ICE: Self type not provided for type alias declarations in impls

The following impls both result in an ICE:

```
struct S {}

impl S {
type D = [u64;1];
}
```

```
struct C {
a: T
}

impl C{
type D = [T; 1];
}
```

The error message is `Self type not provided`.

Start by reproducing the two type-alias examples from the issue and locating the compiler path that handles associated type declarations in impls. Trace why the Self type is missing for both non-generic and generic impls; done means neither example produces an ICE and the compiler reports the intended result or diagnostic.

Written by the indexing model from the issue text.