An aggregation with a $lookup that's pushed to SBE can tassert with 6447401.

The query planner records the index during planning, multiplanning can yield and the SBE plan is built in a different snapshot where the index may no longer be available.

The impact is that a query can tassert if its stage building happens concurrently with an index drop for one of the foreign collections. Note that if an index on the "primary" collection is dropped at runtime, the query uasserts, so the user impact of this bug is fairly minimal.

This is reproduced only on 8.0, since master has had refactors in how it looks up the index. As part of this task, we should figure out whether 9.0 also has the bug. I've included 9.0 in the "affects version" just to be conservative.