`invalid(..)` type description in diagnostic

```rust

struct Slice {
source: ref [u256; 8], // reference to the original array
start: usize,
len: usize,
}

impl Slice {
fn new(source: ref [u256; 8], start: usize, len: usize) -> Slice {
Slice { source, start, len }
}
}

impl Seq for Slice {
type Item = u256

fn len(self) -> usize {
self.len
}

fn get(ref self, i: usize) -> ref Item {
self.source[self.start + I] // ERROR: expected `ref invalid(PathResolutionFailed(Item))`, but `u256` is given
}
}
```

Start by reproducing the diagnostic with the Slice and Seq example in the issue, then trace the compiler path that formats the expected `ref invalid(PathResolutionFailed(Item))` type. Identify where the unresolved associated type is rendered and update the diagnostic so it gives a useful type description; rerun the same example to confirm the output.

Written by the indexing model from the issue text.