You only know there are no mutations if Foo itself does not contain any indirections. Additionally, the compiler generally cannot assume that Foo_bar does not modify Foo as it is legal to cast away const as long as it is not originally a variable declared as const (so in your static Foo example it would be UB to cast away const).

static + const is valuable, but const parameters are merely a convention, there is no actual enforcement around them and due to aliasing the compiler generally can’t assume the parameter doesn’t actually change anyway.