
169   
        : remaining_count_(count)
164   
    {
170   
    {
165   
    }
171   
    }
166  
172  
167   
    /** Atomically claim winner status; exactly one task succeeds. */
173   
    /** Atomically claim winner status; exactly one task succeeds. */
168   
    bool try_win(std::size_t index) noexcept
174   
    bool try_win(std::size_t index) noexcept
169   
    {
175   
    {
170   
        bool expected = false;
176   
        bool expected = false;
171   
        if(has_winner_.compare_exchange_strong(
177   
        if(has_winner_.compare_exchange_strong(
172   
            expected, true, std::memory_order_acq_rel))
178   
            expected, true, std::memory_order_acq_rel))
173   
        {
179   
        {
174   
            winner_index_ = index;
180   
            winner_index_ = index;
175   
            stop_source_.request_stop();
181   
            stop_source_.request_stop();
176   
            return true;
182   
            return true;
177   
        }
183   
        }
178   
        return false;
184   
        return false;
179   
    }
185   
    }
180  
186  
181   
    /** @pre try_win() returned true. */
187   
    /** @pre try_win() returned true. */
182   
    void set_winner_exception(std::exception_ptr ep) noexcept
188   
    void set_winner_exception(std::exception_ptr ep) noexcept
183   
    {
189   
    {
184   
        winner_exception_ = ep;
190   
        winner_exception_ = ep;
185   
    }
191   
    }
186  
192  
187   
    // Runners signal completion directly via final_suspend; no member function needed.
193   
    // Runners signal completion directly via final_suspend; no member function needed.
188   
};
194   
};
189  
195  
190   
/** Shared state for heterogeneous when_any operation.
196   
/** Shared state for heterogeneous when_any operation.
191  
197  
192   
    Coordinates winner selection, result storage, and completion tracking
198   
    Coordinates winner selection, result storage, and completion tracking
193   
    for all child tasks in a when_any operation. Uses composition with
199   
    for all child tasks in a when_any operation. Uses composition with
194   
    when_any_core for shared functionality.
200   
    when_any_core for shared functionality.
195  
201  
196   
    @par Lifetime
202   
    @par Lifetime
197   
    Allocated on the parent coroutine's frame, outlives all runners.
203   
    Allocated on the parent coroutine's frame, outlives all runners.
198  
204  
199 - 
    @tparam Ts Task result types.
205 + 
    @tparam T0 First task's result type.
 
206 + 
    @tparam Ts Remaining tasks' result types.
200   
*/
207   
*/
201 - 
template<typename... Ts>
208 + 
template<typename T0, typename... Ts>
202   
struct when_any_state
209   
struct when_any_state
203   
{
210   
{
204 - 
    static constexpr std::size_t task_count = sizeof...(Ts);
211 + 
    static constexpr std::size_t task_count = 1 + sizeof...(Ts);
205 - 
    using variant_type = std::variant<void_to_monostate_t<Ts>...>;
212 + 
    using variant_type = when_any_variant_t<T0, Ts...>;
206  
213  
207   
    when_any_core core_;
214   
    when_any_core core_;
208   
    std::optional<variant_type> result_;
215   
    std::optional<variant_type> result_;
209   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
216   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
210  
217  
211   
    when_any_state()
218   
    when_any_state()
212   
        : core_(task_count)
219   
        : core_(task_count)
213   
    {
220   
    {
214   
    }
221   
    }
215  
222  
216   
    // Runners self-destruct in final_suspend. No destruction needed here.
223   
    // Runners self-destruct in final_suspend. No destruction needed here.
217  
224  
218   
    /** @pre core_.try_win() returned true.
225   
    /** @pre core_.try_win() returned true.
219   
        @note Uses in_place_index (not type) for positional variant access.
226   
        @note Uses in_place_index (not type) for positional variant access.
220   
    */
227   
    */
221   
    template<std::size_t I, typename T>
228   
    template<std::size_t I, typename T>
222   
    void set_winner_result(T value)
229   
    void set_winner_result(T value)
223   
        noexcept(std::is_nothrow_move_constructible_v<T>)
230   
        noexcept(std::is_nothrow_move_constructible_v<T>)
224   
    {
231   
    {
225   
        result_.emplace(std::in_place_index<I>, std::move(value));
232   
        result_.emplace(std::in_place_index<I>, std::move(value));
226   
    }
233   
    }
227  
234  
228   
    /** @pre core_.try_win() returned true. */
235   
    /** @pre core_.try_win() returned true. */
229   
    template<std::size_t I>
236   
    template<std::size_t I>
230   
    void set_winner_void() noexcept
237   
    void set_winner_void() noexcept
231   
    {
238   
    {
232   
        result_.emplace(std::in_place_index<I>, std::monostate{});
239   
        result_.emplace(std::in_place_index<I>, std::monostate{});
233   
    }
240   
    }
234   
};
241   
};
235  
242  
236   
/** Wrapper coroutine that runs a single child task for when_any.
243   
/** Wrapper coroutine that runs a single child task for when_any.
237  
244  
238   
    Propagates executor/stop_token to the child, attempts to claim winner
245   
    Propagates executor/stop_token to the child, attempts to claim winner
239   
    status on completion, and signals completion for cleanup coordination.
246   
    status on completion, and signals completion for cleanup coordination.
240  
247  
241   
    @tparam StateType The state type (when_any_state or when_any_homogeneous_state).
248   
    @tparam StateType The state type (when_any_state or when_any_homogeneous_state).
242   
*/
249   
*/
243   
template<typename StateType>
250   
template<typename StateType>
244   
struct when_any_runner
251   
struct when_any_runner
245   
{
252   
{
246   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
253   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
247   
    {
254   
    {
248   
        StateType* state_ = nullptr;
255   
        StateType* state_ = nullptr;
249   
        std::size_t index_ = 0;
256   
        std::size_t index_ = 0;
250   
        io_env env_;
257   
        io_env env_;
251  
258  
252   
        when_any_runner get_return_object() noexcept
259   
        when_any_runner get_return_object() noexcept
253   
        {
260   
        {
254   
            return when_any_runner(std::coroutine_handle<promise_type>::from_promise(*this));
261   
            return when_any_runner(std::coroutine_handle<promise_type>::from_promise(*this));
255   
        }
262   
        }
256  
263  
257   
        // Starts suspended; launcher sets up state/ex/token then resumes
264   
        // Starts suspended; launcher sets up state/ex/token then resumes
258   
        std::suspend_always initial_suspend() noexcept
265   
        std::suspend_always initial_suspend() noexcept
259   
        {
266   
        {
260   
            return {};
267   
            return {};
261   
        }
268   
        }
262  
269  
263   
        auto final_suspend() noexcept
270   
        auto final_suspend() noexcept
264   
        {
271   
        {
265   
            struct awaiter
272   
            struct awaiter
266   
            {
273   
            {
267   
                promise_type* p_;
274   
                promise_type* p_;
268   
                bool await_ready() const noexcept { return false; }
275   
                bool await_ready() const noexcept { return false; }
269 - 
                auto await_suspend(std::coroutine_handle<> h) noexcept
276 + 
                std::coroutine_handle<> await_suspend(std::coroutine_handle<> h) noexcept
270   
                {
277   
                {
271   
                    // Extract everything needed before self-destruction.
