SERVER-129906 handled the simplest cases. However, we should also extend the projection-removal logic to remove cases like this one:

[
    {$project: {a: {$add: ["$b", "$c"]}, b:1}},
    {$group: {_id: "$b"}}
] 

The trick here is that the $project involves the $b field that the $group uses, but its output it not read by the $group.