LWG-4090 Underspecified use of locale facets for locale-dependent `std::format`

[StephanTLavavej]

LWG-4090 Underspecified use of locale facets for locale-dependent std::format

[frederick-vs-ja]

The numpunct parts were already implemented and tested in #1767 (merged in #1821, later fixed in #1862). Seems like that we still need to implement the remaining (with mechanisms like those in #2977).

[cpplearner]

Do we support non-ASCII thousand separators and decimal points (for which the transcoding will make a difference), actually?

[statementreply]

When digit separators are U+00A0 (C2 A0 in UTF-8), the wchar_t version is correct, but the char version isn't.

C:\Users\He\source\temp>chcp 65001
Active code page: 65001

C:\Users\He\source\temp>type nbsp_separator.cpp

#include <cstdio>
#include <locale>
#include <print>
using namespace std;

int main() {
    locale::global(locale(""));
    fputs(format("{:L}\n", 12345).c_str(), stdout);
    fputws(format(L"{:L}\n", 12345).c_str(), stdout);
    return 0;
}

C:\Users\He\source\temp>cl /W4 /EHsc /std:c++latest /utf-8 nbsp_separator.cpp
用于 x64 的 Microsoft (R) C/C++ 优化编译器 19.52.36502 版(预览)
版权所有(C) Microsoft Corporation。保留所有权利。

/std:c++latest 作为最新的 C++
working 草稿中的语言功能预览提供。我们希望你提供有关 bug 和改进建议的反馈。
但是，请注意，这些功能按原样提供，没有支持，并且会随着工作草稿的变化
而更改或移除。有关详细信息，请参阅
https://go.microsoft.com/fwlink/?linkid=2045807。

nbsp_separator.cpp
Microsoft (R) Incremental Linker Version 14.52.36502.0 (PREVIEW)
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:nbsp_separator.exe
nbsp_separator.obj

C:\Users\He\source\temp>nbsp_separator.exe
1212 345

C:\Users\He\source\temp>nbsp_separator.exe | wsl xxd -g1
00000000: 31 32 c2 33 34 35 0d 0a 31 32 c2 a0 33 34 35 0d  12.345..12..345.
00000010: 0a                                               .