How can I enforce equality of two associated type parameters of traits?

Question

I have a function f which takes two arguments of the same type, and a function g which takes two arguments of different types, but both types have

Answer 1

A where clause works fine:

fn g(a: T, b: U)
where
    T: A,
    U: A, // where T::A is equal to B::A
{
    f(a.getter(), b.getter())
}