Has anyone tried to impl a trait which has a const generic in an impl block? I'm trying to glue together an external lib, where a GPIO trait is parameterized over a const generic - but I can't add the const generic when I constrain the impl block as follows:

```
// In some external crate.
pub trait ExternalGpio<const N: u8> {}

// My output GPIO that needs to work over some internal P's and over `ExternalGpio`.
pub struct Output<P> {
    pin: P,
}

// Trying to implement over the external trait.
impl<P: ExternalGpio<N>, const N: u8> Output<P> {
    ...
```

Adding the `const N: u8` is not allowed, and the error is:

```
error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
  --> src/gpio.rs:27:26
   |
27 | impl<P: ExternalGpio<N>, const N: u8> Output<P> {
   |                          ^^^^^^^^^^^ unconstrained const parameter
   |
   = note: expressions using a const parameter must map each value to a distinct output value
   = note: proving the result of expressions other than the parameter are unique is not supported
```

It feels like I'm missing something basic?