This is correct, as I could write the following impl:

impl Trait for () {
  type Res = ();
}

Now, Self::Res is (), which is not an integer.

Yeah, well, it's correct to reject the code at least, it just occurred to me that default associated types shouldn't be accessible from default trait methods (broadly: the trait method might be inherited by impls where the associated type has changed and violated the assumptions that the default method made about what the type would be.)

I guess this is an error reporting problem, then. It took me like, maybe 3 hours to figure out what was actually going on. The message it gives in this situation is basically a falsehood. I'm not sure what to recommend instead. At minimum, a note: Default trait methods cannot access associated default types. See explanation: <link to explanation>, this note would only need to be shown in type errors concerning associated types that have defaults, coming from default trait methods.

As to how I got into that situation: I wanted to define a default associated type that I actually didn't expect impls to ever override, because it was just a sort of convenience function of other associated types (that impls would define).

pub trait Graph<K, V, E> {
    type Index: Debug   Display;
    type Res<T> = Result<T, GraphError<Self::Index>>;
    ...
}

Of course, that can run into the same problem, but the code that I had written, where the error was firing, wasn't running afoul of that problem, so it didn't occur to me that the problem existed, or that it was actually about it.