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 - 
   TYPE DEDUPLICATION:
65 + 
   POSITIONAL VARIANT:
66   
   -------------------
66   
   -------------------
67 - 
   std::variant requires unique alternative types. Since when_any can race
67 + 
   The variadic overload returns a std::variant with one alternative per
68 - 
   tasks with identical return types (e.g., three task<int>), we must
68 + 
   input task, preserving positional correspondence. Use .index() on
69 - 
   deduplicate types before constructing the variant.
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 - 
     - Deduplicated variant: std::variant<int, string>
73 + 
     - Result variant: std::variant<int, string, int>
74 - 
     - Return: pair<size_t, variant<int, string>>
74 + 
     - variant.index() tells you which task won (0, 1, or 2)
75 -
 
76 - 
   The winner_index tells you which task won (0, 1, or 2), while the variant
 
77 - 
   holds the result. Use the index to determine how to interpret the variant.
 
78  
75  
79   
   VOID HANDLING:
76   
   VOID HANDLING:
80   
   --------------
77   
   --------------
81 - 
   void tasks contribute std::monostate to the variant (then deduplicated).
78 + 
   void tasks contribute std::monostate to the variant.
82 - 
   All-void tasks result in: pair<size_t, variant<monostate>>
79 + 
   All-void tasks result in: variant<monostate, monostate, monostate>
83  
80  
84   
   MEMORY MODEL:
81   
   MEMORY MODEL:
85   
   -------------
82   
   -------------
86   
   Synchronization chain from winner's write to parent's read:
83   
   Synchronization chain from winner's write to parent's read:
87  
84  
88   
   1. Winner thread writes result_/winner_exception_ (non-atomic)
85   
   1. Winner thread writes result_/winner_exception_ (non-atomic)
89   
   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_
90   
   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()
91   
      → fetch_sub(acq_rel) on remaining_count_, observing count becomes 0
88   
      → fetch_sub(acq_rel) on remaining_count_, observing count becomes 0
92   
   4. Last task returns caller_ex_.dispatch(continuation_) via symmetric transfer
89   
   4. Last task returns caller_ex_.dispatch(continuation_) via symmetric transfer
93   
   5. Parent coroutine resumes and reads result_/winner_exception_
90   
   5. Parent coroutine resumes and reads result_/winner_exception_
94  
91  
95   
   Synchronization analysis:
92   
   Synchronization analysis:
96   
   - All fetch_sub operations on remaining_count_ form a release sequence
93   
   - All fetch_sub operations on remaining_count_ form a release sequence
97   
   - Winner's fetch_sub releases; subsequent fetch_sub operations participate
94   
   - Winner's fetch_sub releases; subsequent fetch_sub operations participate
98   
     in the modification order of remaining_count_
95   
     in the modification order of remaining_count_
99   
   - 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
100   
     modification order, establishing happens-before from winner's writes
97   
     modification order, establishing happens-before from winner's writes
101   
   - Executor dispatch() is expected to provide queue-based synchronization
98   
   - Executor dispatch() is expected to provide queue-based synchronization
102   
     (release-on-post, acquire-on-execute) completing the chain to parent
99   
     (release-on-post, acquire-on-execute) completing the chain to parent
103   
   - Even inline executors work (same thread = sequenced-before)
100   
   - Even inline executors work (same thread = sequenced-before)
104  
101  
105   
   Alternative considered: Adding winner_ready_ atomic (set with release after
102   
   Alternative considered: Adding winner_ready_ atomic (set with release after
106   
   storing winner data, acquired before reading) would make synchronization
103   
   storing winner data, acquired before reading) would make synchronization
107   
   self-contained and not rely on executor implementation details. Current
104   
   self-contained and not rely on executor implementation details. Current
108   
   approach is correct but requires careful reasoning about release sequences
105   
   approach is correct but requires careful reasoning about release sequences
109   
   and executor behavior.
106   
   and executor behavior.
110  
107  
111   
   EXCEPTION SEMANTICS:
108   
   EXCEPTION SEMANTICS:
112   
   --------------------
109   
   --------------------
113   
   Unlike when_all (which captures first exception, discards others), when_any
110   
   Unlike when_all (which captures first exception, discards others), when_any
114   
   treats exceptions as valid completions. If the winning task threw, that
111   
   treats exceptions as valid completions. If the winning task threw, that
115   
   exception is rethrown. Exceptions from non-winners are silently discarded.
112   
   exception is rethrown. Exceptions from non-winners are silently discarded.
116   
*/
113   
*/
117  
114  
118   
namespace boost {
115   
namespace boost {
119   
namespace capy {
116   
namespace capy {
120  
117  
121   
namespace detail {
118   
namespace detail {
122  
119  
123   
/** Convert void to monostate for variant storage.
120   
/** Convert void to monostate for variant storage.
124  
121  
125   
    std::variant<void, ...> is ill-formed, so void tasks contribute
122   
    std::variant<void, ...> is ill-formed, so void tasks contribute
126   
    std::monostate to the result variant instead. Non-void types
123   
    std::monostate to the result variant instead. Non-void types
127   
    pass through unchanged.
124   
    pass through unchanged.
128  
125  
129   
    @tparam T The type to potentially convert (void becomes monostate).
126   
    @tparam T The type to potentially convert (void becomes monostate).
130   
*/
127   
*/
131   
template<typename T>
128   
template<typename T>
132   
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>;
133  
130  
134 - 
// Type deduplication: std::variant requires unique alternative types.
131 + 
// Result variant: one alternative per task, preserving positional
135 - 
// Fold left over the type list, appending each type only if not already present.
132 + 
// correspondence. Use .index() to identify which task won.
136 - 
template<typename Variant, typename T>
133 + 
// void results become monostate.
137 - 
struct variant_append_if_unique;
 
138 -
 
139 - 
template<typename... Vs, typename T>
 
140 - 
struct variant_append_if_unique<std::variant<Vs...>, T>
 
141 - 
{
 
142 - 
    using type = std::conditional_t<
 
143 - 
        (std::is_same_v<T, Vs> || ...),
 
144 - 
        std::variant<Vs...>,
 
145 - 
        std::variant<Vs..., T>>;
 
146 - 
};
 
147 -
 
148 - 
template<typename Accumulated, typename... Remaining>
 
149 - 
struct deduplicate_impl;
 
150 -
 
151 - 
template<typename Accumulated>
 
152 - 
struct deduplicate_impl<Accumulated>
 
153 - 
{
 
154 - 
    using type = Accumulated;
 
155 - 
};
 
156 -
 
157 - 
template<typename Accumulated, typename T, typename... Rest>
 
158 - 
struct deduplicate_impl<Accumulated, T, Rest...>
 
159 - 
{
 
160 - 
    using next = typename variant_append_if_unique<Accumulated, T>::type;
 
161 - 
    using type = typename deduplicate_impl<next, Rest...>::type;
 
162 - 
};
 
163 -
 
164 - 
// Deduplicated variant; void types become monostate before deduplication
 
165 - 
template<typename T0, typename... Ts>
 
166 - 
using unique_variant_t = typename deduplicate_impl<
 
167 - 
    std::variant<void_to_monostate_t<T0>>,
 
168 - 
    void_to_monostate_t<Ts>...>::type;
 
169 -
 
170 - 
// Result: (winner_index, deduplicated_variant). Use index to disambiguate
 
171 - 
// when multiple tasks share the same return type.
 
