Created attachment 41387 [details]
I'm using gcc 7.1.0 from Debian/experimental (Debian package gcc-7 version 7.1.0-5).
I ran into a case where an uninitialized variable could be returned from a function without triggering a warning. With the attached minimized source tst.c the following does not yield a warning:
gcc-7 -DCALL -DTEST2 -Wall -Wextra -o /dev/null -c tst.c
Here I initialize some parts of the S structure (s.x), but not others (s.y). And I return the uninitialized piece: s.y. The extra f() call at the beginning of g() triggers the bug: taking that call out with
gcc-7 -DTEST2 -Wall -Wextra -o /dev/null -c tst.c
produces the warning as it should.
Furthermore, initializing s.x in a slightly different way makes the bug go away as well: this produces the warning also:
gcc-7 -DCALL -Wall -Wextra -o /dev/null -c tst.c
Finally, it looks like gcc-6 had a similar issue where the extra call to f() makes the warning go away, but it was fixed in gcc-7: the following misses the warning:
gcc-6 -DCALL -DTEST2 -Wall -Wextra -o /dev/null -c tst.c
Maybe the fix to THIS bug would be similar. Thanks!
There might be another bug already for this one.
I just tried this with Debian builds of gcc8 and gcc9: 8.3.0-19 and 9.2.1-8. This bug still exists in both.