Add precision modifier for seconds in chrono format

[SappyJoy]

Add precision modifier for seconds in chrono format and test for it.

It's became important for custom datetime types that store date, time and subseconds. I want to use chrono format for those types, but I have no way to control subsecond precision.

Use case:

EXPECT_EQ(fmt::format("{:%Y-%m-%d %H:%M:%.6S %Z}", datetime), "2022-09-28 12:14:25.123456 MSK");

[vitaut]

Thanks for the PR. Is this feature part of the standard?

[SappyJoy]

No, it is not in standard. The standard only supports the precision modifier for strings and floating point numbers. Precision available only for particular argument, but not for format-spec.

[vitaut]

Thanks for the PR. Adding precision support to durations sounds reasonable but for consistency with the standard it should go at the beginning, not in %S (https://eel.is/c++draft/time.format), e.g.

fmt::print("{:.6%S}", std::chrono::nanoseconds{1234});

[SappyJoy]

{:.6%S} looks like precision for argument. There is test for this:

  EXPECT_EQ("1.2 ms   ", fmt::format("{:7.1%Q %q}", dms(1.234)));

It is not obvious for user to use {:.6%S} and expect that this precision modifier belong to %S. But it can be done this way if the specifier has a higher priority on getting the modifier than the argument.

[vitaut]

But it can be done this way if the specifier has a higher priority on getting the modifier than the argument.

We should make existing precision apply to %S and not introduce a new one to avoid diverging too much from the standard.

[SappyJoy]

Oh, I squashed last commits to resolve conflict. It led to close this PR. @vitaut, please reopen it, I made changes. Now it should work.

[vitaut]

I am not able to reopen this diff for some reason. Please submit a new one.