<format>: The width of output is miscalculated when formatting a floating-point number in the locale-specific form

[cpplearner]

Describe the bug

Revealed by libc++ test std/utilities/format/format.functions/locale-specific_form.pass.cpp.

When formatting a floating-point number, digit separators are not taken into account when computing the width of output, which may cause too many fill characters in the output.

For integers, digit separators are correctly handled:

STL/stl/inc/format

Lines 2873 to 2878 in 3eac329

	if (_Specs._Localized) {
	_Groups = _STD use_facet<numpunct<_CharT>>(_Locale._Get()).grouping();
	_Separators = _Count_separators(static_cast<size_t>(_End - _Buffer_start), _Groups);
	// TRANSITION, separators may be wider for wide chars
	_Width += _Separators;
	}

For floating-point numbers, it seems that the width is not adjusted:

STL/stl/inc/format

Lines 3099 to 3102 in 3eac329

	if (_Specs._Localized) {
	_Groups = _STD use_facet<numpunct<_CharT>>(_Locale._Get()).grouping();
	_Separators = _Count_separators(static_cast<size_t>(_Integral_end - _Buffer_start), _Groups);
	}

Command-line test case

D:\test>type test-format.cpp
#include <format>
#include <iostream>

int main() {
    std::cout << std::format(std::locale("en_US"), "{:@>8L}\n", 12345);
    std::cout << std::format(std::locale("en_US"), "{:@>8L}\n", 12345.0);
}

D:\test>cl /std:c++20 /utf-8 /EHs test-format.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.39.33321 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test-format.cpp
Microsoft (R) Incremental Linker Version 14.39.33321.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test-format.exe
test-format.obj

D:\test>test-format.exe
@@12,345
@@@12,345

Expected behavior

The output should be @@12,345 instead of @@@12,345.

STL version

https://github.com/microsoft/STL/commit/3eac329d1f614ecf138d96c22a3b02f87076bc4a