AssertIsBoolean doesn't work with frontend.Variables that were declared using cs.Constant

[OlivierBBB]

The first two test circuits below don't work: visibility of zero and sum is 0.

package circuits

import (
	"testing"

	"github.com/consensys/gnark-crypto/ecc"
	"github.com/consensys/gnark/backend"
	"github.com/consensys/gnark/frontend"
	"github.com/stretchr/testify/assert"
)

// empty circuits
type IsBool1 struct{}
type IsBool2 struct{}
type IsBool3 struct{}

func TestIsBool1(t *testing.T) {

	var circuit IsBool1

	_, err := frontend.Compile(ecc.BN254, backend.GROTH16, &circuit)
	assert.Nil(t, err, "Circuit compilation failed")
}

func TestIsBool2(t *testing.T) {

	var circuit IsBool2

	_, err := frontend.Compile(ecc.BN254, backend.GROTH16, &circuit)
	assert.Nil(t, err, "Circuit compilation failed")
}

func TestIsBool3(t *testing.T) {

	var circuit IsBool3

	_, err := frontend.Compile(ecc.BN254, backend.GROTH16, &circuit)
	assert.Nil(t, err, "Circuit compilation failed")
}

func (c *IsBool1) Define(curve ecc.ID, cs *frontend.ConstraintSystem) error {

	zero := cs.Constant(0)
	one := cs.Constant(1)
	cs.AssertIsBoolean(zero)
	cs.AssertIsBoolean(one)
	return nil
}

func (c *IsBool2) Define(curve ecc.ID, cs *frontend.ConstraintSystem) error {

	zero := cs.Constant(0)
	one := cs.Constant(1)
	sum := cs.Add(zero, one)
	cs.AssertIsBoolean(sum)
	return nil
}

func (c *IsBool3) Define(curve ecc.ID, cs *frontend.ConstraintSystem) error {

	zero := cs.Constant(0)
	one := cs.Constant(1)
	prod := cs.Mul(zero, one)
	cs.AssertIsBoolean(prod)

	return nil
}