[Bug middle-end/104069] Wuse-after-free=2 -O0 false positive "may be used"

["mkaracsony81 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104069

--- Comment #31 from Miklos Karacsony <mkaracsony81 at gmail dot com> ---
(In reply to Sergei Trofimovich from comment #30)
> > configure: WARNING: unrecognized options: --disable-werror
> > In function 'bigger_buffer',
> >     inlined from '__libdw_gunzip' at gzip.c:374:12:
> > gzip.c:96:9: error: pointer may be used after 'realloc'
> > [-Werror=use-after-free]
> >    96 |     b = realloc (state->buffer, more -= 1024);
> >       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > gzip.c:94:13: note: call to 'realloc' here
> >    94 |   char *b = realloc (state->buffer, more);
> >       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > cc1: all warnings being treated as errors
> > make[2]: *** [Makefile:715: gzip.o] Error 1
> 
> You should be able to extract preprocessed file using
> https://gcc.gnu.org/bugs/#need: you will need to find exact `gcc` invocation
> that causes build failure, add `-save-temps` parameter to it, grab '*.i'
> file and attach it along with exact command you used to produce it.

That gcc page does not say how to extract the command from the makefile that
triggers the error, so I've tried to reproduce this bug manually and after like
the 8th attempt when all the header files were finally found by the compiler,
the linker errored out with some undefined references, so no luck. The good new
is that this error is reproducible if the cflags/ldflags/configure...etc
switches set up like in c#29