172   
template<typename T0, typename... Ts>
134   
template<typename T0, typename... Ts>
173 - 
using when_any_result_t = std::pair<std::size_t, unique_variant_t<T0, Ts...>>;
135 + 
using when_any_variant_t = std::variant<void_to_monostate_t<T0>, void_to_monostate_t<Ts>...>;
174  
136  
175   
/** Core shared state for when_any operations.
137   
/** Core shared state for when_any operations.
176  
138  
177   
    Contains all members and methods common to both heterogeneous (variadic)
139   
    Contains all members and methods common to both heterogeneous (variadic)
178   
    and homogeneous (range) when_any implementations. State classes embed
140   
    and homogeneous (range) when_any implementations. State classes embed
179   
    this via composition to avoid CRTP destructor ordering issues.
141   
    this via composition to avoid CRTP destructor ordering issues.
180  
142  
181   
    @par Thread Safety
143   
    @par Thread Safety
182   
    Atomic operations protect winner selection and completion count.
144   
    Atomic operations protect winner selection and completion count.
183   
*/
145   
*/
184   
struct when_any_core
146   
struct when_any_core
185   
{
147   
{
186   
    std::atomic<std::size_t> remaining_count_;
148   
    std::atomic<std::size_t> remaining_count_;
187   
    std::size_t winner_index_{0};
149   
    std::size_t winner_index_{0};
188   
    std::exception_ptr winner_exception_;
150   
    std::exception_ptr winner_exception_;
189   
    std::stop_source stop_source_;
151   
    std::stop_source stop_source_;
190  
152  
191   
    // Bridges parent's stop token to our stop_source
153   
    // Bridges parent's stop token to our stop_source
192   
    struct stop_callback_fn
154   
    struct stop_callback_fn
193   
    {
155   
    {
194   
        std::stop_source* source_;
156   
        std::stop_source* source_;
195   
        void operator()() const noexcept { source_->request_stop(); }
157   
        void operator()() const noexcept { source_->request_stop(); }
196   
    };
158   
    };
197   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
159   
    using stop_callback_t = std::stop_callback<stop_callback_fn>;
198   
    std::optional<stop_callback_t> parent_stop_callback_;
160   
    std::optional<stop_callback_t> parent_stop_callback_;
199  
161  
200   
    std::coroutine_handle<> continuation_;
162   
    std::coroutine_handle<> continuation_;
201   
    io_env const* caller_env_ = nullptr;
163   
    io_env const* caller_env_ = nullptr;
202  
164  
203   
    // 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)
204   
    std::atomic<bool> has_winner_{false};
166   
    std::atomic<bool> has_winner_{false};
205  
167  
206   
    explicit when_any_core(std::size_t count) noexcept
168   
    explicit when_any_core(std::size_t count) noexcept
207   
        : remaining_count_(count)
169   
        : remaining_count_(count)
208   
    {
170   
    {
209   
    }
171   
    }
210  
172  
211   
    /** Atomically claim winner status; exactly one task succeeds. */
173   
    /** Atomically claim winner status; exactly one task succeeds. */
212   
    bool try_win(std::size_t index) noexcept
174   
    bool try_win(std::size_t index) noexcept
213   
    {
175   
    {
214   
        bool expected = false;
176   
        bool expected = false;
215   
        if(has_winner_.compare_exchange_strong(
177   
        if(has_winner_.compare_exchange_strong(
216   
            expected, true, std::memory_order_acq_rel))
178   
            expected, true, std::memory_order_acq_rel))
217   
        {
179   
        {
218   
            winner_index_ = index;
180   
            winner_index_ = index;
219   
            stop_source_.request_stop();
181   
            stop_source_.request_stop();
220   
            return true;
182   
            return true;
221   
        }
183   
        }
222   
        return false;
184   
        return false;
223   
    }
185   
    }
224  
186  
225   
    /** @pre try_win() returned true. */
187   
    /** @pre try_win() returned true. */
226   
    void set_winner_exception(std::exception_ptr ep) noexcept
188   
    void set_winner_exception(std::exception_ptr ep) noexcept
227   
    {
189   
    {
228   
        winner_exception_ = ep;
190   
        winner_exception_ = ep;
229   
    }
191   
    }
230  
192  
231   
    // Runners signal completion directly via final_suspend; no member function needed.
193   
    // Runners signal completion directly via final_suspend; no member function needed.
232   
};
194   
};
233  
195  
234   
/** Shared state for heterogeneous when_any operation.
196   
/** Shared state for heterogeneous when_any operation.
235  
197  
236   
    Coordinates winner selection, result storage, and completion tracking
198   
    Coordinates winner selection, result storage, and completion tracking
237   
    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
238   
    when_any_core for shared functionality.
200   
    when_any_core for shared functionality.
239  
201  
240   
    @par Lifetime
202   
    @par Lifetime
241   
    Allocated on the parent coroutine's frame, outlives all runners.
203   
    Allocated on the parent coroutine's frame, outlives all runners.
242  
204  
243   
    @tparam T0 First task's result type.
205   
    @tparam T0 First task's result type.
244   
    @tparam Ts Remaining tasks' result types.
206   
    @tparam Ts Remaining tasks' result types.
245   
*/
207   
*/
246   
template<typename T0, typename... Ts>
208   
template<typename T0, typename... Ts>
247   
struct when_any_state
209   
struct when_any_state
248   
{
210   
{
249   
    static constexpr std::size_t task_count = 1 + sizeof...(Ts);
211   
    static constexpr std::size_t task_count = 1 + sizeof...(Ts);
250 - 
    using variant_type = unique_variant_t<T0, Ts...>;
212 + 
    using variant_type = when_any_variant_t<T0, Ts...>;
251  
213  
252   
    when_any_core core_;
214   
    when_any_core core_;
253   
    std::optional<variant_type> result_;
215   
    std::optional<variant_type> result_;
254   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
216   
    std::array<std::coroutine_handle<>, task_count> runner_handles_{};
255  
217  
256   
    when_any_state()
218   
    when_any_state()
257   
        : core_(task_count)
219   
        : core_(task_count)
258   
    {
220   
    {
259   
    }
221   
    }
260  
222  
261   
    // Runners self-destruct in final_suspend. No destruction needed here.
223   
    // Runners self-destruct in final_suspend. No destruction needed here.
262  
224  
263   
    /** @pre core_.try_win() returned true.
225   
    /** @pre core_.try_win() returned true.
264 - 
        @note Uses in_place_type (not index) because variant is deduplicated.
226 + 
        @note Uses in_place_index (not type) for positional variant access.
265   
    */
227   
    */
266 - 
    template<typename T>
228 + 
    template<std::size_t I, typename T>
267   
    void set_winner_result(T value)
229   
    void set_winner_result(T value)
268   
        noexcept(std::is_nothrow_move_constructible_v<T>)
230   
        noexcept(std::is_nothrow_move_constructible_v<T>)
269   
    {
231   
    {
270 - 
        result_.emplace(std::in_place_type<T>, std::move(value));
232 + 
        result_.emplace(std::in_place_index<I>, std::move(value));
271   
    }
233   
    }
272  
234  
273   
    /** @pre core_.try_win() returned true. */
235   
    /** @pre core_.try_win() returned true. */
 
