Associated type `Self::Dim` in trait constraint yields "unsupported cyclic reference"

[bluss]

Should this not be ok when the associated item exists in the current trait itself?

Erroring code:

pub trait ShapeBuilder : Into<Shape<Self::Dim>> {
    type Dim;
}
pub struct Shape<D> {
    dim: D,
}

src/impl_constructors.rs:25:37: 25:46 error: unsupported cyclic reference between types/traits detected [E0391]
src/impl_constructors.rs:25 pub trait ShapeBuilder : Into<Shape<Self::Dim>>
                                                                ^~~~~~~~~

Modifying it slightly compiles:

pub trait ShapeBuilder : Into<Shape<<Self as ShapeBuilder>::Dim>> {
    type Dim;
}

pub struct Shape<D> {
    dim: D,
}

[Mark-Simulacrum]

Closing in favor of #35237, since it has more detailed explanation.