Very confusing error messages related to variables/aliases

The following (broken) SQL++ query:

USE hw5;

FROM world AS x,
     x.mondial.country AS z,
{{     x.mondial.mountain AS y }}
GROUP BY z.name
SELECT z.`-car_code` AS country_code,
       z.name AS country_name,
       y.name AS mountain, y.height AS height;

Gives this very confusing error message:

"Cannot resolve ambiguous alias reference for undefined identifier z in [#1, $name][CompilationException]"
 

This is confusing in multiple ways:
1. What the heck is #1?  And ditto for $name?
2. What is the meaning of the use of the array constructor in the error message?  () Note that these users are making heavy use of array constructor syntax, so [ ]'s are common in their queries - we shouldn't use it in our error messages ever unless we're complaining about the user's array constructors IMO.
3. What line is the error on - is this before or after the GROUP BY?  (It's after.)
4. "I defined z, so why is it undefined?" (Asked the user. )

For setup, do this:

DROP DATAVERSE hw5 IF EXISTS;
CREATE DATAVERSE hw5;
USE hw5;
CREATE TYPE worldType AS {auto_id:uuid };
CREATE DATASET world(worldType) PRIMARY KEY auto_id AUTOGENERATED;