Which characters are transformed by toupper() in the default C++ locale?

With the default "C" locale only a-z get transformed by std::toupper() as is documented for example here. Which characters exactly get transformed by std::ctype<CharT>::toupper() with the default C++ locale?

I'm asking because std::toupper(L'ω', std::locale::classic()) returns L'Ω' on Windows and I'm wondering for which other characters the C++ locale also returns an upper case form. In the "C" locale the same character is not transformed: static_cast<wchar_t>(std::towupper(static_cast<std::wint_t>(L'ω'))) returns L'ω' as expected.

I used the following program to verify this:

#include <cwctype>
#include <fstream>
#include <locale>

int main()
{
  std::wofstream fs("out.txt");
  fs.imbue(std::locale("en_US.UTF8"));
  fs << L"std::toupper(L'ω', std::locale::classic()): " << std::toupper(L'ω', std::locale::classic()) << std::endl;
  fs << L"static_cast<wchar_t>(std::towupper(static_cast<std::wint_t>(L'ω'))): "
     << static_cast<wchar_t>(std::towupper(static_cast<std::wint_t>(L'ω'))) << std::endl;

  return 0;
}

Content of out.txt when compiled with Visual Studio 2019 (save source file with UTF-8 encoding and add compiler switch /utf-8) and executed on Windows 10:

std::toupper(L'ω', std::locale::classic()): Ω
static_cast<wchar_t>(std::towupper(static_cast<std::wint_t>(L'ω'))): ω

Output with gcc version 8.4.0 (Ubuntu 8.4.0-1ubuntu1~18.04):

std::toupper(L'ω', std::locale::classic()): ω
static_cast<wchar_t>(std::towupper(static_cast<std::wint_t>(L'ω'))): ω