Workaround a broken C locale on Windows

include/fmt/format.h

@@ -1453,8 +1453,9 @@ template <typename Range> class basic_writer {
     };
 
     void on_num() {
-      int num_digits = internal::count_digits(abs_value);
       char_type sep = internal::thousands_sep<char_type>(writer.locale_);
+      if (!sep) return on_dec();
+      int num_digits = internal::count_digits(abs_value);
       int size = num_digits + sep_size * ((num_digits - 1) / 3);
       writer.write_int(size, get_prefix(), specs,
                        num_writer{abs_value, size, sep});

test/locale-test.cc

@@ -44,5 +44,9 @@ TEST(LocaleTest, WFormat) {
   EXPECT_EQ(L"1~234~567", fmt::format(loc, L"{:n}", 1234567));
   fmt::format_arg_store<fmt::wformat_context, int> as{1234567};
   EXPECT_EQ(L"1~234~567", fmt::vformat(loc, L"{:n}", fmt::wformat_args(as)));
+  auto sep =
+      std::use_facet<std::numpunct<wchar_t>>(std::locale("C")).thousands_sep();
+  auto result = sep == ',' ? L"1,234,567" : L"1234567";
+  EXPECT_EQ(result, fmt::format(std::locale("C"), L"{:n}", 1234567));
 }
 #endif  // FMT_STATIC_THOUSANDS_SEPARATOR