$in match expressions will in the future be translated into the eqMember op type – for example, $in: [1, 2, 3] becomes the ABT filter:

PathTraverse []
PathCompare [EqMember]
Const [[1, 2, 3]] 

This breaks this ABT unit test. The old implementation of $in was supported with indexing, as it used to be chain of PathCompares with op type eq and that has index support. However, the new implementation involves a new op type, and thus after passing through the optimizer, the ABT cannot use an index.