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Type: Task
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Resolution: Won't Fix
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Priority: Major - P3
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None
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Affects Version/s: None
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Component/s: Querying
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Labels:None
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Query
In 4.2, $-prefixed field names were banned everywhere in agg.
In 4.4, there is a small whitelist of field names permitted to start with $.
If a user were to create a view, or a validator (and use $expr) with a $-prefixed field name in FCV 4.2, they may not be able to downgrade without dropping views/collections with a 4.4. binary.