Function IsXDigit

tests/gtest/gtest.h:2235–2237

  return isupper(static_cast<unsigned char>(ch)) != 0;
}
inline bool IsXDigit(char ch) {
  return isxdigit(static_cast<unsigned char>(ch)) != 0;
}
inline bool IsXDigit(wchar_t ch) {
  const unsigned char low_byte = static_cast<unsigned char>(ch);
  return ch == low_byte && isxdigit(low_byte) != 0;