Jonathan Wakely <cxx@kayari.org> writes:

On Tue, 3 Jun 2025 at 23:24, Jonathan Wakely <cxx@kayari.org> wrote:

On Tue, 3 Jun 2025 at 21:58, Howard Hinnant via Std-Proposals <
std-proposals@lists.isocpp.org> wrote:

I view this as a bug in the spec.  It was not intended to outlaw unsigned
durations for hh_mm_ss.

Upon experimentation I found that my example implementation does not
outlaw it, gcc does not, msvc does, and llvm does.

A revised wording might look something like:  let abs_d have the value -d
if is_neg is true, else d.  Then replace abs(d) with abs_d.

This change in wording will not break gcc.  And it will turn cases that
don’t compile on MSVC and LLVM into cases that do compile, so I believe the
“breakage” would be acceptable.

It's no accident that it works with GCC:

commit r13-5002-ge36e57b032b2d70eaa1294d5921e4fd8ce12a74d <https://gcc.gnu.org/cgit/gcc/commit/?id=e36e57b032b2d70eaa1294d5921e4fd8ce12a74d>
Author: Jonathan Wakely
Date:   Wed Jan 4 16:43:51 2023 +0000

   libstdc++: Fix std::chrono::hh_mm_ss with unsigned rep [PR108265 <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108265>]

   libstdc++-v3/ChangeLog:

           PR libstdc++/108265 <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108265>
           * include/std/chrono (hh_mm_ss): Do not use chrono::abs if
           duration rep is unsigned.
           * testsuite/std/time/hh_mm_ss/1.cc: Check unsigned rep.


In response to https://gcc.gnu.org/PR108265 reported by one Michael
Duggan :-)

I think this is a conforming extension (no valid program can detect the
difference) and I agree with Howard that it doesn't cause any real
breakage. The fact that chrono::hh_mm_ss uses chrono::abs internally is not
detectable using SFINAE or concepts, because
std::is_constructible_v<hh_mm_ss<seconds>, duration<unsigned>> is true,
even for implementations where it doesn't actually compile if you try to
construct that.

Changing the spec to make this work seems like a defect fix, it wouldn't
break anything.

Okay.  I have never submitted a DR, so I would like a review of the
message I propose to send to lwgchair@gmail.com.  I removed the TeX
formatting, but I could re-add it if you think it is best.

The chrono::hh_mm_ss constructor is poorly defined for negative
durations.

In [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 this is unintentional.

Proposed wording:

Constructs an object of type hh_mm_ss which represents the Duration d
with precision precision.  Let ABS_D represent -d if is_neg is true and
d otherwise.

 Initializes is_neg with d < Duration::zero().

 Initializes h with duration_cast<chrono::hours>(ABS_D).

 Initializes m with duration_cast<chrono::minutes>(ABS_D - hours()).

 Initializes s with duration_cast<chrono::seconds>(ABS_D - hours() -
 minutes()).

 If treat_as_floating_point_v<precision::rep> is true, initializes ss
 with ABS_D - hours() - minutes() - seconds().

 Otherwise, initializes ss with duration_cast<precision>(ABS_D(d) -
 hours() - minutes()- seconds()).