correct is_cyclic again for non-variable ref cells (#2116)

src/machine/gc.rs

@@ -273,8 +273,10 @@ impl<'a, UMP: UnmarkPolicy> StacklessPreOrderHeapIter<'a, UMP> {
     fn is_cyclic(&self, var_current: usize, var_next: usize) -> bool {
         if self.heap[var_next].is_var() {
             !self.heap[var_next].get_forwarding_bit() && var_current != var_next
+        } else if self.heap[var_next].is_ref() {
+            self.heap[var_next].get_mark_bit()
         } else {
-            self.heap[var_next].is_ref()
+            false
         }
     }
 

src/tests/acyclic_term.pl

@@ -173,6 +173,10 @@ test("acyclic_term#2114", (
     A=B*B, acyclic_term(A)
 )).
 
+test("acyclic_term#2116", (
+    A=B*B,B=[]*[], acyclic_term(A)
+)).
+
 main :-
     findall(test(Name, Goal), test(Name, Goal), Tests),
     run_tests(Tests, Failed),