Why shouldn't the operator change the value of its parameters?

Question

C++ / C#

Hello, please explain how option 1 differs from option 2:

Why should the operator not change the value of his parameters?

Answer 1

You write like this in code
6 = 4 + 2
. And you have a bam and 2 turned into 6, no other way is magic.

Answer 2

It is not necessary to think that if the operator is unary, then its call should lead to a change in the original object.

var valid = true;
if (!valid) {
  ...
}
if (valid){
  ...
}

and what would happen if the operator ! would change the original valid variable?