samuel.mercier@mongodb.com noted on Friday that it's possible to vary identifier used in an array filter to change the query shape, even though such a change doesn't change the semantics of the update statement.

Example: these two update statements behave identically:

{ 
        q: { x: 3 },  
        u: {$inc: { "grades.$[t].questions.score": 2 } },
        arrayFilters: [ { "t.type": "quiz" }  ], 
}

{ 
        q: { x: 3 },  
        u: {$inc: { "grades.$[v].questions.score": 2 } },
        arrayFilters: [ { "v.type": "quiz" }  ], 
}

In these two statements, only the variable in the array filter is different, t vs v. But they do exactly the same thing.

If these two statements behave identically, it stands to reason that they should have identical query shapes.

Note: a similar case exists for let expressions and aggregation pipelines, but we don't currently normalize based on variable names there either. In any case, it seems worth a discussion what the right behavior is here.