Specifier 'n' does not consider the grouping() from the locale

[dlaugt]

C++ locale has two configurations (thousands_sep() and grouping()) to display thousand separator. fmt is using only thousands_sep(). It would be nice if fmt uses both as stringstream.

struct no_grouping : std::numpunct<char>
{
  std::string do_grouping () const {return "";}
};
std::locale loc (locale ("en-US"), new no_grouping);

std::stringstream ss;
ss.imbue (loc);
ss << 100000;
// gives 100000

fmt::format (loc, "{:n}", 100000);
// gives 100,000

struct special_grouping : std::numpunct<char>
{
  std::string do_grouping () const {return "\03\02";}
};
std::locale loc (locale ("en-US"), new special_grouping);

std::stringstream ss;
ss.imbue (loc);
ss << 100000;
// gives 1,00,000

fmt::format (loc, "{:n}", 100000);
// gives 100,000

For example, the Indian locale is a practical usage in python:

>>> from locale import setlocale, LC_ALL
>>> setlocale(LC_ALL, 'en_IN')
'en_IN'
>>> '{:n}'.format(100000)
'1,00,000'