236 + 
    template<std::size_t I>
274   
    void set_winner_void() noexcept
237   
    void set_winner_void() noexcept
275   
    {
238   
    {
276 - 
        result_.emplace(std::in_place_type<std::monostate>, std::monostate{});
239 + 
        result_.emplace(std::in_place_index<I>, std::monostate{});
277   
    }
240   
    }
278   
};
241   
};
279  
242  
280   
/** Wrapper coroutine that runs a single child task for when_any.
243   
/** Wrapper coroutine that runs a single child task for when_any.
281  
244  
282   
    Propagates executor/stop_token to the child, attempts to claim winner
245   
    Propagates executor/stop_token to the child, attempts to claim winner
283   
    status on completion, and signals completion for cleanup coordination.
246   
    status on completion, and signals completion for cleanup coordination.
284  
247  
285   
    @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).
286   
*/
249   
*/
287   
template<typename StateType>
250   
template<typename StateType>
288   
struct when_any_runner
251   
struct when_any_runner
289   
{
252   
{
290   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
253   
    struct promise_type // : frame_allocating_base  // DISABLED FOR TESTING
291   
    {
254   
    {
292   
        StateType* state_ = nullptr;
255   
        StateType* state_ = nullptr;
293   
        std::size_t index_ = 0;
256   
        std::size_t index_ = 0;
294   
        io_env env_;
257   
        io_env env_;
295  
258  
296   
        when_any_runner get_return_object() noexcept
259   
        when_any_runner get_return_object() noexcept
297   
        {
260   
        {
298   
            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));
299   
        }
262   
        }
300  
263  
301   
        // Starts suspended; launcher sets up state/ex/token then resumes
264   
        // Starts suspended; launcher sets up state/ex/token then resumes
302   
        std::suspend_always initial_suspend() noexcept
265   
        std::suspend_always initial_suspend() noexcept
303   
        {
266   
        {
304   
            return {};
267   
            return {};
305   
        }
268   
        }
306  
269  
307   
        auto final_suspend() noexcept
270   
        auto final_suspend() noexcept
308   
        {
271   
        {
309   
            struct awaiter
272   
            struct awaiter
310   
            {
273   
            {
311   
                promise_type* p_;
274   
                promise_type* p_;
312   
                bool await_ready() const noexcept { return false; }
275   
                bool await_ready() const noexcept { return false; }
313   
                std::coroutine_handle<> await_suspend(std::coroutine_handle<> h) noexcept
276   
                std::coroutine_handle<> await_suspend(std::coroutine_handle<> h) noexcept
314   
                {
277   
                {
315   
                    // Extract everything needed before self-destruction.
278   
                    // Extract everything needed before self-destruction.
316   
                    auto& core = p_->state_->core_;
279   
                    auto& core = p_->state_->core_;
317   
                    auto* counter = &core.remaining_count_;
280   
                    auto* counter = &core.remaining_count_;
318   
                    auto* caller_env = core.caller_env_;
281   
                    auto* caller_env = core.caller_env_;
319   
                    auto cont = core.continuation_;
282   
                    auto cont = core.continuation_;
320  
283  
321   
                    h.destroy();
284   
                    h.destroy();
322  
285  
323   
                    // If last runner, dispatch parent for symmetric transfer.
286   
                    // If last runner, dispatch parent for symmetric transfer.
324   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
287   
                    auto remaining = counter->fetch_sub(1, std::memory_order_acq_rel);
325   
                    if(remaining == 1)
288   
                    if(remaining == 1)
326   
                        return caller_env->executor.dispatch(cont);
289   
                        return caller_env->executor.dispatch(cont);
327   
                    return std::noop_coroutine();
290   
                    return std::noop_coroutine();
328   
                }
291   
                }
329   
                void await_resume() const noexcept {}
292   
                void await_resume() const noexcept {}
330   
            };
293   
            };
331   
            return awaiter{this};
294   
            return awaiter{this};
332   
        }
295   
        }
333  
296  
334   
        void return_void() noexcept {}
297   
        void return_void() noexcept {}
335  
298  
336   
        // Exceptions are valid completions in when_any (unlike when_all)
299   
        // Exceptions are valid completions in when_any (unlike when_all)
337   
        void unhandled_exception()
300   
        void unhandled_exception()
338   
        {
301   
        {
339   
            if(state_->core_.try_win(index_))
302   
            if(state_->core_.try_win(index_))
340   
                state_->core_.set_winner_exception(std::current_exception());
303   
                state_->core_.set_winner_exception(std::current_exception());
341   
        }
304   
        }
342  
305  
343   
        /** Injects executor and stop token into child awaitables. */
306   
        /** Injects executor and stop token into child awaitables. */
344   
        template<class Awaitable>
307   
        template<class Awaitable>
345   
        struct transform_awaiter
308   
        struct transform_awaiter
346   
        {
309   
        {
347   
            std::decay_t<Awaitable> a_;
310   
            std::decay_t<Awaitable> a_;
348   
            promise_type* p_;
311   
            promise_type* p_;
349  
312  
350   
            bool await_ready() { return a_.await_ready(); }
313   
            bool await_ready() { return a_.await_ready(); }
351   
            auto await_resume() { return a_.await_resume(); }
314   
            auto await_resume() { return a_.await_resume(); }
352  
315  
353   
            template<class Promise>
316   
            template<class Promise>
354   
            auto await_suspend(std::coroutine_handle<Promise> h)
317   
            auto await_suspend(std::coroutine_handle<Promise> h)
355   
            {
318   
            {
356   
#ifdef _MSC_VER
319   
#ifdef _MSC_VER
357   
                using R = decltype(a_.await_suspend(h, &p_->env_));
320   
                using R = decltype(a_.await_suspend(h, &p_->env_));
358   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
321   
                if constexpr (std::is_same_v<R, std::coroutine_handle<>>)
359   
                    a_.await_suspend(h, &p_->env_).resume();
322   
                    a_.await_suspend(h, &p_->env_).resume();
360   
                else
323   
                else
361   
                    return a_.await_suspend(h, &p_->env_);
324   
                    return a_.await_suspend(h, &p_->env_);
362   
#else
325   
#else
363   
                return a_.await_suspend(h, &p_->env_);
326   
                return a_.await_suspend(h, &p_->env_);
364   
#endif
327   
#endif
365   
            }
328   
            }
366   
        };
329   
        };
367  
330  
368   
        template<class Awaitable>
331   
        template<class Awaitable>
369   
        auto await_transform(Awaitable&& a)
332   
        auto await_transform(Awaitable&& a)
370   
        {
333   
        {
371   
            using A = std::decay_t<Awaitable>;
334   
            using A = std::decay_t<Awaitable>;
372   
            if constexpr (IoAwaitable<A>)
335   
            if constexpr (IoAwaitable<A>)
373   
            {
336   
            {
374   
                return transform_awaiter<Awaitable>{
337   
                return transform_awaiter<Awaitable>{
375   
                    std::forward<Awaitable>(a), this};
338   
                    std::forward<Awaitable>(a), this};
376   
            }
339   
            }
377   
            else
340   
            else
378   
            {
341   
            {
379   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
342   
                static_assert(sizeof(A) == 0, "requires IoAwaitable");
380   
            }
343   
            }
381   
        }
344   
        }
382   
    };
345   
    };
383  
346  
384   
    std::coroutine_handle<promise_type> h_;
347   
    std::coroutine_handle<promise_type> h_;
385  
348  
386   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
349   
    explicit when_any_runner(std::coroutine_handle<promise_type> h) noexcept
387   
        : h_(h)
350   
        : h_(h)
388   
    {
351   
    {
389   
    }
352   
    }
390  
353  
391   
    // Enable move for all clang versions - some versions need it
354   
    // Enable move for all clang versions - some versions need it
392   
    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)) {}
393  
356  
394   
    // Non-copyable
357   
    // Non-copyable
395   
    when_any_runner(when_any_runner const&) = delete;
358   
    when_any_runner(when_any_runner const&) = delete;
396   
    when_any_runner& operator=(when_any_runner const&) = delete;
359   
    when_any_runner& operator=(when_any_runner const&) = delete;
397   
    when_any_runner& operator=(when_any_runner&&) = delete;
360   
    when_any_runner& operator=(when_any_runner&&) = delete;
398  
361  
399   
    auto release() noexcept
362   
    auto release() noexcept
400   
    {
363   
    {
401   
        return std::exchange(h_, nullptr);
364   
        return std::exchange(h_, nullptr);
402   
    }
365   
    }
403   
};
366   
};
404  
367  
405 - 
/** Wraps a child awaitable, attempts to claim winner on completion.
368 + 
/** Indexed overload for heterogeneous when_any (compile-time index).
 
369 +
 
370 + 
    Uses compile-time index I for variant construction via in_place_index.
 
371 + 
    Called from when_any_launcher::launch_one<I>().
 
372 + 
*/
 
373 + 
template<std::size_t I, IoAwaitable Awaitable, typename StateType>
 
374 + 
when_any_runner<StateType>
 
375 + 
make_when_any_runner(Awaitable inner, StateType* state)
 
376 + 
{
 
377 + 
    using T = awaitable_result_t<Awaitable>;
 
378 + 
    if constexpr (std::is_void_v<T>)
 
379 + 
    {
 
380 + 
        co_await std::move(inner);
 
381 + 
        if(state->core_.try_win(I))
 
382 + 
            state->template set_winner_void<I>();
 
383 + 
    }
 
384 + 
    else
 
385 + 
    {
 
386 + 
        auto result = co_await std::move(inner);
 
387 + 
        if(state->core_.try_win(I))
 
388 + 
        {
 
389 + 
            try
 
390 + 
            {
 
391 + 
                state->template set_winner_result<I>(std::move(result));
 
392 + 
            }
 
393 + 
            catch(...)
 
