when_all: replace void filtering with monostate to preserve task-index-to-result-index mapping

[mvandeberg]

Problem

when_any (variadic) returns a std::variant<void_to_monostate_t<Ts>...>, replacing void-returning tasks with std::monostate so that .index() maps directly to the input task index. This is correct and useful.

when_all (variadic) uses filter_void_tuple_t which removes void types entirely from the result tuple via tuple_cat. This breaks the task-index-to-result-index invariant.

Example

auto [a, b] = co_await when_all(
    fetch_int(),     // task<int>   — index 0
    log_something(), // task<void>  — index 1
    fetch_str()      // task<string> — index 2
);
// a = int (from task 0) ✓
// b = string (from task 2), but accessed via get<1> ✗
// No entry for task 1 — tuple indices don't correspond to task indices

With monostate replacement (matching when_any's approach):

auto [a, b, c] = co_await when_all(
    fetch_int(),     // task<int>        — index 0
    log_something(), // task<void>       — index 1
    fetch_str()      // task<string>     — index 2
);
// a = int (from task 0)        — get<0> ✓
// b = monostate (from task 1)  — get<1> ✓
// c = string (from task 2)     — get<2> ✓

Proposed Fix

Replace the void-filtering logic in when_all with void_to_monostate_t (already defined in when_any.hpp), mirroring the approach used by when_any:

• Result type: std::tuple<void_to_monostate_t<awaitable_result_t<As>>...>
• result_holder<void> stores/returns std::monostate instead of being empty
• extract_results returns monostate for void tasks instead of filtering them out
• All-void case returns std::tuple<std::monostate, ...> instead of void

This is a breaking API change for any code using when_all with a mix of void and non-void tasks, or relying on the all-void → task<void> return.

Affected Files

• include/boost/capy/when_all.hpp
• test/unit/when_all.cpp
• Potentially doc examples