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### 3232. Inconsistency in `zoned_time` deduction guides

Section: 27.11.7.1 [time.zone.zonedtime.overview] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-30 Last modified: 2021-02-25

Priority: 0

View all other issues in [time.zone.zonedtime.overview].

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Discussion:

Currently, the `time_zone` is always storing the time with the precision not coarser that `seconds`, as consequence any instance with the duration with coarser precision with `seconds`, is de-facto equivalent to one instantiated to the duration of seconds. This is caused by the fact, that time zone offset can be defined up to seconds precision. To illustrate both of following specialization has the same behavior (keep `seconds`):

>     zoned_time<minutes> zt(current_zone(), floor<minutes>(system_clock::now());
>     zoned_time<hours>   zt(current_zone(), floor<hours>(system_clock::now());
>     zoned_time<seconds> zt(current_zone(), floor<seconds>(system_clock::now());

To avoid unnecessary code bloat caused by above, most deduction guides are instantiating `zoned_time` with at least `seconds` precision (i.e. `zoned_time<seconds>` will be deduced for all of above):

>     template<class TimeZonePtr, class Duration>
>       zoned_time(TimeZonePtr, zoned_time<Duration>, choose = choose::earliest)
>         -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;

However there is single exception:

>     template<class Duration, class TimeZonePtr, class TimeZonePtr2>
>       zoned_time(TimeZonePtr, zoned_time<Duration, TimeZonePtr2>, choose = choose::earliest)
>          -> zoned_time<Duration, TimeZonePtr>;

This deduction guide should be updated to preserve the consistency of design.

[2019-07 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4820.

1. Modify 27.11.7.1 [time.zone.zonedtime.overview] as indicated:

   >     […]
   >     template<class Duration, class TimeZonePtr, class TimeZonePtr2>
   >       zoned_time(TimeZonePtr, zoned_time<Duration, TimeZonePtr2>, choose = choose::earliest)
   >         -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;
   >     […]