394 + 
            {
 
395 + 
                state->core_.set_winner_exception(std::current_exception());
 
396 + 
            }
 
397 + 
        }
 
398 + 
    }
 
399 + 
}
 
400 +
 
401 + 
/** Runtime-index overload for homogeneous when_any (range path).
406  
402  
407   
    Uses requires-expressions to detect state capabilities:
403   
    Uses requires-expressions to detect state capabilities:
408   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
404   
    - set_winner_void(): for heterogeneous void tasks (stores monostate)
409   
    - set_winner_result(): for non-void tasks
405   
    - set_winner_result(): for non-void tasks
410   
    - Neither: for homogeneous void tasks (no result storage)
406   
    - Neither: for homogeneous void tasks (no result storage)
411   
*/
407   
*/
412   
template<IoAwaitable Awaitable, typename StateType>
408   
template<IoAwaitable Awaitable, typename StateType>
413   
when_any_runner<StateType>
409   
when_any_runner<StateType>
414   
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)
415   
{
411   
{
416   
    using T = awaitable_result_t<Awaitable>;
412   
    using T = awaitable_result_t<Awaitable>;
417   
    if constexpr (std::is_void_v<T>)
413   
    if constexpr (std::is_void_v<T>)
418   
    {
414   
    {
419   
        co_await std::move(inner);
415   
        co_await std::move(inner);
420   
        if(state->core_.try_win(index))
416   
        if(state->core_.try_win(index))
421 - 
            // Heterogeneous void tasks store monostate in the variant
 
422   
        {
417   
        {
423   
            if constexpr (requires { state->set_winner_void(); })
418   
            if constexpr (requires { state->set_winner_void(); })
424 - 
            // Homogeneous void tasks have no result to store
 
425   
                state->set_winner_void();
419   
                state->set_winner_void();
426   
        }
420   
        }
427   
    }
421   
    }
428   
    else
422   
    else
429   
    {
423   
    {
430   
        auto result = co_await std::move(inner);
424   
        auto result = co_await std::move(inner);
431   
        if(state->core_.try_win(index))
425   
        if(state->core_.try_win(index))
432 - 
            // Defensive: move should not throw (already moved once), but we
 
433 - 
            // catch just in case since an uncaught exception would be devastating.
 
434   
        {
426   
        {
435   
            try
427   
            try
436   
            {
428   
            {
437   
                state->set_winner_result(std::move(result));
429   
                state->set_winner_result(std::move(result));
438   
            }
430   
            }
439   
            catch(...)
431   
            catch(...)
440   
            {
432   
            {
441   
                state->core_.set_winner_exception(std::current_exception());
433   
                state->core_.set_winner_exception(std::current_exception());
442   
            }
434   
            }
443   
        }
435   
        }
444   
    }
436   
    }
445   
}
437   
}
446  
438  
447   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
439   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
448   
template<IoAwaitable... Awaitables>
440   
template<IoAwaitable... Awaitables>
449   
class when_any_launcher
441   
class when_any_launcher
450   
{
442   
{
451   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
443   
    using state_type = when_any_state<awaitable_result_t<Awaitables>...>;
452  
444  
453   
    std::tuple<Awaitables...>* tasks_;
445   
    std::tuple<Awaitables...>* tasks_;
454   
    state_type* state_;
446   
    state_type* state_;
455  
447  
456   
public:
448   
public:
457   
    when_any_launcher(
449   
    when_any_launcher(
458   
        std::tuple<Awaitables...>* tasks,
450   
        std::tuple<Awaitables...>* tasks,
459   
        state_type* state)
451   
        state_type* state)
460   
        : tasks_(tasks)
452   
        : tasks_(tasks)
461   
        , state_(state)
453   
        , state_(state)
462   
    {
454   
    {
463   
    }
455   
    }
464  
456  
465   
    bool await_ready() const noexcept
457   
    bool await_ready() const noexcept
466   
    {
458   
    {
467   
        return sizeof...(Awaitables) == 0;
459   
        return sizeof...(Awaitables) == 0;
468   
    }
460   
    }
469  
461  
470   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
462   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
471   
        destroys this object before await_suspend returns. Must not reference
463   
        destroys this object before await_suspend returns. Must not reference
472   
        `this` after the final launch_one call.
464   
        `this` after the final launch_one call.
473   
    */
465   
    */
474   
    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)
475   
    {
467   
    {
476   
        state_->core_.continuation_ = continuation;
468   
        state_->core_.continuation_ = continuation;
477   
        state_->core_.caller_env_ = caller_env;
469   
        state_->core_.caller_env_ = caller_env;
478  
470  
479   
        if(caller_env->stop_token.stop_possible())
471   
        if(caller_env->stop_token.stop_possible())
480   
        {
472   
        {
481   
            state_->core_.parent_stop_callback_.emplace(
473   
            state_->core_.parent_stop_callback_.emplace(
482   
                caller_env->stop_token,
474   
                caller_env->stop_token,
483   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
475   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
484  
476  
485   
            if(caller_env->stop_token.stop_requested())
477   
            if(caller_env->stop_token.stop_requested())
486   
                state_->core_.stop_source_.request_stop();
478   
                state_->core_.stop_source_.request_stop();
487   
        }
479   
        }
488  
480  
489   
        auto token = state_->core_.stop_source_.get_token();
481   
        auto token = state_->core_.stop_source_.get_token();
490   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
482   
        [&]<std::size_t... Is>(std::index_sequence<Is...>) {
491   
            (..., launch_one<Is>(caller_env->executor, token));
483   
            (..., launch_one<Is>(caller_env->executor, token));
492   
        }(std::index_sequence_for<Awaitables...>{});
484   
        }(std::index_sequence_for<Awaitables...>{});
493  
485  
494   
        return std::noop_coroutine();
486   
        return std::noop_coroutine();
495   
    }
487   
    }
496  
488  
497   
    void await_resume() const noexcept
489   
    void await_resume() const noexcept
498   
    {
490   
    {
499   
    }
491   
    }
500  
492  
501   
private:
493   
private:
502   
    /** @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). */
503   
    template<std::size_t I>
495   
    template<std::size_t I>
504   
    void launch_one(executor_ref caller_ex, std::stop_token token)
496   
    void launch_one(executor_ref caller_ex, std::stop_token token)
505   
    {
497   
    {
506 - 
        auto runner = make_when_any_runner(
498 + 
        auto runner = make_when_any_runner<I>(
507 - 
            std::move(std::get<I>(*tasks_)), state_, I);
499 + 
            std::move(std::get<I>(*tasks_)), state_);
508  
500  
509   
        auto h = runner.release();
501   
        auto h = runner.release();
510   
        h.promise().state_ = state_;
502   
        h.promise().state_ = state_;
511   
        h.promise().index_ = I;
503   
        h.promise().index_ = I;
512   
        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};
513  
505  
514   
        std::coroutine_handle<> ch{h};
506   
        std::coroutine_handle<> ch{h};
515   
        state_->runner_handles_[I] = ch;
507   
        state_->runner_handles_[I] = ch;
516   
        caller_ex.post(ch);
508   
        caller_ex.post(ch);
517   
    }
509   
    }
518   
};
510   
};
519  
511  
520   
} // namespace detail
512   
} // namespace detail
521  
513  
522   
/** Wait for the first awaitable to complete.
514   
/** Wait for the first awaitable to complete.
523  
515  
524   
    Races multiple heterogeneous awaitables concurrently and returns when the
516   
    Races multiple heterogeneous awaitables concurrently and returns when the
525 - 
    first one completes. The result includes the winner's index and a
517 + 
    first one completes. The result is a variant with one alternative per
526 - 
    deduplicated variant containing the result value.
518 + 
    input task, preserving positional correspondence.
527  
519  
528   
    @par Suspends
520   
    @par Suspends
529   
    The calling coroutine suspends when co_await is invoked. All awaitables
521   
    The calling coroutine suspends when co_await is invoked. All awaitables
530   
    are launched concurrently and execute in parallel. The coroutine resumes
522   
    are launched concurrently and execute in parallel. The coroutine resumes
531   
    only after all awaitables have completed, even though the winner is
523   
    only after all awaitables have completed, even though the winner is
532   
    determined by the first to finish.
524   
    determined by the first to finish.
533  
525  
534   
    @par Completion Conditions
526   
    @par Completion Conditions
535   
    @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)
536   
    @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
537   
    @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
538   
    @li Parent coroutine resumes only after all siblings acknowledge completion
530   
    @li Parent coroutine resumes only after all siblings acknowledge completion
539   
    @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
540  
532  
541   
    @par Cancellation Semantics
533   
    @par Cancellation Semantics
542   
    Cancellation is supported via stop_token propagated through the
534   
    Cancellation is supported via stop_token propagated through the
543   
    IoAwaitable protocol:
535   
    IoAwaitable protocol:
544   
    @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
545   
    @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
546   
    @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
547   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
539   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
548   
    @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
549  
541  
550   
    @par Concurrency/Overlap
542   
    @par Concurrency/Overlap
551   
    All awaitables are launched concurrently before any can complete.
543   
    All awaitables are launched concurrently before any can complete.
552   
    The launcher iterates through the arguments, starting each task on the
544   
    The launcher iterates through the arguments, starting each task on the
553   
    caller's executor. Tasks may execute in parallel on multi-threaded
545   
    caller's executor. Tasks may execute in parallel on multi-threaded
554   
    executors or interleave on single-threaded executors. There is no
546   
    executors or interleave on single-threaded executors. There is no
555   
    guaranteed ordering of task completion.
547   
    guaranteed ordering of task completion.
556  
548  
557   
    @par Notable Error Conditions
549   
    @par Notable Error Conditions
558   
    @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
559   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
551   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
560   
    @li Cancellation: tasks may complete via cancellation without throwing
552   
    @li Cancellation: tasks may complete via cancellation without throwing
561  
553  
562   
    @par Example
554   
    @par Example
563   
    @code
555   
    @code
564   
    task<void> example() {
556   
    task<void> example() {
565 - 
        auto [index, result] = co_await when_any(
557 + 
        auto result = co_await when_any(
566 - 
            fetch_from_primary(),   // task<Response>
 
