A query like

[{$project: {a: {$meta: "sortKey"}}}]

fails with the error "cannot use sortKey $meta projection without a sort", but this query succeeds:

[{$group: {_id: "$a", vals: {$addToSet: {$meta: "sortKey"}}}}] 

This validation is inconsistent with the other meta fields. It may be because this validation takes place here within canonical_query.cpp, rather than as part of the DepsTracker, like is used for the other meta fields.