LWG4090 Underspecified use of locale facets for locale-dependent std::format

[jwakely]

https://cplusplus.github.io/LWG/issue4090

LWG would like SG16 to look at this issue please

[tahonermann]

SG16 discussed this issue during its 2024-06-12 meeting. No polls were taken. SG16 will review again in the new year.

[tahonermann]

Now that I've finally published a meeting summary for the 2024-06-12 SG16 meeting, I can detail the next steps for resolving this issue.

SG16 concluded that the std::numpunct facet should be used to format integer, floating-point, and bool types and that this matches existing implementation behavior for libc++, libstdc++, and MS STL as exhibited at https://godbolt.org/z/GM6Mzv6vG. This program:

#include <format>
#include <iostream>
#include <locale>

struct test_numpunct : std::numpunct<char> {
  char do_decimal_point() const override {
	return '-';
  }
  char do_thousands_sep() const override {
	return '\'';
  }
  std::string do_grouping() const override {
	return "\1";
  }
  std::string do_truename() const override {
	return "eurt";
  }
  std::string do_falsename() const override {
	return "eslaf";
  }
};

int main() {
  std::locale loc;
  std::cout << std::format(loc, "{:L} {:L} {:L} {:L}\n", 123456, 123.456, true, false);
  loc = std::locale(loc, new test_numpunct);
  std::cout << std::format(loc, "{:L} {:L} {:L} {:L}\n", 123456, 123.456, true, false);
}

produces the following output for all three implementations.

123456 123.456 true false
1'2'3'4'5'6 1'2'3-456 eurt eslaf

Despite implementations doing the right thing, the intended behavior is under specified in the standard and [format.string.std] should be updated accordingly. A proposed resolution is needed to make further progress on this issue. I'll ask SG16 participants for a volunteer to draft wording. Once a PR materializes, I'll have SG16 confirm it matches the intent.

[jwakely]

https://godbolt.org/z/PexbszhWs extends the example to show a custom num_put facet which is not used by the std::format calls (but is used when writing to a stream directly after imbuing the stream with the locale). This confirms that implementations use numpunct directly, not via num_put.