567 - 
            fetch_from_backup()     // task<Response>
 
568 - 
        );
 
569 - 
        // index is 0 or 1, result holds the winner's Response
 
570 - 
        auto response = std::get<Response>(result);
 
571 - 
    }
 
572 - 
    @endcode
 
573 -
 
574 - 
    @par Example with Heterogeneous Types
 
575 - 
    @code
 
576 - 
    task<void> mixed_types() {
 
577 - 
        auto [index, result] = co_await when_any(
 
578   
            fetch_int(),      // task<int>
558   
            fetch_int(),      // task<int>
579   
            fetch_string()    // task<std::string>
559   
            fetch_string()    // task<std::string>
580   
        );
560   
        );
581 - 
        if (index == 0)
561 + 
        // result.index() is 0 or 1
582 - 
            std::cout << "Got int: " << std::get<int>(result) << "\n";
562 + 
        if (result.index() == 0)
 
563 + 
            std::cout << "Got int: " << std::get<0>(result) << "\n";
583   
        else
564   
        else
584 - 
            std::cout << "Got string: " << std::get<std::string>(result) << "\n";
565 + 
            std::cout << "Got string: " << std::get<1>(result) << "\n";
585   
    }
566   
    }
586   
    @endcode
567   
    @endcode
587  
568  
588   
    @tparam A0 First awaitable type (must satisfy IoAwaitable).
569   
    @tparam A0 First awaitable type (must satisfy IoAwaitable).
589   
    @tparam As Remaining awaitable types (must satisfy IoAwaitable).
570   
    @tparam As Remaining awaitable types (must satisfy IoAwaitable).
590   
    @param a0 The first awaitable to race.
571   
    @param a0 The first awaitable to race.
591   
    @param as Additional awaitables to race concurrently.
572   
    @param as Additional awaitables to race concurrently.
592 - 
    @return A task yielding a pair of (winner_index, result_variant).
573 + 
    @return A task yielding a variant with one alternative per awaitable.
 
574 + 
        Use .index() to identify the winner. Void awaitables contribute
 
575 + 
        std::monostate.
593  
576  
594   
    @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.
595  
578  
596   
    @par Remarks
579   
    @par Remarks
597   
    Awaitables are moved into the coroutine frame; original objects become
580   
    Awaitables are moved into the coroutine frame; original objects become
598 - 
    empty after the call. When multiple awaitables share the same return type,
581 + 
    empty after the call. The variant preserves one alternative per input
599 - 
    the variant is deduplicated to contain only unique types. Use the winner
582 + 
    task. Use .index() to determine which awaitable completed first.
600 - 
    index to determine which awaitable completed first. Void awaitables
583 + 
    Void awaitables contribute std::monostate to the variant.
601 - 
    contribute std::monostate to the variant.
 
602  
584  
603   
    @see when_all, IoAwaitable
585   
    @see when_all, IoAwaitable
604   
*/
586   
*/
605   
template<IoAwaitable A0, IoAwaitable... As>
587   
template<IoAwaitable A0, IoAwaitable... As>
606   
[[nodiscard]] auto when_any(A0 a0, As... as)
588   
[[nodiscard]] auto when_any(A0 a0, As... as)
607 - 
    -> task<detail::when_any_result_t<
589 + 
    -> task<detail::when_any_variant_t<
608   
        detail::awaitable_result_t<A0>,
590   
        detail::awaitable_result_t<A0>,
609   
        detail::awaitable_result_t<As>...>>
591   
        detail::awaitable_result_t<As>...>>
610 - 
    using result_type = detail::when_any_result_t<
 
611 - 
        detail::awaitable_result_t<A0>,
 
612 - 
        detail::awaitable_result_t<As>...>;
 
