`invalid(..)` type description in diagnostic

[sbillig]

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
    }
}