gprolog builtin phrase/3 incorrectly succeeds when matching constant lists

[ais523]

gprolog seems to be producing incorrect results when using the phrase/3 builtin. In particular, the symptom is that when phrase/3 is used to match a DCG that requires its input to start with a particular string/list, and the input actually starts with something else, phrase/3 reports that there was a match (returning an unbound list for the remaining unconsumed input), rather than reporting that there was no match.

Example to reproduce: start gprolog, and give it the input line

phrase([2], [1,2], T).

Expected behaviour: gprolog answers "no": this query is equivalent to "remove the prefix [2] from the list [1,2] and bind the remainder to T", and [1,2] does not start with [2], so this task should be impossible. I've verified that this is the expected/correct output against both gprolog's documentation, and against swipl (a different Prolog implementation).

Actual behaviour: gprolog answers "yes", without binding anything to T (it remains an unbound variable).

The problem seems to be specific to the case of there being no match; with the comparable

phrase([1], [1,2], T).

gprolog will produce the output of "T=[2]" and "yes" (the expected output), and will not suggest a second solution with T unbound.

It's also specific to the use of phrase/3 for evaluating DCG literals directly; using the --> operator to give a name to the grammar consisting only of [2], then executing that grammar (whether directly or with phrase/3), produces the correct results.

phrase/2 seems to have the same buggy behaviour as phrase/3 (probably because it's implemented as calling phrase/3 behind the scenes).

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gprolog 1.4.5.0-3
ProcVersionSignature: Ubuntu 5.4.0-84.94-generic 5.4.133
Uname: Linux 5.4.0-84-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.20
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: X-Cinnamon
Date: Wed Oct 6 08:42:51 2021
ExecutablePath: /usr/lib/gprolog/bin/gprolog
InstallationDate: Installed on 2019-11-11 (694 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
SourcePackage: gprolog
UpgradeStatus: Upgraded to focal on 2020-10-02 (368 days ago)