613 -
 
614   
{
592   
{
615   
    detail::when_any_state<
593   
    detail::when_any_state<
616   
        detail::awaitable_result_t<A0>,
594   
        detail::awaitable_result_t<A0>,
617   
        detail::awaitable_result_t<As>...> state;
595   
        detail::awaitable_result_t<As>...> state;
618   
    std::tuple<A0, As...> awaitable_tuple(std::move(a0), std::move(as)...);
596   
    std::tuple<A0, As...> awaitable_tuple(std::move(a0), std::move(as)...);
619  
597  
620   
    co_await detail::when_any_launcher<A0, As...>(&awaitable_tuple, &state);
598   
    co_await detail::when_any_launcher<A0, As...>(&awaitable_tuple, &state);
621  
599  
622   
    if(state.core_.winner_exception_)
600   
    if(state.core_.winner_exception_)
623   
        std::rethrow_exception(state.core_.winner_exception_);
601   
        std::rethrow_exception(state.core_.winner_exception_);
624  
602  
625 - 
    co_return result_type{state.core_.winner_index_, std::move(*state.result_)};
603 + 
    co_return std::move(*state.result_);
626   
}
604   
}
627  
605  
628   
/** Concept for ranges of full I/O awaitables.
606   
/** Concept for ranges of full I/O awaitables.
629  
607  
630   
    A range satisfies `IoAwaitableRange` if it is a sized input range
608   
    A range satisfies `IoAwaitableRange` if it is a sized input range
631   
    whose value type satisfies @ref IoAwaitable. This enables when_any
609   
    whose value type satisfies @ref IoAwaitable. This enables when_any
632   
    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.
633  
611  
634   
    @tparam R The range type.
612   
    @tparam R The range type.
635  
613  
636   
    @par Requirements
614   
    @par Requirements
637   
    @li `R` must satisfy `std::ranges::input_range`
615   
    @li `R` must satisfy `std::ranges::input_range`
638   
    @li `R` must satisfy `std::ranges::sized_range`
616   
    @li `R` must satisfy `std::ranges::sized_range`
639   
    @li `std::ranges::range_value_t<R>` must satisfy @ref IoAwaitable
617   
    @li `std::ranges::range_value_t<R>` must satisfy @ref IoAwaitable
640  
618  
641   
    @par Syntactic Requirements
619   
    @par Syntactic Requirements
642   
    Given `r` of type `R`:
620   
    Given `r` of type `R`:
643   
    @li `std::ranges::begin(r)` is valid
621   
    @li `std::ranges::begin(r)` is valid
644   
    @li `std::ranges::end(r)` is valid
622   
    @li `std::ranges::end(r)` is valid
645   
    @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>`
646   
    @li `*std::ranges::begin(r)` satisfies @ref IoAwaitable
624   
    @li `*std::ranges::begin(r)` satisfies @ref IoAwaitable
647  
625  
648   
    @par Example
626   
    @par Example
649   
    @code
627   
    @code
650   
    template<IoAwaitableRange R>
628   
    template<IoAwaitableRange R>
651   
    task<void> race_all(R&& awaitables) {
629   
    task<void> race_all(R&& awaitables) {
652   
        auto winner = co_await when_any(std::forward<R>(awaitables));
630   
        auto winner = co_await when_any(std::forward<R>(awaitables));
653   
        // Process winner...
631   
        // Process winner...
654   
    }
632   
    }
655   
    @endcode
633   
    @endcode
656  
634  
657   
    @see when_any, IoAwaitable
635   
    @see when_any, IoAwaitable
658   
*/
636   
*/
659   
template<typename R>
637   
template<typename R>
660   
concept IoAwaitableRange =
638   
concept IoAwaitableRange =
661   
    std::ranges::input_range<R> &&
639   
    std::ranges::input_range<R> &&
662   
    std::ranges::sized_range<R> &&
640   
    std::ranges::sized_range<R> &&
663   
    IoAwaitable<std::ranges::range_value_t<R>>;
641   
    IoAwaitable<std::ranges::range_value_t<R>>;
664  
642  
665   
namespace detail {
643   
namespace detail {
666  
644  
667   
/** Shared state for homogeneous when_any (range overload).
645   
/** Shared state for homogeneous when_any (range overload).
668  
646  
669   
    Uses composition with when_any_core for shared functionality.
647   
    Uses composition with when_any_core for shared functionality.
670   
    Simpler than heterogeneous: optional<T> instead of variant, vector
648   
    Simpler than heterogeneous: optional<T> instead of variant, vector
671   
    instead of array for runner handles.
649   
    instead of array for runner handles.
672   
*/
650   
*/
673   
template<typename T>
651   
template<typename T>
674   
struct when_any_homogeneous_state
652   
struct when_any_homogeneous_state
675   
{
653   
{
676   
    when_any_core core_;
654   
    when_any_core core_;
677   
    std::optional<T> result_;
655   
    std::optional<T> result_;
678   
    std::vector<std::coroutine_handle<>> runner_handles_;
656   
    std::vector<std::coroutine_handle<>> runner_handles_;
679  
657  
680   
    explicit when_any_homogeneous_state(std::size_t count)
658   
    explicit when_any_homogeneous_state(std::size_t count)
681   
        : core_(count)
659   
        : core_(count)
682   
        , runner_handles_(count)
660   
        , runner_handles_(count)
683   
    {
661   
    {
684   
    }
662   
    }
685  
663  
686   
    // Runners self-destruct in final_suspend. No destruction needed here.
664   
    // Runners self-destruct in final_suspend. No destruction needed here.
687  
665  
688   
    /** @pre core_.try_win() returned true. */
666   
    /** @pre core_.try_win() returned true. */
689   
    void set_winner_result(T value)
667   
    void set_winner_result(T value)
690   
        noexcept(std::is_nothrow_move_constructible_v<T>)
668   
        noexcept(std::is_nothrow_move_constructible_v<T>)
691   
    {
669   
    {
692   
        result_.emplace(std::move(value));
670   
        result_.emplace(std::move(value));
693   
    }
671   
    }
694   
};
672   
};
695  
673  
696   
/** Specialization for void tasks (no result storage needed). */
674   
/** Specialization for void tasks (no result storage needed). */
697   
template<>
675   
template<>
698   
struct when_any_homogeneous_state<void>
676   
struct when_any_homogeneous_state<void>
699   
{
677   
{
700   
    when_any_core core_;
678   
    when_any_core core_;
701   
    std::vector<std::coroutine_handle<>> runner_handles_;
679   
    std::vector<std::coroutine_handle<>> runner_handles_;
702  
680  
703   
    explicit when_any_homogeneous_state(std::size_t count)
681   
    explicit when_any_homogeneous_state(std::size_t count)
704   
        : core_(count)
682   
        : core_(count)
705   
        , runner_handles_(count)
683   
        , runner_handles_(count)
706   
    {
684   
    {
707   
    }
685   
    }
708  
686  
709   
    // Runners self-destruct in final_suspend. No destruction needed here.
687   
    // Runners self-destruct in final_suspend. No destruction needed here.
710  
688  
711   
    // No set_winner_result - void tasks have no result to store
689   
    // No set_winner_result - void tasks have no result to store
712   
};
690   
};
713  
691  
714   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
692   
/** Launches all runners concurrently; see await_suspend for lifetime concerns. */
715   
template<IoAwaitableRange Range>
693   
template<IoAwaitableRange Range>
716   
class when_any_homogeneous_launcher
694   
class when_any_homogeneous_launcher
717   
{
695   
{
718   
    using Awaitable = std::ranges::range_value_t<Range>;
696   
    using Awaitable = std::ranges::range_value_t<Range>;
719   
    using T = awaitable_result_t<Awaitable>;
697   
    using T = awaitable_result_t<Awaitable>;
720  
698  
721   
    Range* range_;
699   
    Range* range_;
722   
    when_any_homogeneous_state<T>* state_;
700   
    when_any_homogeneous_state<T>* state_;
723  
701  
724   
public:
702   
public:
725   
    when_any_homogeneous_launcher(
703   
    when_any_homogeneous_launcher(
726   
        Range* range,
704   
        Range* range,
727   
        when_any_homogeneous_state<T>* state)
705   
        when_any_homogeneous_state<T>* state)
728   
        : range_(range)
706   
        : range_(range)
729   
        , state_(state)
707   
        , state_(state)
730   
    {
708   
    {
731   
    }
709   
    }
732  
710  
733   
    bool await_ready() const noexcept
711   
    bool await_ready() const noexcept
734   
    {
712   
    {
735   
        return std::ranges::empty(*range_);
713   
        return std::ranges::empty(*range_);
736   
    }
714   
    }
737  
715  
738   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
716   
    /** CRITICAL: If the last task finishes synchronously, parent resumes and
739   
        destroys this object before await_suspend returns. Must not reference
717   
        destroys this object before await_suspend returns. Must not reference
740   
        `this` after dispatching begins.
718   
        `this` after dispatching begins.
741  
719  
742   
        Two-phase approach:
720   
        Two-phase approach:
743   
        1. Create all runners (safe - no dispatch yet)
721   
        1. Create all runners (safe - no dispatch yet)
744   
        2. Dispatch all runners (any may complete synchronously)
722   
        2. Dispatch all runners (any may complete synchronously)
745   
    */
723   
    */
746   
    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)
747   
    {
725   
    {
748   
        state_->core_.continuation_ = continuation;
726   
        state_->core_.continuation_ = continuation;
749   
        state_->core_.caller_env_ = caller_env;
727   
        state_->core_.caller_env_ = caller_env;
750  
728  
751   
        if(caller_env->stop_token.stop_possible())
729   
        if(caller_env->stop_token.stop_possible())
752   
        {
730   
        {
753   
            state_->core_.parent_stop_callback_.emplace(
731   
            state_->core_.parent_stop_callback_.emplace(
754   
                caller_env->stop_token,
732   
                caller_env->stop_token,
755   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
733   
                when_any_core::stop_callback_fn{&state_->core_.stop_source_});
756  
734  
757   
            if(caller_env->stop_token.stop_requested())
735   
