Consider the following document:

{b: {[{c: 2}, {c: 3}]}}

If it is referred from an agg expression such as [\{$project: {a: "$b.c"}}]

then the result (assigned at field "a") will be (unsurprisingly) [2, 3].

Now consider this document which adds extra array nesting:

{b: {[[{c: 2}, {c: 3}]]}}

Currently in classic, the expression returns an empty array [], instead of the more arguably rational result of [[2, 3]]

This bug is about clarifying this behavior and specifically determining if it makes sense to adopt it going forward, particularly in the context of the new optimizer.