Default format prints nothing for std::chrono::system_clock::time_point

[toomuchsalt]

Tested with both current head and version 7.1.3( godbolt )

The following code yields an empty string

#include <fmt/chrono.h>

#include <chrono>

int main() {
  fmt::print("Default format: {} ", std::chrono::system_clock::now());
}

https://godbolt.org/z/E33jW916o

[vitaut]

Looks like the default format should be as described in https://eel.is/c++draft/time#clock.system.nonmembers-2, but this is not implemented yet. PRs are welcome.

[sunmy2019]

This part is a little bit triky.
In the C++ stardard, we need a locale object to format year_month_day and hh_mm_ss. Thus, directly using fmt::print lacks the information of which locale to use.
Even with fmt::format(locale, format, args), there is a problem: we cannot squeeze the date/time format direcly from locale (unless we use some internal API). So a call to the time_put is inevitable if we stick to the standard.

There are some workarounds.

1. We can throw away the locale object, using {:%Y-%m-%d %H:%M:%S} as default format.
2. We can keep the locale object, and use the global locale if unsepecified. Making every output go through the function time_put. Just like the how we handle weekday's format.

The first is easy to implement. (I already implemented.) While the latter is more complicated, and of course, more time/memory consuming.

[vitaut]

The default should be locale-independent (the first option). There is a Library Working Group issue to fix it in the standard as well.

[vitaut]

Default format has been added in #2345. Thanks @sunmy2019.