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[time.clock.cast.sys],[time.clock.cast.utc] Use Duration2 for clarity. #4565

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Merged
merged 2 commits into from
May 29, 2021
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[time.clock.cast.sys],[time.clock.cast.utc] Use Duration2 for clarity. #4565

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7953c23
[time.clock.cast.sys],[time.clock.cast.utc] Use Duration2 for clarity.
StephanTLavavej Mar 28, 2021
9a3fe91
Break `Duration2` with hyphenation to correct overfull \hbox (#1)
CaseyCarter Mar 29, 2021
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16 changes: 8 additions & 8 deletions source/time.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -3717,8 +3717,8 @@

\pnum
\mandates
\tcode{SourceClock::to_sys(t)} returns a \tcode{sys_time<Duration>},
where \tcode{Duration} is a valid \tcode{chrono::duration} specialization.
\tcode{SourceClock::to_sys(t)} returns a \tcode{sys_time<Duration2>},
where \tcode{Duration2} is a valid \tcode{chrono::duration} specialization.
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This seems like a normative change to me. Before, I read this as requiring Duration to be a valid specialization of chrono::duration and requiring SourceClock::to_sys(t) to return sys_time<Duration>. The after condition is obviously quite different, allowing SourceClock::to_sys(t) to return a sys_time parameterized with a completely different specialization of chrono::duration.

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I read this as [...] requiring SourceClock::to_sys(t) to return sys_time<Duration>.

That's a plausible reading on its face, but based on my experience implementing this, I think it's too restrictive to be the intended meaning. There's no guarantee that the input Duration will be able to represent the difference between the two clock epochs, so in general there must be some flexibility to return a different duration specialization. That's why all the built-in conversions return a common_type_t<Duration, seconds>. Otherwise, something reasonable like clock_cast<gps_clock>(sys_days{1d / January / 2020y}) won't work if the input and output Duration have to be the same.

But I'm happy to help, if desired, if people prefer to see this escalated to an LWG issue.

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But I'm happy to help, if desired, if people prefer to see this escalated to an LWG issue.

We've already gone to the trouble to prepare a PR, I suggest waiting for word from one of the editorial team before dropping this and resubmitting as an LWG issue. My instincts for what the Project Editor will consider to be an editorial change are not infallible.


\pnum
\returns
Expand Down Expand Up @@ -3748,8 +3748,8 @@

\pnum
\mandates
\tcode{DestClock::from_sys(t)} returns a \tcode{time_point<DestClock, Duration>},
where \tcode{Duration} is a valid \tcode{chrono::duration} specialization.
\tcode{DestClock::from_sys(t)} returns a \tcode{time_point<DestClock, Duration2>},
where \tcode{Dura\-tion2} is a valid \tcode{chrono::duration} specialization.

\pnum
\returns
Expand Down Expand Up @@ -3781,8 +3781,8 @@

\pnum
\mandates
\tcode{SourceClock::to_utc(t)} returns a \tcode{utc_time<Duration>},
where \tcode{Duration} is a valid \tcode{chrono::duration} specialization.
\tcode{SourceClock::to_utc(t)} returns a \tcode{utc_time<Duration2>},
where \tcode{Duration2} is a valid \tcode{chrono::duration} specialization.

\pnum
\returns
Expand Down Expand Up @@ -3812,8 +3812,8 @@

\pnum
\mandates
\tcode{DestClock::from_utc(t)} returns a \tcode{time_point<DestClock, Duration>},
where \tcode{Duration} is a valid \tcode{chrono::duration} specialization.
\tcode{DestClock::from_utc(t)} returns a \tcode{time_point<DestClock, Duration2>},
where \tcode{Dura\-tion2} is a valid \tcode{chrono::duration} specialization.

\pnum
\returns
Expand Down