deal with cancellation in qe for #6500

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/qe/qe.cpp

@@ -1437,13 +1437,12 @@ namespace qe {
                 res = m_solver.check();
                 if (res == l_true && has_uninterpreted(m, m_fml))
                     res = l_undef;
-                if (res == l_true) {
+                if (res == l_true)
+                    res = final_check();
+                if (res == l_true)
                     is_sat = true;
-                    final_check();
-                }
-                else {
+                else 
                     break;
-                }
             }
             if (res == l_undef) {
                 free_vars.append(num_vars, vars);
@@ -1501,30 +1500,33 @@ namespace qe {
 
     private:
 
-        void final_check() {
-            model_ref model;
+        lbool final_check() {
+            model_ref model;            
             m_solver.get_model(model);
+            if (!model)
+                return l_undef;
             scoped_ptr<model_evaluator> model_eval = alloc(model_evaluator, *model);
 
-            while (true) {
+            while (m.inc()) {
                 TRACE("qe", model_v2_pp(tout, *model););
-                while (can_propagate_assignment(*model_eval)) {
+                while (can_propagate_assignment(*model_eval)) 
                     propagate_assignment(*model_eval);
-                }
                 VERIFY(CHOOSE_VAR == update_current(*model_eval, true));
                 SASSERT(m_current->fml());
                 if (l_true != m_solver.check()) {
-                    return;
+                    return l_true;
                 }
                 m_solver.get_model(model);
                 model_eval = alloc(model_evaluator, *model);
                 search_tree* st = m_current;
                 update_current(*model_eval, false);
-                if (st == m_current) {
+                if (st == m_current) 
                     break;
-                }
-            }            
-            pop(*model_eval);                    
+            }
+            if (!m.inc())
+                return l_undef;
+            pop(*model_eval);
+            return l_true;            
         } 
 
         ast_manager& get_manager() override { return m; }