Memory corruption bug causes crash in bison 3.6.4

I have been able to reproduce this bug in the latest version and in
versions as old as 3.0.4, the oldest version I have tried.

I am attaching the smallest grammar I was able to produce that
generates the error. I'm aware that this grammar is rather contrived;
I can provide a larger, more complete-but-self-contained example upon
request, but the bug reporting instructions recommended I minimize the
grammar as much as possible, so I chose to provide the smallest
grammar I could construct that triggered the crash, even though in
practice it looks nothing like a real production grammar.

The bug manifests with the following error message, followed by bison
aborting without completing successfully or generating any output:
"double free or corrruption (!prev)". I am on Ubuntu 18.04, and I
built bison with gcc 7.5.0. I am also attaching the log generated from
running bison through valgrind on this example, which seems to show a
number of use-after-free violations.

-- 
Dwight Guth

Director of Engineering


Email: [email protected]