Improve timezone tests

[phprus]

Issues present after commit 115001a

1. macOS has broken std::time_put:

[ RUN      ] chrono_test.system_clock_time_point
/Users/phprus/Devel/fmt/test/chrono-test.cc:287: Failure
Expected equality of these values:
  sys_output
    Which is: "+0500"
  fmt::format(fmt::runtime(fmt_spec), t1)
    Which is: "+0000"
/Users/phprus/Devel/fmt/test/chrono-test.cc:288: Failure
Expected equality of these values:
  sys_output
    Which is: "+0500"
  fmt::format(fmt::runtime(fmt_spec), tm)
    Which is: "+0000"
/Users/phprus/Devel/fmt/test/chrono-test.cc:299: Failure
Expected equality of these values:
  sys_output
    Which is: "+05:00"
  fmt::format("{:%Ez}", t1)
    Which is: "+00:00"
/Users/phprus/Devel/fmt/test/chrono-test.cc:300: Failure
Expected equality of these values:
  sys_output
    Which is: "+05:00"
  fmt::format("{:%Ez}", tm)
    Which is: "+00:00"
/Users/phprus/Devel/fmt/test/chrono-test.cc:302: Failure
Expected equality of these values:
  sys_output
    Which is: "+05:00"
  fmt::format("{:%Oz}", t1)
    Which is: "+00:00"
/Users/phprus/Devel/fmt/test/chrono-test.cc:303: Failure
Expected equality of these values:
  sys_output
    Which is: "+05:00"
  fmt::format("{:%Oz}", tm)
    Which is: "+00:00"
[  FAILED  ] chrono_test.system_clock_time_point (0 ms)

2. Windows has not normal support UTC timezone for std::tm.
   We need to use std::chrono to format time with timezone.

[phprus]

Demo:
See errors in CI for branch https://github.com/phprus/fmt/tree/chrono-tz-test-demo

[phprus]

Rebased

[vitaut]

Thanks for the PR! I noticed the timezone issues too but didn't have time to look into them myself.

What is the purpose of setting the timezone in CI and can we avoid it to keep configs simpler?

[vitaut]

I suggest moving this code to a separate function and reuse here and elsewhere in chrono-test.

[vitaut]

A comment clarifying why these are handled separately would be nice too.

[phprus]

For loops are not equivalent.

First for loop tests std::chrono::time_point (std::chrono::system_clock::now()) and std::tm (in UTC).
Second for loop tests timezone formatters only for std::tm (in localtime).

I think we should not do this, since timezone support is a weak point of fmt and the standard library.

[phprus]

@vitaut
Comments added.

[vitaut]

I didn't mean merging the for loops, just moving the selected block, namely

#if defined(__MINGW32__) && !defined(_UCRT)
  spec_list = {"%Z"};
#else
  spec_list = {"%z", "%Z"};
#endif

into a separate function (say, get_timezone_specs) to keep the system-specific conditional compilation in one place.

Other than that, all looks good.

[phprus]

@vitaut
I don't understand how this will simplify the code.
The condition defined(__MINGW32__) && !defined(_UCRT) is used 4 times in this test. Partially with precondition ifndef _WIN32.

[vitaut]

It is not perfect but would be slightly cleaner because we could reuse this function in two of these places. Another option is to move the defined(__MINGW32__) && !defined(_UCRT) condition into one place, define our own macro (e.g. FMT_HAS_C99_STRFTIME) and reuse that in all places.

[phprus]

Done.
I added macro FMT_HAS_C99_STRFTIME.

[phprus]

@vitaut
By default GitHub Actions uses UTC and possible time zone related issues are not tested in CI.
I think this change makes sense for testing future versions of fmt.

[vitaut]

By default GitHub Actions uses UTC and possible time zone related issues are not tested in CI.
I think this change makes sense for testing future versions of fmt.

Sounds reasonable but is it possible to move changing the timezone to code instead of CI?

[phprus]

@vitaut

Sounds reasonable but is it possible to move changing the timezone to code instead of CI?

Unfortunately, I am not aware of a cross-platform and universal API for changing the timezone settings. So I changed it at the CI settings level. I think it's more reliable.

[phprus]

I understand that this is not are best solution, but I have no other universal solution.
Chrono supports timezones only in C++20 and only in MSVC.

[vitaut]

Thank you!