When the return type of a default trait method returns an associated type (that has a default), there's always a mismatched type error

[makoConstruct]

playground

#![feature(
    associated_type_defaults,
)]


pub trait Trait {
    type Res = isize;
    fn f() -> Self::Res {
        2
    }
}

At line of '2': error[E0308]: mismatched types expected associated type, found integer

[Noratrieb]

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

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

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

[makoConstruct]

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.

[makoConstruct]

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.