278   
                    // Extract everything needed before self-destruction.
272   
                    auto& core = p_->state_->core_;
279   
                    auto& core = p_->state_->core_;
273   
                    auto* counter = &core.remaining_count_;
280   
                    auto* counter = &core.remaining_count_;
274   
                    auto* caller_env = core.caller_env_;
281   
                    auto* caller_env = core.caller_env_;
275   
                    auto cont = core.continuation_;
282   
                    auto cont = core.continuation_;
276  
283  
277   
                    h.destroy();
284   
                    h.destroy();
278  
285  
279   
                    // If last runner, dispatch parent for symmetric transfer.
286   
                    // If last runner, dispatch parent for symmetric transfer.
280   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
287   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
281   
                    if(remaining == 1)
288   
                    if(remaining == 1)
282 - 
                        return detail::symmetric_transfer(caller_env->executor.dispatch(cont));
289 + 
                        return caller_env->executor.dispatch(cont);
283 - 
                    return detail::symmetric_transfer(std::noop_coroutine());
290 + 
                    return std::noop_coroutine();
284   
                }
291   
                }
285   
                void await_resume() const noexcept {}
292   
                void await_resume() const noexcept {}
286   
            };
293   
            };
287   
            return awaiter{this};
294   
            return awaiter{this};
288   
        }
295   
        }
289  
296  
290   
        void return_void() noexcept {}
297   
        void return_void() noexcept {}
291  
298  
292   
        // Exceptions are valid completions in when_any (unlike when_all)
299   
        // Exceptions are valid completions in when_any (unlike when_all)
293   
        void unhandled_exception()
300   
        void unhandled_exception()
294   
        {
301   
        {
295   
            if(state_->core_.try_win(index_))
302   
            if(state_->core_.try_win(index_))
296   
                state_->core_.set_winner_exception(std::current_exception());
303   
                state_->core_.set_winner_exception(std::current_exception());
297   
        }
304   
        }
298  
305  
299   
        /** Injects executor and stop token into child awaitables. */
306   
        /** Injects executor and stop token into child awaitables. */
300   
        template<class Awaitable>
307   
        template<class Awaitable>
301   
        struct transform_awaiter
308   
        struct transform_awaiter
302   
        {
309   
        {
303   
            std::decay_t<Awaitable> a_;
310   
            std::decay_t<Awaitable> a_;
304   
            promise_type* p_;
311   
            promise_type* p_;
305  
312  
306   
            bool await_ready() { return a_.await_ready(); }
313   
            bool await_ready() { return a_.await_ready(); }
307   
            auto await_resume() { return a_.await_resume(); }
314   
            auto await_resume() { return a_.await_resume(); }
308  
315  
309   
            template<class Promise>
316   
            template<class Promise>
310   
            auto await_suspend(std::coroutine_handle<Promise> h)
317   
            auto await_suspend(std::coroutine_handle<Promise> h)
311   
            {
318   
            {
 
319 + 
#ifdef _MSC_VER
312   
                using R = decltype(a_.await_suspend(h, &p_->env_));
320   
                using R = decltype(a_.await_suspend(h, &p_->env_));
313   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
321   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
314 - 
                    return detail::symmetric_transfer(a_.await_suspend(h, &p_->env_));
322 + 
                    a_.await_suspend(h, &p_->env_).resume();
315   
                else
323   
                else
316   
                    return a_.await_suspend(h, &p_->env_);
324   
                    return a_.await_suspend(h, &p_->env_);
 
325 + 
#else
 
326 + 
                return a_.await_suspend(h, &p_->env_);
 
327 + 
#endif
317   
            }
328   
            }
318   
        };
329   
        };
319  
330  
320   
        template<class Awaitable>
331   
        template<class Awaitable>
321   
        auto await_transform(Awaitable&& a)
332   
        auto await_transform(Awaitable&& a)
322   
        {
333   
        {
323   
            using A = std::decay_t<Awaitable>;
334   
            using A = std::decay_t<Awaitable>;
324   
            if constexpr (IoAwaitable<A>)
335   
            if constexpr (IoAwaitable<A>)
325   
            {
336   
            {
326   
                return transform_awaiter<Awaitable>{
337   
                return transform_awaiter<Awaitable>{
327   
                    std::forward<Awaitable>(a), this};
338   
                    std::forward<Awaitable>(a), this};
328   
            }
339   
            }
329   
            else
340   
            else
330   
            {
341   
            {
331   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
342   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
332   
            }
343   
            }
333   
        }
344   
        }
334   
    };
345   
    };
335  
346  
336   
    std::coroutine_handle<promise_type> h_;
347   
    std::coroutine_handle<promise_type> h_;
337  
348  
338   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
349   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
339   
        : h_(h)
350   
        : h_(h)
340   
    {
351   
    {
341   
    }
352   
    }
342  
353  
343   
    // Enable move for all clang versions - some versions need it
354   
    // 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)) {}
355   
    when_any_runner(when_any_runner&& other) noexcept : h_(std::exchange(other.h_, nullptr)) {}
345  
356  
346   
    // Non-copyable
357   
    // Non-copyable
347   
    when_any_runner(when_any_runner const&) = delete;
358   
    when_any_runner(when_any_runner const&) = delete;
348   
    when_any_runner& operator=(when_any_runner const&) = delete;
359   
    when_any_runner& operator=(when_any_runner const&) = delete;
349   
    when_any_runner& operator=(when_any_runner&&) = delete;
360   
    when_any_runner& operator=(when_any_runner&&) = delete;
350  
361  
351   
    auto release() noexcept
362   
    auto release() noexcept
352   
    {
363   
    {
353   
        return std::exchange(h_, nullptr);
364   
        return std::exchange(h_, nullptr);
354   
    }
365   
    }
355   
};
366   
};
356  
367  
357   
/** Indexed overload for heterogeneous when_any (compile-time index).
368   
/** Indexed overload for heterogeneous when_any (compile-time index).
358  
369  
359   
    Uses compile-time index I for variant construction via in_place_index.
370   
    Uses compile-time index I for variant construction via in_place_index.
360   
    Called from when_any_launcher::launch_one<I>().
371   
    Called from when_any_launcher::launch_one<I>().
361   
*/
372   
*/
362   
template<std::size_t I, IoAwaitable Awaitable, typename StateType>
373   
template<std::size_t I, IoAwaitable Awaitable, typename StateType>
363   
when_any_runner<StateType>
374   
when_any_runner<StateType>
364   
make_when_any_runner(Awaitable inner, StateType* state)
375   
make_when_any_runner(Awaitable inner, StateType* state)
365   
{
376   
{
366   
    using T = awaitable_result_t<Awaitable>;
377   
    using T = awaitable_result_t<Awaitable>;
367   
    if constexpr (std::is_void_v<T>)
378   
    if constexpr (std::is_void_v<T>)
368   
    {
379   
    {
369   
        co_await std::move(inner);
380   
        co_await std::move(inner);
370   
        if(state->core_.try_win(I))
381   
        if(state->core_.try_win(I))
371   
            state->template set_winner_void<I>();
382   
            state->template set_winner_void<I>();
372   
    }
383   
    }
373   
    else
384   
    else
374   
    {
385   
    {
375   
        auto result = co_await std::move(inner);
386   
        auto result = co_await std::move(inner);
376   
        if(state->core_.try_win(I))
387   
        if(state->core_.try_win(I))
377   
        {
388   
        {
378   
            try
389   
            try
379   
            {
390   
            {
380   
                state->template set_winner_result<I>(std::move(result));
391   
                state->template set_winner_result<I>(std::move(result));
381   
            }
392   
            }
382   
            catch(...)
