fix: minimum 1 bit for constant binary decomposition (Consensys#1229)

* fix: minimum 1 bit for constant binary decomposition

* fix: remove incorrect optim path for constant

* test: apply ivokub suggested edit

frontend/cs/r1cs/api_assertions.go

@@ -130,12 +130,6 @@ func (builder *builder) mustBeLessOrEqVar(a, bound frontend.Variable) {
 	// but a can be either constant or a wire.
 
 	nbBits := builder.cs.FieldBitLen()
-	if ca, aConst := builder.constantValue(a); aConst {
-		// if a is a constant, we only need the number of bits of a;
-		// the binary decomposition of the bound will fail if nbBits(bound) > nbBits(a)
-		ba := builder.cs.ToBigInt(ca)
-		nbBits = ba.BitLen()
-	}
 
 	aBits := bits.ToBinary(builder, a, bits.WithNbDigits(nbBits), bits.WithUnconstrainedOutputs(), bits.OmitModulusCheck())
 	boundBits := bits.ToBinary(builder, bound, bits.WithNbDigits(nbBits))

internal/regression_tests/issue1227/issue_1227_test.go

@@ -8,17 +8,22 @@ import (
 )
 
 type Circuit struct {
-	X frontend.Variable
+	constVal int
+	X        frontend.Variable
 }
 
 func (circuit *Circuit) Define(api frontend.API) error {
-	api.AssertIsLessOrEqual(1, circuit.X)
+	api.AssertIsLessOrEqual(circuit.constVal, circuit.X)
 	return nil
 }
 
 func TestConstantPath(t *testing.T) {
 	assert := test.NewAssert(t)
-	assert.CheckCircuit(&Circuit{},
+	assert.CheckCircuit(&Circuit{constVal: 1},
 		test.WithValidAssignment(&Circuit{X: 1}),   // 1 <= 1 --> true
 		test.WithInvalidAssignment(&Circuit{X: 0})) // 1 <= 0 --> false
+	// test edge case where constant is 0
+	assert.CheckCircuit(&Circuit{constVal: 0},
+		test.WithValidAssignment(&Circuit{X: 1}), // 0 <= 1 --> true
+		test.WithValidAssignment(&Circuit{X: 0})) // 0 <= 0 --> true
 }