Fix constraint satisfaction check (#409)

* Fix constraint satisfaction check

* Address comments

Author: Pratyush

relations/src/gr1cs/predicate/mod.rs

@@ -183,12 +183,20 @@ impl<F: Field> PredicateConstraintSystem<F> {
     /// Check if the constraints enforced by this predicate are satisfied
     /// i.e. `L(x_1, x_2, ..., x_n) == 0`.
     pub fn which_constraint_is_unsatisfied(&self, cs: &ConstraintSystem<F>) -> Option<usize> {
+        let panic_msg = |v| panic!("Variable {v:?} is not assigned; did you run `cs.finalize()`?");
         for (i, constraint) in self.iter_constraints().enumerate() {
             let variables: Vec<F> = constraint
                 .into_iter()
-                .map(|variable| cs.assigned_value(variable).unwrap())
+                .map(|v| {
+                    cs.assigned_value(v).unwrap_or_else(|| {
+                        cs.get_lc(v)
+                            .iter()
+                            .map(|&(c, v)| c * cs.assigned_value(v).unwrap_or_else(|| panic_msg(v)))
+                            .sum()
+                    })
+                })
                 .collect();
-            if self.predicate.is_satisfied(&variables) {
+            if !self.predicate.is_satisfied(&variables) {
                 return Some(i);
             }
         }

relations/src/gr1cs/predicate/polynomial_constraint.rs

@@ -44,8 +44,7 @@ impl<F: Field> PolynomialPredicate<F> {
 impl<F: Field> PolynomialPredicate<F> {
     /// Check if the predicate is satisfied by the given variables.
     pub fn is_satisfied(&self, variables: &[F]) -> bool {
-        // TODO: Change the polynomial eval to get a slice as an evaluation point
-        !self.polynomial.evaluate(&variables.to_vec()).is_zero()
+        self.polynomial.evaluate(&variables.to_vec()).is_zero()
     }
 
     /// Evaluate the predicate on the given variables.