393   
            catch(...)
383   
            {
394   
            {
384   
                state->core_.set_winner_exception(std::current_exception());
395   
                state->core_.set_winner_exception(std::current_exception());
385   
            }
396   
            }
386   
        }
397   
        }
387   
    }
398   
    }
388   
}
399   
}
389  
400  
390   
/** Runtime-index overload for homogeneous when_any (range path).
401   
/** Runtime-index overload for homogeneous when_any (range path).
391  
402  
392   
    Uses requires-expressions to detect state capabilities:
403   
    Uses requires-expressions to detect state capabilities:
393   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
404   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
394   
    - set_winner_result(): for non-void tasks
405   
    - set_winner_result(): for non-void tasks
395   
    - Neither: for homogeneous void tasks (no result storage)
406   
    - Neither: for homogeneous void tasks (no result storage)
396   
*/
407   
*/
397   
template<IoAwaitable Awaitable, typename StateType>
408   
template<IoAwaitable Awaitable, typename StateType>
398   
when_any_runner<StateType>
409   
when_any_runner<StateType>
399   
make_when_any_runner(Awaitable inner, StateType* state, std::size_t index)
410   
make_when_any_runner(Awaitable inner, StateType* state, std::size_t index)
400   
{
411   
{
401   
    using T = awaitable_result_t<Awaitable>;
412   
    using T = awaitable_result_t<Awaitable>;
402   
    if constexpr (std::is_void_v<T>)
413   
    if constexpr (std::is_void_v<T>)
403   
    {
414   
    {
404   
        co_await std::move(inner);
415   
        co_await std::move(inner);
405   
        if(state->core_.try_win(index))
416   
        if(state->core_.try_win(index))
406   
        {
417   
        {
407   
            if constexpr (requires { state->set_winner_void(); })
418   
            if constexpr (requires { state->set_winner_void(); })
408   
                state->set_winner_void();
419   
                state->set_winner_void();
409   
        }
420   
        }
410   
    }
421   
    }
411   
    else
422   
    else
412   
    {
423   
    {
413   
        auto result = co_await std::move(inner);
424   
        auto result = co_await std::move(inner);
414   
        if(state->core_.try_win(index))
425   
        if(state->core_.try_win(index))
415   
        {
426   
        {
416   
            try
427   
            try
417   
            {
428   
            {
418   
                state->set_winner_result(std::move(result));
429   
                state->set_winner_result(std::move(result));
419   
            }
430   
            }
420   
            catch(...)
431   
            catch(...)
421   
            {
432   
            {
422   
                state->core_.set_winner_exception(std::current_exception());
433   
                state->core_.set_winner_exception(std::current_exception());
423   
            }
434   
            }
424   
        }
435   
        }
425   
    }
436   
    }
426   
}
437   
}
427  
438  
428   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
439   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
429   
template<IoAwaitable... Awaitables>
440   
template<IoAwaitable... Awaitables>
430   
class when_any_launcher
441   
class when_any_launcher
431   
{
442   
{
432   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
443   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
433  
444  
434   
    std::tuple<Awaitables...>* tasks_;
445   
    std::tuple<Awaitables...>* tasks_;
435   
    state_type* state_;
446   
    state_type* state_;
436  
447  
437   
public:
448   
public:
438   
    when_any_launcher(
449   
    when_any_launcher(
439   
        std::tuple<Awaitables...>* tasks,
450   
        std::tuple<Awaitables...>* tasks,
440   
        state_type* state)
451   
        state_type* state)
441   
        : tasks_(tasks)
452   
        : tasks_(tasks)
442   
        , state_(state)
453   
        , state_(state)
443   
    {
454   
    {
444   
    }
455   
    }
445  
456  
446   
    bool await_ready() const noexcept
457   
    bool await_ready() const noexcept
447   
    {
458   
    {
448   
        return sizeof...(Awaitables) == 0;
459   
        return sizeof...(Awaitables) == 0;
449   
    }
460   
    }
450  
461  
451   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
462   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
452   
        destroys this object before await_suspend returns. Must not reference
463   
        destroys this object before await_suspend returns. Must not reference
453   
        `this` after the final launch_one call.
464   
        `this` after the final launch_one call.
454   
    */
465   
    */
455   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
466   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
456   
    {
467   
    {
457   
        state_->core_.continuation_ = continuation;
468   
        state_->core_.continuation_ = continuation;
458   
        state_->core_.caller_env_ = caller_env;
469   
        state_->core_.caller_env_ = caller_env;
459  
470  
460   
        if(caller_env->stop_token.stop_possible())
471   
        if(caller_env->stop_token.stop_possible())
461   
        {
472   
        {
462   
            state_->core_.parent_stop_callback_.emplace(
473   
            state_->core_.parent_stop_callback_.emplace(
463   
                caller_env->stop_token,
474   
                caller_env->stop_token,
464   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
475   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
465  
476  
466   
            if(caller_env->stop_token.stop_requested())
477   
            if(caller_env->stop_token.stop_requested())
467   
                state_->core_.stop_source_.request_stop();
478   
                state_->core_.stop_source_.request_stop();
468   
        }
479   
        }
469  
480  
470   
        auto token = state_->core_.stop_source_.get_token();
481   
        auto token = state_->core_.stop_source_.get_token();
471   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
482   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
472   
            (..., launch_one<Is>(caller_env->executor, token));
483   
            (..., launch_one<Is>(caller_env->executor, token));
473   
        }(std::index_sequence_for<Awaitables...>{});
484   
        }(std::index_sequence_for<Awaitables...>{});
474  
485  
475   
        return std::noop_coroutine();
486   
        return std::noop_coroutine();
476   
    }
487   
    }
477  
488  
478   
    void await_resume() const noexcept
489   
    void await_resume() const noexcept
479   
    {
490   
    {
480   
    }
491   
    }
481  
492  
482   
private:
493   
private:
483   
    /** @pre Ex::dispatch() and std::coroutine_handle<>::resume() must not throw (handle may leak). */
494   
    /** @pre Ex::dispatch() and std::coroutine_handle<>::resume() must not throw (handle may leak). */
484   
    template<std::size_t I>
495   
    template<std::size_t I>
485   
    void launch_one(executor_ref caller_ex, std::stop_token token)
496   
    void launch_one(executor_ref caller_ex, std::stop_token token)
486   
    {
497   
    {
487   
        auto runner = make_when_any_runner<I>(
498   
        auto runner = make_when_any_runner<I>(
488   
            std::move(std::get<I>(*tasks_)), state_);
499   
            std::move(std::get<I>(*tasks_)), state_);
489  
500  
490   
        auto h = runner.release();
501   
        auto h = runner.release();
491   
        h.promise().state_ = state_;
502   
        h.promise().state_ = state_;
492   
        h.promise().index_ = I;
503   
        h.promise().index_ = I;
493   
        h.promise().env_ = io_env{caller_ex, token, state_->core_.caller_env_->frame_allocator};
504   
        h.promise().env_ = io_env{caller_ex, token, state_->core_.caller_env_->frame_allocator};
494  
505  
495   
        std::coroutine_handle<> ch{h};
506   
        std::coroutine_handle<> ch{h};
496   
        state_->runner_handles_[I] = ch;
507   
        state_->runner_handles_[I] = ch;
497   
        caller_ex.post(ch);
508   
        caller_ex.post(ch);
498   
    }
509   
    }
499   
};
510   
};
500  
511  
501   
} // namespace detail
512   
} // namespace detail
502  
513  
503   
/** Wait for the first awaitable to complete.
