From cb79e83510066c2ef14d96ff441d844869222f1e Mon Sep 17 00:00:00 2001
From: bakaq <bakaq@bakaq.net>
Date: Thu, 21 Sep 2023 14:00:37 -0300
Subject: [PATCH] Avoid dif/1 attribute with empty list

Closes #1956
---
 src/lib/dif.pl   | 5 ++++-
 src/tests/dif.pl | 7 +++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/lib/dif.pl b/src/lib/dif.pl
index 4fe35b80..fedb320c 100644
--- a/src/lib/dif.pl
+++ b/src/lib/dif.pl
@@ -35,7 +35,10 @@ vars_remove_goal([], _).
 vars_remove_goal([Var|Vars], Goal0) :-
     get_atts(Var, +dif(Goals0)),
     remove_goal(Goals0, Goal0, Goals),
-    put_atts(Var, +dif(Goals)),
+    (   Goals = [] ->
+        put_atts(Var, -dif(_))
+    ;   put_atts(Var, +dif(Goals))
+    ),
     vars_remove_goal(Vars, Goal0).
 
 reinforce_goal(Goal0, Goal) :-
diff --git a/src/tests/dif.pl b/src/tests/dif.pl
index 752458b5..bb1c76d6 100644
--- a/src/tests/dif.pl
+++ b/src/tests/dif.pl
@@ -189,6 +189,13 @@ test("dif#12 but with multiple variables in the residuals",(
     Res = []
 )).
 
+% https://github.com/mthom/scryer-prolog/issues/1956
+test("scryer-prolog#1956",(
+    call_residue_vars((dif(a-a,X-_),X=b), Res),
+    X == b,
+    Res = []
+)).
+
 main :-
     findall(test(Name, Goal), test(Name, Goal), Tests),
     run_tests(Tests, Failed),
-- 
2.54.0