            if(caller_env->stop_token.stop_requested())
758   
                state_->core_.stop_source_.request_stop();
736   
                state_->core_.stop_source_.request_stop();
759   
        }
737   
        }
760  
738  
761   
        auto token = state_->core_.stop_source_.get_token();
739   
        auto token = state_->core_.stop_source_.get_token();
762  
740  
763   
        // Phase 1: Create all runners without dispatching.
741   
        // Phase 1: Create all runners without dispatching.
764   
        // This iterates over *range_ safely because no runners execute yet.
742   
        // This iterates over *range_ safely because no runners execute yet.
765   
        std::size_t index = 0;
743   
        std::size_t index = 0;
766   
        for(auto&& a : *range_)
744   
        for(auto&& a : *range_)
767   
        {
745   
        {
768   
            auto runner = make_when_any_runner(
746   
            auto runner = make_when_any_runner(
769   
                std::move(a), state_, index);
747   
                std::move(a), state_, index);
770  
748  
771   
            auto h = runner.release();
749   
            auto h = runner.release();
772   
            h.promise().state_ = state_;
750   
            h.promise().state_ = state_;
773   
            h.promise().index_ = index;
751   
            h.promise().index_ = index;
774   
            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};
775  
753  
776   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
754   
            state_->runner_handles_[index] = std::coroutine_handle<>{h};
777   
            ++index;
755   
            ++index;
778   
        }
756   
        }
779  
757  
780   
        // Phase 2: Post all runners. Any may complete synchronously.
758   
        // Phase 2: Post all runners. Any may complete synchronously.
781   
        // After last post, state_ and this may be destroyed.
759   
        // After last post, state_ and this may be destroyed.
782   
        // Use raw pointer/count captured before posting.
760   
        // Use raw pointer/count captured before posting.
783   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
761   
        std::coroutine_handle<>* handles = state_->runner_handles_.data();
784   
        std::size_t count = state_->runner_handles_.size();
762   
        std::size_t count = state_->runner_handles_.size();
785   
        for(std::size_t i = 0; i < count; ++i)
763   
        for(std::size_t i = 0; i < count; ++i)
786   
            caller_env->executor.post(handles[i]);
764   
            caller_env->executor.post(handles[i]);
787  
765  
788   
        return std::noop_coroutine();
766   
        return std::noop_coroutine();
789   
    }
767   
    }
790  
768  
791   
    void await_resume() const noexcept
769   
    void await_resume() const noexcept
792   
    {
770   
    {
793   
    }
771   
    }
794   
};
772   
};
795  
773  
796   
} // namespace detail
774   
} // namespace detail
797  
775  
798   
/** Wait for the first awaitable to complete (range overload).
776   
/** Wait for the first awaitable to complete (range overload).
799  
777  
800   
    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
801   
    sized input range of IoAwaitable types, enabling use with arrays,
779   
    sized input range of IoAwaitable types, enabling use with arrays,
802   
    spans, or custom containers.
780   
    spans, or custom containers.
803  
781  
804   
    @par Suspends
782   
    @par Suspends
805   
    The calling coroutine suspends when co_await is invoked. All awaitables
783   
    The calling coroutine suspends when co_await is invoked. All awaitables
806   
    in the range are launched concurrently and execute in parallel. The
784   
    in the range are launched concurrently and execute in parallel. The
807   
    coroutine resumes only after all awaitables have completed, even though
785   
    coroutine resumes only after all awaitables have completed, even though
808   
    the winner is determined by the first to finish.
786   
    the winner is determined by the first to finish.
809  
787  
810   
    @par Completion Conditions
788   
    @par Completion Conditions
811   
    @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)
812   
    @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
813   
    @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
814   
    @li Parent coroutine resumes only after all siblings acknowledge completion
792   
    @li Parent coroutine resumes only after all siblings acknowledge completion
815   
    @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
816  
794  
817   
    @par Cancellation Semantics
795   
    @par Cancellation Semantics
818   
    Cancellation is supported via stop_token propagated through the
796   
    Cancellation is supported via stop_token propagated through the
819   
    IoAwaitable protocol:
797   
    IoAwaitable protocol:
820   
    @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
821   
    @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
822   
    @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
823   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
801   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
824   
    @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
825  
803  
826   
    @par Concurrency/Overlap
804   
    @par Concurrency/Overlap
827   
    All awaitables are launched concurrently before any can complete.
805   
    All awaitables are launched concurrently before any can complete.
828   
    The launcher iterates through the range, starting each task on the
806   
    The launcher iterates through the range, starting each task on the
829   
    caller's executor. Tasks may execute in parallel on multi-threaded
807   
    caller's executor. Tasks may execute in parallel on multi-threaded
830   
    executors or interleave on single-threaded executors. There is no
808   
    executors or interleave on single-threaded executors. There is no
831   
    guaranteed ordering of task completion.
809   
    guaranteed ordering of task completion.
832  
810  
833   
    @par Notable Error Conditions
811   
    @par Notable Error Conditions
834   
    @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)
835   
    @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
836   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
814   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
837   
    @li Cancellation: tasks may complete via cancellation without throwing
815   
    @li Cancellation: tasks may complete via cancellation without throwing
838  
816  
839   
    @par Example
817   
    @par Example
840   
    @code
818   
    @code
841   
    task<void> example() {
819   
    task<void> example() {
842   
        std::array<task<Response>, 3> requests = {
820   
        std::array<task<Response>, 3> requests = {
843   
            fetch_from_server(0),
821   
            fetch_from_server(0),
844   
            fetch_from_server(1),
822   
            fetch_from_server(1),
845   
            fetch_from_server(2)
823   
            fetch_from_server(2)
846   
        };
824   
        };
847  
825  
848   
        auto [index, response] = co_await when_any(std::move(requests));
826   
        auto [index, response] = co_await when_any(std::move(requests));
849   
    }
827   
    }
850   
    @endcode
828   
    @endcode
851  
829  
852   
    @par Example with Vector
830   
    @par Example with Vector
853   
    @code
831   
    @code
854   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
832   
    task<Response> fetch_fastest(std::vector<Server> const& servers) {
855   
        std::vector<task<Response>> requests;
833   
        std::vector<task<Response>> requests;
856   
        for (auto const& server : servers)
834   
        for (auto const& server : servers)
857   
            requests.push_back(fetch_from(server));
835   
            requests.push_back(fetch_from(server));
858  
836  
859   
        auto [index, response] = co_await when_any(std::move(requests));
837   
        auto [index, response] = co_await when_any(std::move(requests));
860   
        co_return response;
838   
        co_return response;
861   
    }
839   
    }
862   
    @endcode
840   
    @endcode
863  
841  
864   
    @tparam R Range type satisfying IoAwaitableRange.
842   
    @tparam R Range type satisfying IoAwaitableRange.
865   
    @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).
866   
    @return A task yielding a pair of (winner_index, result).
844   
    @return A task yielding a pair of (winner_index, result).
867  
845  
868   
    @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).
869   
    @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.
870  
848  
871   
    @par Remarks
849   
    @par Remarks
872   
    Elements are moved from the range; for lvalue ranges, the original
850   
    Elements are moved from the range; for lvalue ranges, the original
873   
    container will have moved-from elements after this call. The range
851   
    container will have moved-from elements after this call. The range
874   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
852   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
875   
    the variadic overload, no variant wrapper is needed since all tasks
853   
    the variadic overload, no variant wrapper is needed since all tasks
876   
    share the same return type.
854   
    share the same return type.
877  
855  
878   
    @see when_any, IoAwaitableRange
856   
    @see when_any, IoAwaitableRange
879   
*/
857   
*/
880   
template<IoAwaitableRange R>
858   
template<IoAwaitableRange R>
881   
    requires (!std::is_void_v<detail::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>>>)
882   
[[nodiscard]] auto when_any(R&& awaitables)
860   
[[nodiscard]] auto when_any(R&& awaitables)
883   
    -> task<std::pair<std::size_t, detail::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>>>>
884   
{
862   
{
885   
    using Awaitable = std::ranges::range_value_t<R>;
863   
    using Awaitable = std::ranges::range_value_t<R>;
886   