514   
/** Wait for the first awaitable to complete.
504  
515  
505   
    Races multiple heterogeneous awaitables concurrently and returns when the
516   
    Races multiple heterogeneous awaitables concurrently and returns when the
506   
    first one completes. The result is a variant with one alternative per
517   
    first one completes. The result is a variant with one alternative per
507   
    input task, preserving positional correspondence.
518   
    input task, preserving positional correspondence.
508  
519  
509   
    @par Suspends
520   
    @par Suspends
510   
    The calling coroutine suspends when co_await is invoked. All awaitables
521   
    The calling coroutine suspends when co_await is invoked. All awaitables
511   
    are launched concurrently and execute in parallel. The coroutine resumes
522   
    are launched concurrently and execute in parallel. The coroutine resumes
512   
    only after all awaitables have completed, even though the winner is
523   
    only after all awaitables have completed, even though the winner is
513   
    determined by the first to finish.
524   
    determined by the first to finish.
514  
525  
515   
    @par Completion Conditions
526   
    @par Completion Conditions
516   
    @li Winner is determined when the first awaitable completes (success or exception)
527   
    @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
528   
    @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
529   
    @li Once a winner exists, stop is requested for all remaining siblings
519   
    @li Parent coroutine resumes only after all siblings acknowledge completion
530   
    @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
531   
    @li The winner's result is returned; if the winner threw, the exception is rethrown
521  
532  
522   
    @par Cancellation Semantics
533   
    @par Cancellation Semantics
523   
    Cancellation is supported via stop_token propagated through the
534   
    Cancellation is supported via stop_token propagated through the
524   
    IoAwaitable protocol:
535   
    IoAwaitable protocol:
525   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
536   
    @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
537   
    @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
538   
    @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
539   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
529   
    @li Stop requests are cooperative; tasks must check and respond to them
540   
    @li Stop requests are cooperative; tasks must check and respond to them
530  
541  
531   
    @par Concurrency/Overlap
542   
    @par Concurrency/Overlap
532   
    All awaitables are launched concurrently before any can complete.
543   
    All awaitables are launched concurrently before any can complete.
533   
    The launcher iterates through the arguments, starting each task on the
544   
    The launcher iterates through the arguments, starting each task on the
534   
    caller's executor. Tasks may execute in parallel on multi-threaded
545   
    caller's executor. Tasks may execute in parallel on multi-threaded
535   
    executors or interleave on single-threaded executors. There is no
546   
    executors or interleave on single-threaded executors. There is no
536   
    guaranteed ordering of task completion.
547   
    guaranteed ordering of task completion.
537  
548  
538   
    @par Notable Error Conditions
549   
    @par Notable Error Conditions
539   
    @li Winner exception: if the winning task threw, that exception is rethrown
550   
    @li Winner exception: if the winning task threw, that exception is rethrown
540   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
551   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
541   
    @li Cancellation: tasks may complete via cancellation without throwing
552   
    @li Cancellation: tasks may complete via cancellation without throwing
542  
553  
543   
    @par Example
554   
    @par Example
544   
    @code
555   
    @code
545   
    task<void> example() {
556   
    task<void> example() {
546   
        auto result = co_await when_any(
557   
        auto result = co_await when_any(
547   
            fetch_int(),      // task<int>
558   
            fetch_int(),      // task<int>
548   
            fetch_string()    // task<std::string>
559   
            fetch_string()    // task<std::string>
549   
        );
560   
        );
550   
        // result.index() is 0 or 1
561   
        // result.index() is 0 or 1
551   
        if (result.index() == 0)
562   
        if (result.index() == 0)
552   
            std::cout << "Got int: " << std::get<0>(result) << "\n";
563   
            std::cout << "Got int: " << std::get<0>(result) << "\n";
553   
        else
564   
        else
554   
            std::cout << "Got string: " << std::get<1>(result) << "\n";
565   
            std::cout << "Got string: " << std::get<1>(result) << "\n";
555   
    }
566   
    }
556   
    @endcode
567   
    @endcode
557  
568  
558 - 
    @param as Awaitables to race concurrently (at least one required; each
569 + 
    @tparam A0 First awaitable type (must satisfy IoAwaitable).
559 - 
        must satisfy IoAwaitable).
570 + 
    @tparam As Remaining awaitable types (must satisfy IoAwaitable).
560 - 
    @return A task yielding a std::variant with one alternative per awaitable.
571 + 
    @param a0 The first awaitable to race.
 
572 + 
    @param as Additional awaitables to race concurrently.
 
573 + 
    @return A task yielding a variant with one alternative per awaitable.
561   
        Use .index() to identify the winner. Void awaitables contribute
574   
        Use .index() to identify the winner. Void awaitables contribute
562   
        std::monostate.
575   
        std::monostate.
563  
576  
564   
    @throws Rethrows the winner's exception if the winning task threw an exception.
577   
    @throws Rethrows the winner's exception if the winning task threw an exception.
565  
578  
566   
    @par Remarks
579   
    @par Remarks
567   
    Awaitables are moved into the coroutine frame; original objects become
580   
    Awaitables are moved into the coroutine frame; original objects become
568   
    empty after the call. The variant preserves one alternative per input
581   
    empty after the call. The variant preserves one alternative per input
569   
    task. Use .index() to determine which awaitable completed first.
582   
    task. Use .index() to determine which awaitable completed first.
570   
    Void awaitables contribute std::monostate to the variant.
583   
    Void awaitables contribute std::monostate to the variant.
