fix: fixes #1246 ensure cond is boolean in api.Select

frontend/cs/scs/api.go

@@ -415,6 +415,9 @@ func (builder *builder) Select(b frontend.Variable, i1, i2 frontend.Variable) fr
 		return i1
 	}
 
+	// ensure the condition is a boolean
+	builder.AssertIsBoolean(b)
+
 	u := builder.Sub(i1, i2)
 	l := builder.Mul(u, b)
 

internal/regression_tests/issue1246/issue1246_test.go

@@ -0,0 +1,37 @@
+package issue1246_test
+
+import (
+	"testing"
+
+	"github.com/consensys/gnark/frontend"
+	"github.com/consensys/gnark/test"
+)
+
+// Circuit definition
+// here we aim to catch the case where the API doesn't enforce the condition to be a boolean
+type notBoolCond struct {
+	Condition, Y1, Y2 frontend.Variable
+}
+
+func (circuit *notBoolCond) Define(api frontend.API) error {
+	d := api.Select(circuit.Condition, circuit.Y1, circuit.Y2)
+
+	// per api definition, d should hold either Y1 or Y2.
+	// we have y1 = 2, y2 = 4, condition = 2 (non boolean)
+	// r = condition(y1-y2) + y2
+	api.AssertIsEqual(d, 0)
+
+	return nil
+}
+
+func TestSelectConditionNonBool(t *testing.T) {
+	assert := test.NewAssert(t)
+
+	assert.CheckCircuit(&notBoolCond{},
+		test.WithInvalidAssignment(&notBoolCond{
+			Condition: 2,
+			Y1:        2,
+			Y2:        4,
+		}),
+	)
+}