circom does not create a constraint for addition

Here is a simple circuit that outputs the sum of two inputs:

pragma circom 2.1.4;

template Add() {
   signal input a, b;
   signal output c;

   c <== a + b;
}

component main  = Add();

I compiled the circuit with the following command.

circom Add.circom --r1cs --sym --wasm

And got this result.

template instances: 1
non-linear constraints: 0
linear constraints: 0
public inputs: 0
private inputs: 2 (none belong to witness)
public outputs: 1
wires: 2
labels: 4
Written successfully: ./Add.r1cs
Written successfully: ./Add.sym
Written successfully: ./Add_js/Add.wasm
Everything went okay

Why there was no linear and non-linear constraint for this expression? However, when I change from addition to multiplication (i.e., c <== a*b), there is 1 non-linear constraint as circom produced the following output.

template instances: 1
non-linear constraints: 1
linear constraints: 0
public inputs: 0
private inputs: 2
public outputs: 1
wires: 4
labels: 4
Written successfully: ./Add.r1cs
Written successfully: ./Add.sym
Written successfully: ./Add_js/Add.wasm
Everything went okay

it's because --O2 is the default. Add --O1 or --O0 at the end to avoid addition constraint deletion.