> You can't have Foo<N> * Foo<M> -> Foo<N+M> or similar.

You can though, unless I totally misunderstand your syntax

    use std::{marker::PhantomData, ops::{Add, Mul}};

    pub struct Foo<T> {
        _p: PhantomData<T>,
    }

    impl<N, M> Mul<Foo<M>> for Foo<N> 
    where
        N: Add<M>
    {
        type Output = Foo<<N as Add<M>>::Output>;
        fn mul(self, rhs: Foo<M>) -> Self::Output {
            todo!()
        }
    }