Replace void filtering with monostate in when_all

[mvandeberg]

Closes #204

Summary by CodeRabbit

Release Notes

• Improvements
  • Improved when_all to consistently handle void-returning tasks by including them in the result tuple as std::monostate values, preserving input indices and enabling uniform tuple aggregation for mixed task combinations.
  • Enhanced type utilities for better void handling across asynchronous operations.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: 8981cb67-0c73-4e58-a071-de74b2491f2b

📥 Commits

Reviewing files that changed from the base of the PR and between 0cea4bb and 0a0bb31.

⛔ Files ignored due to path filters (4)

• doc/modules/ROOT/pages/4.coroutines/4f.composition.adoc is excluded by !**/doc/**
• doc/unlisted/coroutines-when-all.adoc is excluded by !**/doc/**
• doc/unlisted/library-when-all.adoc is excluded by !**/doc/**
• test/unit/when_all.cpp is excluded by !**/test/**

📒 Files selected for processing (4)

• example/parallel-fetch/parallel_fetch.cpp
• include/boost/capy/detail/void_to_monostate.hpp
• include/boost/capy/when_all.hpp
• include/boost/capy/when_any.hpp

📝 Walkthrough

Walkthrough

The PR extracts a shared void_to_monostate_t type alias into a dedicated detail header and updates when_all to replace void-filtering logic with monostate replacement, preserving task-index-to-result-index mapping across both when_any and when_all APIs.

Changes

Cohort / File(s)	Summary
New Detail Header include/boost/capy/detail/void_to_monostate.hpp	Introduces void_to_monostate_t template alias that conditionally maps void to std::monostate while passing through other types unchanged, guarded by <type_traits> and <variant> includes.
when_all Result Type Refactor include/boost/capy/when_all.hpp	Replaces non_void_tuple_t with when_all_result_t to return std::tuple<void_to_monostate_t<Ts>...>. Updates result_holder<void> to provide get() returning std::monostate. Simplifies extraction logic to always construct tuples with index-preserving monostate values for void results.
when_any Header Consolidation include/boost/capy/when_any.hpp	Removes inline void_to_monostate_t definition and adds include of new shared detail header to source the alias from the centralized location.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related PRs

• Refactor variadic when_any to return positional variant #190: Directly related refactoring of when_any implementation that also affects the void_to_monostate_t alias and variant-handling logic.
• Update when_all to operate on IoAwaitable #146: Modifies when_all's type-level result computation using awaitable_result_t, touching the same type machinery being extended in this PR.
• Add public type aliases for detail types in when_any/when_all APIs #203: Updates when_all/when_any API signatures and result-type aliases, overlapping with the type system changes introduced here.

Poem

🐰 A monostate hops where void once hid,
Indices now map to tasks as they did!
No filtering, just placeholder replacements—
Tuples aligned with crystal-clear tracements.
When all tasks done, order stays put, behold! 🌟

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Replace void filtering with monostate in when_all' accurately describes the main change: switching from void filtering to monostate in the when_all implementation.
Linked Issues check	✅ Passed	The pull request implements all required changes from issue #204: introduces void_to_monostate_t type alias, updates when_all to use monostate instead of filtering void, modifies result_holder to return monostate, and updates when_any to use the shared type alias.
Out of Scope Changes check	✅ Passed	All changes directly support the stated objective of replacing void filtering with monostate in when_all to preserve task-index-to-result-index mapping; no unrelated modifications detected.

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[cppalliance-bot]

An automated preview of the documentation is available at https://227.capy.prtest3.cppalliance.org/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-03-11 22:28:27 UTC

[cppalliance-bot]

GCOVR code coverage report https://227.capy.prtest3.cppalliance.org/gcovr/index.html
LCOV code coverage report https://227.capy.prtest3.cppalliance.org/genhtml/index.html
Coverage Diff Report https://227.capy.prtest3.cppalliance.org/diff-report/index.html

Build time: 2026-03-11 22:36:59 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.36%. Comparing base (55efc7f) to head (0a0bb31).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #227      +/-   ##
===========================================
- Coverage    92.41%   92.36%   -0.05%     
===========================================
  Files          162      162              
  Lines         8854     8854              
===========================================
- Hits          8182     8178       -4     
- Misses         672      676       +4     

Flag	Coverage Δ
linux	92.33% <ø> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
include/boost/capy/when_all.hpp	97.95% <100.00%> (ø)
include/boost/capy/when_any.hpp	99.31% <ø> (ø)

... and 4 files with indirect coverage changes

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