test: add regression test for zero mul duplicate

frontend/cs/scs/api_test.go

@@ -147,9 +147,43 @@ func TestExistDiv0(t *testing.T) {
 		Res7: 0, Res8: 55,
 	}, ecc.BN254.ScalarField())
 	assert.NoError(err)
-	solution, err := ccs.Solve(w)
+	_, err = ccs.Solve(w)
+	assert.NoError(err)
+}
+
+type IssueDiv0Circuit2 struct {
+	A1, B1 frontend.Variable
+
+	Res1, Res2 frontend.Variable
+}
+
+func (c *IssueDiv0Circuit2) Define(api frontend.API) error {
+	// case 1
+	b1 := api.Mul(0, c.A1)
+	b2 := api.Mul(4, c.B1)
+	t1 := api.Mul(b1, b2)
+
+	b3 := api.Mul(2, c.A1)
+	b4 := api.Mul(5, c.B1)
+	t2 := api.Mul(b3, b4)
+
+	// test solver
+	api.AssertIsEqual(t1, c.Res1)
+	api.AssertIsEqual(t2, c.Res2)
+	return nil
+}
+
+func TestExistDiv02(t *testing.T) {
+	assert := test.NewAssert(t)
+	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), scs.NewBuilder, &IssueDiv0Circuit2{})
+	assert.NoError(err)
+	w, err := frontend.NewWitness(&IssueDiv0Circuit2{
+		A1: 11, B1: 21,
+		Res1: 0, Res2: 2310,
+	}, ecc.BN254.ScalarField())
+	assert.NoError(err)
+	_, err = ccs.Solve(w)
 	assert.NoError(err)
-	_ = solution
 }
 
 type mulAccFastTrackCircuit struct {