571  
584  
572   
    @see when_all, IoAwaitable
585   
    @see when_all, IoAwaitable
573   
*/
586   
*/
574 - 
template<IoAwaitable... As>
587 + 
template<IoAwaitable A0, IoAwaitable... As>
575 - 
    requires (sizeof...(As) > 0)
588 + 
[[nodiscard]] auto when_any(A0 a0, As... as)
576 - 
[[nodiscard]] auto when_any(As... as)
589 + 
    -> task<detail::when_any_variant_t<
577 - 
    -> task<std::variant<void_to_monostate_t<awaitable_result_t<As>>...>>
590 + 
        detail::awaitable_result_t<A0>,
 
591 + 
        detail::awaitable_result_t<As>...>>
578   
{
592   
{
579 - 
    detail::when_any_state<awaitable_result_t<As>...> state;
593 + 
    detail::when_any_state<
580 - 
    std::tuple<As...> awaitable_tuple(std::move(as)...);
594 + 
        detail::awaitable_result_t<A0>,
 
595 + 
        detail::awaitable_result_t<As>...> state;
 
596 + 
    std::tuple<A0, As...> awaitable_tuple(std::move(a0), std::move(as)...);
581  
597  
582 - 
    co_await detail::when_any_launcher<As...>(&awaitable_tuple, &state);
598 + 
    co_await detail::when_any_launcher<A0, As...>(&awaitable_tuple, &state);
583  
599  
584   
    if(state.core_.winner_exception_)
600   
    if(state.core_.winner_exception_)
585   
        std::rethrow_exception(state.core_.winner_exception_);
601   
        std::rethrow_exception(state.core_.winner_exception_);
586  
602  
587   
    co_return std::move(*state.result_);
603   
    co_return std::move(*state.result_);
588   
}
604   
}
589  
605  
590   
/** Concept for ranges of full I/O awaitables.
606   
/** Concept for ranges of full I/O awaitables.
591  
607  
592   
    A range satisfies `IoAwaitableRange` if it is a sized input range
608   
    A range satisfies `IoAwaitableRange` if it is a sized input range
593   
    whose value type satisfies @ref IoAwaitable. This enables when_any
609   
    whose value type satisfies @ref IoAwaitable. This enables when_any
594   
    to accept any container or view of awaitables, not just std::vector.
610   
    to accept any container or view of awaitables, not just std::vector.
595  
611  
596   
    @tparam R The range type.
612   
    @tparam R The range type.
597  
613  
598   
    @par Requirements
614   
    @par Requirements
599   
    @li `R` must satisfy `std::ranges::input_range`
615   
    @li `R` must satisfy `std::ranges::input_range`
600   
    @li `R` must satisfy `std::ranges::sized_range`
616   
    @li `R` must satisfy `std::ranges::sized_range`
601   
    @li `std::ranges::range_value_t<R>` must satisfy @ref IoAwaitable
617   
    @li `std::ranges::range_value_t<R>` must satisfy @ref IoAwaitable
602  
618  
603   
    @par Syntactic Requirements
619   
    @par Syntactic Requirements
604   
    Given `r` of type `R`:
620   
    Given `r` of type `R`:
605   
    @li `std::ranges::begin(r)` is valid
621   
    @li `std::ranges::begin(r)` is valid
606   
    @li `std::ranges::end(r)` is valid
622   
    @li `std::ranges::end(r)` is valid
607   
    @li `std::ranges::size(r)` returns `std::ranges::range_size_t<R>`
623   
    @li `std::ranges::size(r)` returns `std::ranges::range_size_t<R>`
608   
    @li `*std::ranges::begin(r)` satisfies @ref IoAwaitable
624   
    @li `*std::ranges::begin(r)` satisfies @ref IoAwaitable
609  
625  
610   
    @par Example
626   
    @par Example
611   
    @code
627   
    @code
612   
    template<IoAwaitableRange R>
628   
    template<IoAwaitableRange R>
613   
    task<void> race_all(R&& awaitables) {
629   
    task<void> race_all(R&& awaitables) {
614   
        auto winner = co_await when_any(std::forward<R>(awaitables));
630   
        auto winner = co_await when_any(std::forward<R>(awaitables));
615   
        // Process winner...
631   
        // Process winner...
616   
    }
632   
    }
617   
    @endcode
633   
    @endcode
618  
634  
619   
    @see when_any, IoAwaitable
635   
    @see when_any, IoAwaitable
620   
*/
636   
*/
621   
template<typename R>
637   
template<typename R>
622   
concept IoAwaitableRange =
638   
concept IoAwaitableRange =
623   
    std::ranges::input_range<R> &&
639   
    std::ranges::input_range<R> &&
624   
    std::ranges::sized_range<R> &&
640   
    std::ranges::sized_range<R> &&
625   
    IoAwaitable<std::ranges::range_value_t<R>>;
641   
    IoAwaitable<std::ranges::range_value_t<R>>;
626  
642  
627   
namespace detail {
643   
namespace detail {
628  
644  
629   
/** Shared state for homogeneous when_any (range overload).
645   
/** Shared state for homogeneous when_any (range overload).
630  
646  
631   
    Uses composition with when_any_core for shared functionality.
647   
    Uses composition with when_any_core for shared functionality.
632   
    Simpler than heterogeneous: optional<T> instead of variant, vector
648   
    Simpler than heterogeneous: optional<T> instead of variant, vector
633   
    instead of array for runner handles.
649   
    instead of array for runner handles.
634   
*/
650   
*/
635   
template<typename T>
651   
template<typename T>
636   
struct when_any_homogeneous_state
652   
struct when_any_homogeneous_state
637   
{
653   
{
638   
    when_any_core core_;
654   
    when_any_core core_;
639   
    std::optional<T> result_;
655   
    std::optional<T> result_;
640   
    std::vector<std::coroutine_handle<>> runner_handles_;
656   
    std::vector<std::coroutine_handle<>> runner_handles_;
641  
657  
642   
    explicit when_any_homogeneous_state(std::size_t count)
658   
    explicit when_any_homogeneous_state(std::size_t count)
643   
        : core_(count)
659   
        : core_(count)
644   
        , runner_handles_(count)
660   
        , runner_handles_(count)
645   
    {
661   
    {
646   
    }
662   
    }
647  
663  
648   
    // Runners self-destruct in final_suspend. No destruction needed here.
664   
    // Runners self-destruct in final_suspend. No destruction needed here.
649  
665  
650   
    /** @pre core_.try_win() returned true. */
666   
    /** @pre core_.try_win() returned true. */
651   
    void set_winner_result(T value)
667   
    void set_winner_result(T value)
652   
        noexcept(std::is_nothrow_move_constructible_v<T>)
668   
        noexcept(std::is_nothrow_move_constructible_v<T>)
653   
    {
669   
    {
654   
        result_.emplace(std::move(value));
670   
        result_.emplace(std::move(value));
655   
    }
671   
    }
656   
};
672   
};
657  
673  
658   
/** Specialization for void tasks (no result storage needed). */
674   
/** Specialization for void tasks (no result storage needed). */
659   
template<>
675   
template<>
660   
struct when_any_homogeneous_state<void>
676   
struct when_any_homogeneous_state<void>
661   
{
677   
{
662   
    when_any_core core_;
678   
    when_any_core core_;
663   
    std::vector<std::coroutine_handle<>> runner_handles_;
679   
    std::vector<std::coroutine_handle<>> runner_handles_;
664  
680  
665   
    explicit when_any_homogeneous_state(std::size_t count)
681   
    explicit when_any_homogeneous_state(std::size_t count)
666   
        : core_(count)
682   
        : core_(count)
667   
        , runner_handles_(count)
683   
        , runner_handles_(count)
668   
    {
684   
    {
669   
    }
685   
    }
670  
686  
671   
    // Runners self-destruct in final_suspend. No destruction needed here.
