---
title: when_all() is just just()
document: P4217R0
date: 2026-05-06
audience: SG1, LEWG
reply-to:
  - "Robert Leahy < <rleahy@rleahy.ca>"
paper-type: proposal
---

The standard currently specifies, by fiat, that `std::execution::when_all()` is ill-formed (§33.9.12.12 [exec.when.all]):

*“The expressions* `when_all(sndrs...)` *and* `when_all_with_variant(sndrs...)` *are* *ill-formed if any of the following is* `true`*:*

*** `sizeof...(sndrs)` *is* `0`*, or*
*** *[...]”*

If this restriction were not present the asynchronous operation which results from connecting the result of `std::execution::when_all()` and starting the operation state yielded thereby would hang due to the fact the implementation of `std::execution::start` therefor does not complete the operation eagerly when there are no child senders (ibid.).

## Discussion

`std::execution::when_all` *“adapt[s] multiple input senders into a sender that completes* *when all input senders have* completed” (ibid.). Given zero senders “all input senders” have always trivially completed (in the same way that `std::all_of` returns `true` for empty input) and therefore there’s no reason to ban `std::execution::when_all()`, it is simply equivalent to `std::execution::just()`.

Banning `std::execution::when_all()` (i.e. the status quo) unnecessarily creates a special case when writing generic algorithms.

## Proposal

### [exec.when.all]

[...]

The names `when_all` and `when_all_with_variant` denote customization point objects. Let `sndrs` be a pack of subexpressions and let `Sndrs` be a pack of the types `decltype((sndrs))...`. The expressions `when_all(sndrs...)` and `when_all_with_variant(sndrs...)` are ill-formed if any of the following is `true`:

* `sizeof...(sndrs)` is `0`, or
* `(sender<Sndrs> && ...)` is `false`.

The expression `when_all(sndrs...)` is expression-equivalent to:

* `make-sender(when_all, {}, sndrs...)` if `sizeof`…`(sndrs)` is not `0`, or
* `just()` otherwise.

[...]

## Acknowledgements

The author would like to thank Jonathan Müller for his C++Now 2026 talk which made him aware of this defect.