    using T = detail::awaitable_result_t<Awaitable>;
864   
    using T = detail::awaitable_result_t<Awaitable>;
887   
    using result_type = std::pair<std::size_t, T>;
865   
    using result_type = std::pair<std::size_t, T>;
888   
    using OwnedRange = std::remove_cvref_t<R>;
866   
    using OwnedRange = std::remove_cvref_t<R>;
889  
867  
890   
    auto count = std::ranges::size(awaitables);
868   
    auto count = std::ranges::size(awaitables);
891   
    if(count == 0)
869   
    if(count == 0)
892   
        throw std::invalid_argument("when_any requires at least one awaitable");
870   
        throw std::invalid_argument("when_any requires at least one awaitable");
893  
871  
894   
    // Move/copy range onto coroutine frame to ensure lifetime
872   
    // Move/copy range onto coroutine frame to ensure lifetime
895   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
873   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
896  
874  
897   
    detail::when_any_homogeneous_state<T> state(count);
875   
    detail::when_any_homogeneous_state<T> state(count);
898  
876  
899   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
877   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
900  
878  
901   
    if(state.core_.winner_exception_)
879   
    if(state.core_.winner_exception_)
902   
        std::rethrow_exception(state.core_.winner_exception_);
880   
        std::rethrow_exception(state.core_.winner_exception_);
903  
881  
904   
    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_)};
905   
}
883   
}
906  
884  
907   
/** Wait for the first awaitable to complete (void range overload).
885   
/** Wait for the first awaitable to complete (void range overload).
908  
886  
909   
    Races a range of void-returning awaitables. Since void awaitables have
887   
    Races a range of void-returning awaitables. Since void awaitables have
910   
    no result value, only the winner's index is returned.
888   
    no result value, only the winner's index is returned.
911  
889  
912   
    @par Suspends
890   
    @par Suspends
913   
    The calling coroutine suspends when co_await is invoked. All awaitables
891   
    The calling coroutine suspends when co_await is invoked. All awaitables
914   
    in the range are launched concurrently and execute in parallel. The
892   
    in the range are launched concurrently and execute in parallel. The
915   
    coroutine resumes only after all awaitables have completed, even though
893   
    coroutine resumes only after all awaitables have completed, even though
916   
    the winner is determined by the first to finish.
894   
    the winner is determined by the first to finish.
917  
895  
918   
    @par Completion Conditions
896   
    @par Completion Conditions
919   
    @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)
920   
    @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
921   
    @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
922   
    @li Parent coroutine resumes only after all siblings acknowledge completion
900   
    @li Parent coroutine resumes only after all siblings acknowledge completion
923   
    @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
924  
902  
925   
    @par Cancellation Semantics
903   
    @par Cancellation Semantics
926   
    Cancellation is supported via stop_token propagated through the
904   
    Cancellation is supported via stop_token propagated through the
927   
    IoAwaitable protocol:
905   
    IoAwaitable protocol:
928   
    @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
929   
    @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
930   
    @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
931   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
909   
    @li Siblings must handle cancellation gracefully and complete before parent resumes
932   
    @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
933  
911  
934   
    @par Concurrency/Overlap
912   
    @par Concurrency/Overlap
935   
    All awaitables are launched concurrently before any can complete.
913   
    All awaitables are launched concurrently before any can complete.
936   
    The launcher iterates through the range, starting each task on the
914   
    The launcher iterates through the range, starting each task on the
937   
    caller's executor. Tasks may execute in parallel on multi-threaded
915   
    caller's executor. Tasks may execute in parallel on multi-threaded
938   
    executors or interleave on single-threaded executors. There is no
916   
    executors or interleave on single-threaded executors. There is no
939   
    guaranteed ordering of task completion.
917   
    guaranteed ordering of task completion.
940  
918  
941   
    @par Notable Error Conditions
919   
    @par Notable Error Conditions
942   
    @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)
943   
    @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
944   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
922   
    @li Non-winner exceptions: silently discarded (only winner's result matters)
945   
    @li Cancellation: tasks may complete via cancellation without throwing
923   
    @li Cancellation: tasks may complete via cancellation without throwing
946  
924  
947   
    @par Example
925   
    @par Example
948   
    @code
926   
    @code
949   
    task<void> example() {
927   
    task<void> example() {
950   
        std::vector<task<void>> tasks;
928   
        std::vector<task<void>> tasks;
951   
        for (int i = 0; i < 5; ++i)
929   
        for (int i = 0; i < 5; ++i)
952   
            tasks.push_back(background_work(i));
930   
            tasks.push_back(background_work(i));
953  
931  
954   
        std::size_t winner = co_await when_any(std::move(tasks));
932   
        std::size_t winner = co_await when_any(std::move(tasks));
955   
        // winner is the index of the first task to complete
933   
        // winner is the index of the first task to complete
956   
    }
934   
    }
957   
    @endcode
935   
    @endcode
958  
936  
959   
    @par Example with Timeout
937   
    @par Example with Timeout
960   
    @code
938   
    @code
961   
    task<void> with_timeout() {
939   
    task<void> with_timeout() {
962   
        std::vector<task<void>> tasks;
940   
        std::vector<task<void>> tasks;
963   
        tasks.push_back(long_running_operation());
941   
        tasks.push_back(long_running_operation());
964   
        tasks.push_back(delay(std::chrono::seconds(5)));
942   
        tasks.push_back(delay(std::chrono::seconds(5)));
965  
943  
966   
        std::size_t winner = co_await when_any(std::move(tasks));
944   
        std::size_t winner = co_await when_any(std::move(tasks));
967   
        if (winner == 1) {
945   
        if (winner == 1) {
968   
            // Timeout occurred
946   
            // Timeout occurred
969   
        }
947   
        }
970   
    }
948   
    }
971   
    @endcode
949   
    @endcode
972  
950  
973   
    @tparam R Range type satisfying IoAwaitableRange with void result.
951   
    @tparam R Range type satisfying IoAwaitableRange with void result.
974   
    @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).
975   
    @return A task yielding the winner's index (zero-based).
953   
    @return A task yielding the winner's index (zero-based).
976  
954  
977   
    @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).
978   
    @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.
979  
957  
980   
    @par Remarks
958   
    @par Remarks
981   
    Elements are moved from the range; for lvalue ranges, the original
959   
    Elements are moved from the range; for lvalue ranges, the original
982   
    container will have moved-from elements after this call. The range
960   
    container will have moved-from elements after this call. The range
983   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
961   
    is moved onto the coroutine frame to ensure lifetime safety. Unlike
984   
    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
985   
    produce no value.
963   
    produce no value.
986  
964  
987   
    @see when_any, IoAwaitableRange
965   
    @see when_any, IoAwaitableRange
988   
*/
966   
*/
989   
template<IoAwaitableRange R>
967   
template<IoAwaitableRange R>
990   
    requires std::is_void_v<detail::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>>>
991   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
969   
[[nodiscard]] auto when_any(R&& awaitables) -> task<std::size_t>
992   
{
970   
{
993   
    using OwnedRange = std::remove_cvref_t<R>;
971   
    using OwnedRange = std::remove_cvref_t<R>;
994  
972  
995   
    auto count = std::ranges::size(awaitables);
973   
    auto count = std::ranges::size(awaitables);
996   
    if(count == 0)
974   
    if(count == 0)
997   
        throw std::invalid_argument("when_any requires at least one awaitable");
975   
        throw std::invalid_argument("when_any requires at least one awaitable");
998  
976  
999   
    // Move/copy range onto coroutine frame to ensure lifetime
977   
    // Move/copy range onto coroutine frame to ensure lifetime
1000   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
978   
    OwnedRange owned_awaitables = std::forward<R>(awaitables);
1001  
979  
1002   
    detail::when_any_homogeneous_state<void> state(count);
980   
    detail::when_any_homogeneous_state<void> state(count);
1003  
981  
1004   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
982   
    co_await detail::when_any_homogeneous_launcher<OwnedRange>(&owned_awaitables, &state);
1005  
983  
1006   
    if(state.core_.winner_exception_)
984   
    if(state.core_.winner_exception_)
1007   
        std::rethrow_exception(state.core_.winner_exception_);
985   
        std::rethrow_exception(state.core_.winner_exception_);
1008  
986  
1009   
    co_return state.core_.winner_index_;
987   
    co_return state.core_.winner_index_;
1010   
}
988   
}
1011  
989  
1012   
} // namespace capy
990   
} // namespace capy
1013   
} // namespace boost
991   
} // namespace boost
1014  
992  
1015   
#endif
993   
#endif