687   
    // Runners self-destruct in final_suspend. No destruction needed here.
672  
688  
673   
    // No set_winner_result - void tasks have no result to store
689   
    // No set_winner_result - void tasks have no result to store
674   
};
690   
};
675  
691  
676   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
692   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
677   
template<IoAwaitableRange Range>
693   
template<IoAwaitableRange Range>
678   
class when_any_homogeneous_launcher
694   
class when_any_homogeneous_launcher
679   
{
695   
{
680   
    using Awaitable = std::ranges::range_value_t<Range>;
696   
    using Awaitable = std::ranges::range_value_t<Range>;
681   
    using T = awaitable_result_t<Awaitable>;
697   
    using T = awaitable_result_t<Awaitable>;
682  
698  
683   
    Range* range_;
699   
    Range* range_;
684   
    when_any_homogeneous_state<T>* state_;
700   
    when_any_homogeneous_state<T>* state_;
685  
701  
686   
public:
702   
public:
687   
    when_any_homogeneous_launcher(
703   
    when_any_homogeneous_launcher(
688   
        Range* range,
704   
        Range* range,
689   
        when_any_homogeneous_state<T>* state)
705   
        when_any_homogeneous_state<T>* state)
690   
        : range_(range)
706   
        : range_(range)
691   
        , state_(state)
707   
        , state_(state)
692   
    {
708   
    {
693   
    }
709   
    }
694  
710  
695   
    bool await_ready() const noexcept
711   
    bool await_ready() const noexcept
696   
    {
712   
    {
697   
        return std::ranges::empty(*range_);
713   
        return std::ranges::empty(*range_);
698   
    }
714   
    }
699  
715  
700   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
716   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
701   
        destroys this object before await_suspend returns. Must not reference
717   
        destroys this object before await_suspend returns. Must not reference
702   
        `this` after dispatching begins.
718   
        `this` after dispatching begins.
703  
719  
704   
        Two-phase approach:
720   
        Two-phase approach:
705   
        1. Create all runners (safe - no dispatch yet)
721   
        1. Create all runners (safe - no dispatch yet)
706   
        2. Dispatch all runners (any may complete synchronously)
722   
        2. Dispatch all runners (any may complete synchronously)
707   
    */
723   
    */
708   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
724   
    std::coroutine_handle<> await_suspend(std::coroutine_handle<> continuation, io_env const* caller_env)
709   
    {
725   
    {
710   
        state_->core_.continuation_ = continuation;
726   
        state_->core_.continuation_ = continuation;
711   
        state_->core_.caller_env_ = caller_env;
727   
        state_->core_.caller_env_ = caller_env;
712  
728  
713   
        if(caller_env->stop_token.stop_possible())
729   
        if(caller_env->stop_token.stop_possible())
714   
        {
730   
        {
715   
            state_->core_.parent_stop_callback_.emplace(
731   
            state_->core_.parent_stop_callback_.emplace(
716   
                caller_env->stop_token,
732   
                caller_env->stop_token,
717   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
733   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
718  
734  
719   
            if(caller_env->stop_token.stop_requested())
735   
            if(caller_env->stop_token.stop_requested())
720   
                state_->core_.stop_source_.request_stop();
736   
                state_->core_.stop_source_.request_stop();
721   
        }
737   
        }
722  
738  
723   
        auto token = state_->core_.stop_source_.get_token();
739   
        auto token = state_->core_.stop_source_.get_token();
724  
740  
725   
        // Phase 1: Create all runners without dispatching.
741   
        // Phase 1: Create all runners without dispatching.
726   
        // This iterates over *range_ safely because no runners execute yet.
742   
        // This iterates over *range_ safely because no runners execute yet.
727   
        std::size_t index = 0;
743   
        std::size_t index = 0;
728   
        for(auto&& a : *range_)
744   
        for(auto&& a : *range_)
729   
        {
745   
        {
730   
            auto runner = make_when_any_runner(
746   
            auto runner = make_when_any_runner(
731   
                std::move(a), state_, index);
747   
                std::move(a), state_, index);
732  
748  
733   
            auto h = runner.release();
749   
            auto h = runner.release();
734   
            h.promise().state_ = state_;
750   
            h.promise().state_ = state_;
735   
            h.promise().index_ = index;
751   
            h.promise().index_ = index;
736   
            h.promise().env_ = io_env{caller_env->executor, token, caller_env->frame_allocator};
752   
            h.promise().env_ = io_env{caller_env->executor, token, caller_env->frame_allocator};
737  
753  
738   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
754   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
739   
            ++index;
755   
            ++index;
740   
        }
756   
        }
741  
757  
742   
        // Phase 2: Post all runners. Any may complete synchronously.
758   
        // Phase 2: Post all runners. Any may complete synchronously.
743   
        // After last post, state_ and this may be destroyed.
759   
        // After last post, state_ and this may be destroyed.
744   
        // Use raw pointer/count captured before posting.
760   
        // Use raw pointer/count captured before posting.
745   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
761   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
746   
        std::size_t count = state_->runner_handles_.size();
762   
        std::size_t count = state_->runner_handles_.size();
747   
        for(std::size_t i = 0; i < count; ++i)
763   
        for(std::size_t i = 0; i < count; ++i)
748   
            caller_env->executor.post(handles[i]);
764   
            caller_env->executor.post(handles[i]);
749  
765  
750   
        return std::noop_coroutine();
766   
        return std::noop_coroutine();
751   
    }
767   
    }
752  
768  
753   
    void await_resume() const noexcept
769   
    void await_resume() const noexcept
754   
    {
770   
    {
755   
    }
771   
    }
756   
};
772   
};
757  
773  
758   
} // namespace detail
774   
} // namespace detail
759  
775  
760   
/** Wait for the first awaitable to complete (range overload).
776   
/** Wait for the first awaitable to complete (range overload).
761  
777  
762   
    Races a range of awaitables with the same result type. Accepts any
778   
    Races a range of awaitables with the same result type. Accepts any
763   
    sized input range of IoAwaitable types, enabling use with arrays,
779   
    sized input range of IoAwaitable types, enabling use with arrays,
764   
    spans, or custom containers.
780   
    spans, or custom containers.
765  
781  
766   
    @par Suspends
782   
    @par Suspends
767   
    The calling coroutine suspends when co_await is invoked. All awaitables
783   
    The calling coroutine suspends when co_await is invoked. All awaitables
768   
    in the range are launched concurrently and execute in parallel. The
784   
    in the range are launched concurrently and execute in parallel. The
769   
    coroutine resumes only after all awaitables have completed, even though
785   
    coroutine resumes only after all awaitables have completed, even though
770   
    the winner is determined by the first to finish.
786   
    the winner is determined by the first to finish.
