use std::ops::Add;

trait Scalar: Sized
where
    f64: Add<Self, Output = Self>,
{}

impl Scalar for f64 {}

fn add_one<S>(a: S) -> S
where
    S: Scalar,
    f64: Add<S, Output = S>, // Why is this line necessary?
{
    1.0 + a
}

fn main() {
    println!("1 + 1 = {}", add_one(1.0));
}