_isatty vs GetConsoleMode?

[dimztimz]

In the paper there is this note:

Changes since R3:
Replaced _isatty(_fileno(stream)) with GetConsoleMode(_get_osfhandle(_fileno(stream)), ...) in a note in [format.functions] because the former may return 1 for streams not referring to a terminal.

What is the difference? Which one is more correct? Currently _isatty() is still used:

fmt/include/fmt/format-inl.h

Lines 1482 to 1498 in 81f1cc7

	FMT_FUNC bool write_console(std::FILE* f, string_view text) {
	auto fd = _fileno(f);
	if (_isatty(fd)) {
	detail::utf8_to_utf16 u16(string_view(text.data(), text.size()));
	auto written = detail::dword();
	if (detail::WriteConsoleW(reinterpret_cast<void*>(_get_osfhandle(fd)),
	u16.c_str(), static_cast<uint32_t>(u16.size()),
	&written, nullptr)) {
	return true;
	}
	}
	// We return false if the file descriptor was not TTY, or it was but
	// SetConsoleW failed which can happen if the output has been redirected to
	// NUL. In both cases when we return false, we should attempt to do regular
	// write via fwrite or std::ostream::write.
	return false;
	}

Related issue #2080. Is the fact that we return false when WriteConsoleW() fails good enough solution and there is no need for GetConsoleMode?

[vitaut]

For the purposes of the paper GetConsoleMode is the right thing because _isatty may return true when there is no console in some cases (I don't remember exactly which though). Implementation-wise it shouldn't matter because in the worst case there is extra transcoding in those cases which is then discarded but that should be uncommon.