unify_with_occurs_check/2, re issue #897

infradig · infradig · commit c02dbde2b84e · 2025-10-03T15:39:59.000+10:00
diff --git a/src/unify.c b/src/unify.c
@@ -497,7 +497,7 @@ static bool unify_structs(query *q, cell *p1, pl_ctx p1_ctx, cell *p2, pl_ctx p2
 static bool unify_var(query *q, cell *p1, pl_ctx p1_ctx, cell *p2, pl_ctx p2_ctx, unsigned depth)
 {
 	bool was_cyclic = false;
-	bool check = is_global(p1) || (p1_ctx == 0);
+	bool check = !is_local(p1);
 
 	if (q->flags.occurs_check == OCCURS_CHECK_TRUE) {
 		if (check && is_cyclic_term(q, p2, p2_ctx))
diff --git a/tests/issues/test897.expected b/tests/issues/test897.expected
diff --git a/tests/issues/test897.pl b/tests/issues/test897.pl
@@ -0,0 +1,4 @@
+:- initialization(main).
+
+main :-
+	S = s(s(A,s(B,A)),1), T = s(s(C,C),1), \+ unify_with_occurs_check(S,T).

Original file line number	Diff line number	Diff line change
`@@ -497,7 +497,7 @@ static bool unify_structs(query q, cell p1, pl_ctx p1_ctx, cell *p2, pl_ctx p2`
`497`	`497`	`static bool unify_var(query q, cell p1, pl_ctx p1_ctx, cell *p2, pl_ctx p2_ctx, unsigned depth)`
`498`	`498`	`{`
`499`	`499`	`bool was_cyclic = false;`
`500`		`- bool check = is_global(p1) \|\| (p1_ctx == 0);`
	`500`	`+ bool check = !is_local(p1);`
`501`	`501`
`502`	`502`	`if (q->flags.occurs_check == OCCURS_CHECK_TRUE) {`
`503`	`503`	`if (check && is_cyclic_term(q, p2, p2_ctx))`
-Original file line number
+Diff line change
@@ @@ -0,0 +1,4 @@ @@
 +:- initialization(main).
++
 +main :-
 +	S = s(s(A,s(B,A)),1), T = s(s(C,C),1), \+ unify_with_occurs_check(S,T).