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4274. The chrono::hh_mm_ss constructor is ill-formed for unsigned durations

Section: 30.9.2 [time.hms.members] Status: Tentatively Ready Submitter: Michael Welsh Duggan Opened: 2025-06-04 Last modified: 2025-06-13

Priority: Not Prioritized

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Discussion:

In 30.9.2 [time.hms.members], paragraph 3, the current wording for the constructor of hh_mm_ss expresses some of its requirements in terms of abs(d), which is assumed to be chrono::abs(chrono::duration). chrono::abs is not defined, however, for durations with an unsigned representation. I believe that not being able to create hh_mm_ss objects from unsigned durations is unintentional.

Moreover, is_constructible_v<hh_mm_ss<ud>, ud> is required to be true by the standard for any duration, so making it actually work makes a lot of sense.

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

  1. Modify 30.9.2 [time.hms.members] as indicated:

    constexpr explicit hh_mm_ss(Duration d);

    -3- Effects: Constructs an object of type hh_mm_ss which represents the Duration d with precision precision.

    1. (3.1) — Initializes is_neg with d < Duration::zero(). Let ABS_D represent -d if is_neg is true and d otherwise.

    2. (3.2) — Initializes h with duration_cast<chrono::hours>(abs(d)ABS_D).

    3. (3.3) — Initializes m with duration_cast<chrono::minutes>(abs(d)ABS_D - hours()).

    4. (3.4) — Initializes s with duration_cast<chrono::seconds>(abs(d)ABS_D - hours() - minutes()).

    5. (3.5) — If treat_as_floating_point_v<precision::rep> is true, initializes ss with abs(d)ABS_D - hours() - minutes() - seconds(). Otherwise, initializes ss with duration_cast<precision>(abs(d)ABS_D - hours() - minutes() - seconds()).