Error duplicate definitions for impls on distinct associated types

[dbeckwith]

As a minimal example of the error I'm running into (using the typenum crate although I don't think it's specific to that):

use core::{marker::PhantomData, ops::Add};
use typenum::*;

struct Wrapper<T>(PhantomData<T>);

type X = <U2 as Add<U3>>::Output;
type Y = <U7 as Add<U9>>::Output;

impl Wrapper<X> {
    fn foo() {}
}

impl Wrapper<Y> {
    fn foo() {}
}

gives

error[E0592]: duplicate definitions with name `foo`
  --> .../lib.rs:30:9
   |
30 |         fn foo() {}
   |         ^^^^^^^^^^^ duplicate definitions for `foo`
...
34 |         fn foo() {}
   |         ----------- other definition for `foo`

I think this might be related to #20400, but I'm not sure. I tried making a simpler reproduction on the playground without using typenum, but I wasn't able to.

Clearly, X and Y are distinct types, as they are not assignable to each other, so why can't I define the same function in impls for them?

Playground

[Spoonbender]

Triage: no change