`<filesystem>`: `temp_directory_path(error_code&)` doesn't return an empty `path` for errors

[StephanTLavavej]

WG21-N5008 [fs.op.temp.dir.path]/3:

Returns: The path p. The signature with argument ec returns path() if an error occurs.

That doesn't appear to be what we do:

STL/stl/inc/filesystem

Lines 4042 to 4056 in 7841cf8

	_EXPORT_STD _NODISCARD inline path temp_directory_path(error_code& _Ec) {
	// get a location suitable for temporary storage, and verify that it is a directory
	_Ec.clear(); // for exception safety
	path _Result;
	_Result._Text.resize(__std_fs_temp_path_max);
	const auto _Temp_result = __std_fs_get_temp_path(_Result._Text.data());
	_Result._Text.resize(_Temp_result._Size);
	if (_Temp_result._Error == __std_win_error::_Max) { // path could be retrieved, but was not a directory
	_Ec = _STD make_error_code(errc::not_a_directory);
	} else {
	_Ec = _Make_ec(_Temp_result._Error);
	}
	return _Result;
	}

STL/stl/src/filesystem.cpp

Lines 807 to 835 in 7841cf8

	[[nodiscard]] _Success_(return._Error == __std_win_error::_Success) __std_ulong_and_error
	__stdcall __std_fs_get_temp_path(_Out_writes_z_(__std_fs_temp_path_max) wchar_t* const _Target) noexcept {
	// calls GetTempPath2W if available (Win11+), else calls GetTempPathW
	// If getting the path failed, returns 0 size; otherwise, returns the size of the
	// expected directory. If the path could be resolved to an existing directory,
	// returns __std_win_error::_Success; otherwise, returns __std_win_error::_Max.
	const auto _Size = _Stl_GetTempPath2W(__std_fs_temp_path_max, _Target);
	if (_Size == 0) {
	return {0, __std_win_error{GetLastError()}};
	}
	// Effects: If exists(p) is false or is_directory(p) is false, an error is reported
	const DWORD _Attributes = GetFileAttributesW(_Target);
	if (_Attributes == INVALID_FILE_ATTRIBUTES || (_Attributes & FILE_ATTRIBUTE_DIRECTORY) == 0u) {
	return {_Size, __std_win_error::_Max};
	}
	if ((_Attributes & FILE_ATTRIBUTE_REPARSE_POINT) != 0u) {
	__std_fs_file_handle _Handle;
	const auto _Last_error = __std_fs_open_handle(
	&_Handle, _Target, __std_access_rights::_File_read_attributes, __std_fs_file_flags::_Backup_semantics);
	__std_fs_close_handle(_Handle);
	if (_Last_error != __std_win_error::_Success) {
	return {_Size, __std_win_error::_Max};
	}
	}
	return {_Size, __std_win_error::_Success};
	}