771  
787  
772   
    @par Completion Conditions
788   
    @par Completion Conditions
773   
    @li Winner is determined when the first awaitable completes (success or exception)
789   
    @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
790   
    @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
791   
    @li Once a winner exists, stop is requested for all remaining siblings
776   
    @li Parent coroutine resumes only after all siblings acknowledge completion
792   
    @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
793   
    @li The winner's index and result are returned; if the winner threw, the exception is rethrown
778  
794  
779   
    @par Cancellation Semantics
795   
    @par Cancellation Semantics
780   
    Cancellation is supported via stop_token propagated through the
796   
    Cancellation is supported via stop_token propagated through the
781   
    IoAwaitable protocol:
797   
    IoAwaitable protocol:
782   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
798   
    @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
799   
    @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
800   
    @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
801   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
786   
    @li Stop requests are cooperative; tasks must check and respond to them
802   
    @li Stop requests are cooperative; tasks must check and respond to them
787  
803  
788   
    @par Concurrency/Overlap
804   
    @par Concurrency/Overlap
789   
    All awaitables are launched concurrently before any can complete.
805   
    All awaitables are launched concurrently before any can complete.
790   
    The launcher iterates through the range, starting each task on the
806   
    The launcher iterates through the range, starting each task on the
791   
    caller's executor. Tasks may execute in parallel on multi-threaded
807   
    caller's executor. Tasks may execute in parallel on multi-threaded
792   
    executors or interleave on single-threaded executors. There is no
808   
    executors or interleave on single-threaded executors. There is no
793   
    guaranteed ordering of task completion.
809   
    guaranteed ordering of task completion.
794  
810  
795   
    @par Notable Error Conditions
811   
    @par Notable Error Conditions
796   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
812   
    @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
813   
    @li Winner exception: if the winning task threw, that exception is rethrown
798   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
814   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
799   
    @li Cancellation: tasks may complete via cancellation without throwing
815   
    @li Cancellation: tasks may complete via cancellation without throwing
800  
816  
801   
    @par Example
817   
    @par Example
802   
    @code
818   
    @code
803   
    task<void> example() {
819   
    task<void> example() {
804   
        std::array<task<Response>, 3> requests = {
820   
        std::array<task<Response>, 3> requests = {
805   
            fetch_from_server(0),
821   
            fetch_from_server(0),
806   
            fetch_from_server(1),
822   
            fetch_from_server(1),
807   
            fetch_from_server(2)
823   
            fetch_from_server(2)
808   
        };
824   
        };
809  
825  
810   
        auto [index, response] = co_await when_any(std::move(requests));
826   
        auto [index, response] = co_await when_any(std::move(requests));
811   
    }
827   
    }
812   
    @endcode
828   
    @endcode
813  
829  
814   
    @par Example with Vector
830   
    @par Example with Vector
815   
    @code
831   
    @code
816   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
832   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
817   
        std::vector<task<Response>> requests;
833   
        std::vector<task<Response>> requests;
818   
        for (auto const& server : servers)
834   
        for (auto const& server : servers)
819   
            requests.push_back(fetch_from(server));
835   
            requests.push_back(fetch_from(server));
820  
836  
821   
        auto [index, response] = co_await when_any(std::move(requests));
837   
        auto [index, response] = co_await when_any(std::move(requests));
822   
        co_return response;
838   
        co_return response;
823   
    }
839   
    }
824   
    @endcode
840   
    @endcode
825  
841  
826   
    @tparam R Range type satisfying IoAwaitableRange.
842   
    @tparam R Range type satisfying IoAwaitableRange.
827   
    @param awaitables Range of awaitables to race concurrently (must not be empty).
843   
    @param awaitables Range of awaitables to race concurrently (must not be empty).
828   
    @return A task yielding a pair of (winner_index, result).
844   
    @return A task yielding a pair of (winner_index, result).
829  
845  
830   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
846   
    @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.
847   
    @throws Rethrows the winner's exception if the winning task threw an exception.
832  
848  
833   
    @par Remarks
849   
    @par Remarks
834   
    Elements are moved from the range; for lvalue ranges, the original
850   
    Elements are moved from the range; for lvalue ranges, the original
835   
    container will have moved-from elements after this call. The range
851   
    container will have moved-from elements after this call. The range
836   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
852   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
837   
    the variadic overload, no variant wrapper is needed since all tasks
853   
    the variadic overload, no variant wrapper is needed since all tasks
838   
    share the same return type.
854   
    share the same return type.
839  
855  
840   
    @see when_any, IoAwaitableRange
856   
    @see when_any, IoAwaitableRange
841   
*/
857   
*/
842   
template<IoAwaitableRange R>
858   
template<IoAwaitableRange R>
843 - 
    requires (!std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>)
859 + 
    requires (!std::is_void_v<detail::awaitable_result_t<std::ranges::range_value_t<R>>>)
844   
[[nodiscard]] auto when_any(R&& awaitables)
860   
[[nodiscard]] auto when_any(R&& awaitables)
845 - 
    -> task<std::pair<std::size_t, awaitable_result_t<std::ranges::range_value_t<R>>>>
861 + 
    -> task<std::pair<std::size_t, detail::awaitable_result_t<std::ranges::range_value_t<R>>>>
846   
{
862   
{
847   
    using Awaitable = std::ranges::range_value_t<R>;
863   
    using Awaitable = std::ranges::range_value_t<R>;
848 - 
    using T = awaitable_result_t<Awaitable>;
864 + 
    using T = detail::awaitable_result_t<Awaitable>;
849   
    using result_type = std::pair<std::size_t, T>;
865   
    using result_type = std::pair<std::size_t, T>;
850   
    using OwnedRange = std::remove_cvref_t<R>;
866   
    using OwnedRange = std::remove_cvref_t<R>;
851  
867  
852   
    auto count = std::ranges::size(awaitables);
868   
    auto count = std::ranges::size(awaitables);
853   
    if(count == 0)
869   
    if(count == 0)
854   
        throw std::invalid_argument("when_any requires at least one awaitable");
870   
        throw std::invalid_argument("when_any requires at least one awaitable");
855  
871  
856   
    // Move/copy range onto coroutine frame to ensure lifetime
872   
    // Move/copy range onto coroutine frame to ensure lifetime
857   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
873   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
858  
874  
859   
    detail::when_any_homogeneous_state<T> state(count);
875   
    detail::when_any_homogeneous_state<T> state(count);
860  
876  
861   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
877   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
862  
878  
863   
    if(state.core_.winner_exception_)
879   
    if(state.core_.winner_exception_)
864   
        std::rethrow_exception(state.core_.winner_exception_);
880   
        std::rethrow_exception(state.core_.winner_exception_);
865  
881  
866   
    co_return result_type{state.core_.winner_index_, std::move(*state.result_)};
882   
    co_return result_type{state.core_.winner_index_, std::move(*state.result_)};
867   
}
883   
}
868  
884  
869   
/** Wait for the first awaitable to complete (void range overload).
885   
/** Wait for the first awaitable to complete (void range overload).
870  
886  
871   
    Races a range of void-returning awaitables. Since void awaitables have
887   
    Races a range of void-returning awaitables. Since void awaitables have
872   
    no result value, only the winner's index is returned.
888   
    no result value, only the winner's index is returned.
