Qmake generates uncorrect format of DEPENDPATH

only one file named main.cpp on test\f1\f2\f3\
and run qmake -project on path test

open generated test.pro file, which has a line

DEPENDPATH += . f1\f2\f3

when I run qmake, it shows

WARNING: .....\test\test.pro:7: Unescaped backslashes are deprecated.

I traced the code, the line
v["DEPENDPATH"] += fileFixify(dir);
might need change, in file projectgenerator.cpp