gprolog segmentation violation due to infinite recursion in unification

Paul Eggert <[email protected]> Wed, 13 Dec 2017 22:45:24 -0800

Newsgroups	gmane.comp.gnu.prolog.bugs
Organization	UCLA Computer Science Department
Message-ID	<[email protected]>

This problem was found by my student Trevor Hackett. To reproduce the problem, 
build and run gprolog for x86-64 on Fedora 27, or use the gprolog bundled with 
Fedora 27 x86-64:

$ gprolog
GNU Prolog 1.4.4 (64 bits)
Compiled Aug  3 2017, 08:15:35 with gcc
By Daniel Diaz
Copyright (C) 1999-2013 Daniel Diaz
| ?- X = [A,B,C,Y], Y = [A,B,C,X], X = Y.
Segmentation fault (core dumped)

It looks like Pl_Unify recurses forever at unify.c:117.