873  
889  
874   
    @par Suspends
890   
    @par Suspends
875   
    The calling coroutine suspends when co_await is invoked. All awaitables
891   
    The calling coroutine suspends when co_await is invoked. All awaitables
876   
    in the range are launched concurrently and execute in parallel. The
892   
    in the range are launched concurrently and execute in parallel. The
877   
    coroutine resumes only after all awaitables have completed, even though
893   
    coroutine resumes only after all awaitables have completed, even though
878   
    the winner is determined by the first to finish.
894   
    the winner is determined by the first to finish.
879  
895  
880   
    @par Completion Conditions
896   
    @par Completion Conditions
881   
    @li Winner is determined when the first awaitable completes (success or exception)
897   
    @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
898   
    @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
899   
    @li Once a winner exists, stop is requested for all remaining siblings
884   
    @li Parent coroutine resumes only after all siblings acknowledge completion
900   
    @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
901   
    @li The winner's index is returned; if the winner threw, the exception is rethrown
886  
902  
887   
    @par Cancellation Semantics
903   
    @par Cancellation Semantics
888   
    Cancellation is supported via stop_token propagated through the
904   
    Cancellation is supported via stop_token propagated through the
889   
    IoAwaitable protocol:
905   
    IoAwaitable protocol:
890   
    @li Each child awaitable receives a stop_token derived from a shared stop_source
906   
    @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
907   
    @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
908   
    @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
909   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
894   
    @li Stop requests are cooperative; tasks must check and respond to them
910   
    @li Stop requests are cooperative; tasks must check and respond to them
895  
911  
896   
    @par Concurrency/Overlap
912   
    @par Concurrency/Overlap
897   
    All awaitables are launched concurrently before any can complete.
913   
    All awaitables are launched concurrently before any can complete.
898   
    The launcher iterates through the range, starting each task on the
914   
    The launcher iterates through the range, starting each task on the
899   
    caller's executor. Tasks may execute in parallel on multi-threaded
915   
    caller's executor. Tasks may execute in parallel on multi-threaded
900   
    executors or interleave on single-threaded executors. There is no
916   
    executors or interleave on single-threaded executors. There is no
901   
    guaranteed ordering of task completion.
917   
    guaranteed ordering of task completion.
902  
918  
903   
    @par Notable Error Conditions
919   
    @par Notable Error Conditions
904   
    @li Empty range: throws std::invalid_argument immediately (not via co_return)
920   
    @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
921   
    @li Winner exception: if the winning task threw, that exception is rethrown
906   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
922   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
907   
    @li Cancellation: tasks may complete via cancellation without throwing
923   
    @li Cancellation: tasks may complete via cancellation without throwing
908  
924  
909   
    @par Example
925   
    @par Example
910   
    @code
926   
    @code
911   
    task<void> example() {
927   
    task<void> example() {
912   
        std::vector<task<void>> tasks;
928   
        std::vector<task<void>> tasks;
913   
        for (int i = 0; i < 5; ++i)
929   
        for (int i = 0; i < 5; ++i)
914   
            tasks.push_back(background_work(i));
930   
            tasks.push_back(background_work(i));
915  
931  
916   
        std::size_t winner = co_await when_any(std::move(tasks));
932   
        std::size_t winner = co_await when_any(std::move(tasks));
917   
        // winner is the index of the first task to complete
933   
        // winner is the index of the first task to complete
918   
    }
934   
    }
919   
    @endcode
935   
    @endcode
920  
936  
921   
    @par Example with Timeout
937   
    @par Example with Timeout
922   
    @code
938   
    @code
923   
    task<void> with_timeout() {
939   
    task<void> with_timeout() {
924   
        std::vector<task<void>> tasks;
940   
        std::vector<task<void>> tasks;
925   
        tasks.push_back(long_running_operation());
941   
        tasks.push_back(long_running_operation());
926   
        tasks.push_back(delay(std::chrono::seconds(5)));
942   
        tasks.push_back(delay(std::chrono::seconds(5)));
927  
943  
928   
        std::size_t winner = co_await when_any(std::move(tasks));
944   
        std::size_t winner = co_await when_any(std::move(tasks));
929   
        if (winner == 1) {
945   
        if (winner == 1) {
930   
            // Timeout occurred
946   
            // Timeout occurred
931   
        }
947   
        }
932   
    }
948   
    }
933   
    @endcode
949   
    @endcode
934  
950  
935   
    @tparam R Range type satisfying IoAwaitableRange with void result.
951   
    @tparam R Range type satisfying IoAwaitableRange with void result.
936   
    @param awaitables Range of void awaitables to race concurrently (must not be empty).
952   
    @param awaitables Range of void awaitables to race concurrently (must not be empty).
937   
    @return A task yielding the winner's index (zero-based).
953   
    @return A task yielding the winner's index (zero-based).
938  
954  
939   
    @throws std::invalid_argument if range is empty (thrown before coroutine suspends).
955   
    @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.
956   
    @throws Rethrows the winner's exception if the winning task threw an exception.
941  
957  
942   
    @par Remarks
958   
    @par Remarks
943   
    Elements are moved from the range; for lvalue ranges, the original
959   
    Elements are moved from the range; for lvalue ranges, the original
944   
    container will have moved-from elements after this call. The range
960   
    container will have moved-from elements after this call. The range
945   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
961   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
946   
    the non-void overload, no result storage is needed since void tasks
962   
    the non-void overload, no result storage is needed since void tasks
947   
    produce no value.
963   
    produce no value.
948  
964  
949   
    @see when_any, IoAwaitableRange
965   
    @see when_any, IoAwaitableRange
950   
*/
966   
*/
951   
template<IoAwaitableRange R>
967   
template<IoAwaitableRange R>
952 - 
    requires std::is_void_v<awaitable_result_t<std::ranges::range_value_t<R>>>
968 + 
    requires std::is_void_v<detail::awaitable_result_t<std::ranges::range_value_t<R>>>
953   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
969   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
954   
{
970   
{
955   
    using OwnedRange = std::remove_cvref_t<R>;
971   
    using OwnedRange = std::remove_cvref_t<R>;
956  
972  
957   
    auto count = std::ranges::size(awaitables);
973   
    auto count = std::ranges::size(awaitables);
958   
    if(count == 0)
974   
    if(count == 0)
959   
        throw std::invalid_argument("when_any requires at least one awaitable");
975   
        throw std::invalid_argument("when_any requires at least one awaitable");
960  
976  
961   
    // Move/copy range onto coroutine frame to ensure lifetime
977   
    // Move/copy range onto coroutine frame to ensure lifetime
962   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
978   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
963  
979  
964   
    detail::when_any_homogeneous_state<void> state(count);
980   
    detail::when_any_homogeneous_state<void> state(count);
965  
981  
966   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
982   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
967  
983  
968   
    if(state.core_.winner_exception_)
984   
    if(state.core_.winner_exception_)
969   
        std::rethrow_exception(state.core_.winner_exception_);
985   
        std::rethrow_exception(state.core_.winner_exception_);
970  
986  
971   
    co_return state.core_.winner_index_;
987   
    co_return state.core_.winner_index_;
972   
}
988   
}
973  
989  
974   
} // namespace capy
990   
} // namespace capy
975   
} // namespace boost
991   
} // namespace boost
976  
992  
977   
#endif
993   
#endif