fix bug in conflict::is_valid exposed by testing unit propagation

src/math/polysat/conflict.cpp

@@ -211,8 +211,8 @@ namespace polysat {
     bool conflict::is_valid() const {
         SASSERT(!empty());
         // If m_dep is set, the corresponding constraint was asserted at m_level and is not valid earlier.
-        if (!m_dep.is_null())
-            return m_level <= s.m_level;
+        if (!m_dep.is_null() && m_level >= s.m_level)
+            return false;
         // All conflict constraints must be bool-assigned.
         for (unsigned lit_idx : m_literals)
             if (!s.m_bvars.is_assigned(sat::to_literal(lit_idx)))

src/math/polysat/solver.cpp

@@ -95,10 +95,33 @@ namespace polysat {
     }
 
     lbool solver::unit_propagate() {
+#if 1
         return l_undef;
-        // disabled to allow debugging unsoundness for watched literals
+#elif 1
+
+        unsigned level = m_level;
+        SASSERT(!m_is_solving);
+        backjump(base_level());
+        if (!is_conflict())
+            propagate();
+        VERIFY(level == m_level);
+        if (is_conflict()) {
+            ++m_stats.m_num_conflicts;
+            return l_false;
+        }
+        
+        return l_undef;
+#else
         flet<uint64_t> _max_d(m_config.m_max_conflicts, m_stats.m_num_conflicts + 2);
-        return check_sat();
+        unsigned level = m_level;
+        lbool r = check_sat();
+        if (r != l_false) {
+            backjump(level);
+            m_conflict.reset();
+        }
+        SASSERT(level == m_level);
+        return r;
+#endif
     }
 
     dd::pdd_manager& solver::sz2pdd(unsigned sz) const {

src/sat/smt/bv_polysat.cpp

@@ -271,6 +271,8 @@ namespace bv {
                 core.push_back(sat::to_literal(n.val() / 2));
             else {                
                 auto [v1, v2] = m_var_eqs[n.val() / 2];
+                VERIFY(var2enode(v1)->get_root() == var2enode(v2)->get_root());
+                VERIFY(n.val() <= 2 * m_var_eqs_head);
                 eqs.push_back(euf::enode_pair(var2enode(v1), var2enode(v2)));
             }
         }