We should consider validating the $merge whenMatched pipeline sooner. Currently, it does not get parsed until execution time UNLESS the $merge stage is serialized for query settings/query stats (at which point the pipeline is parsed and re-serialized in the serialize() function).

This doesn't manifest in any kind of user-facing bug but isn't ideal behavior, as it is odd to have the parsing error first be seen when serializing the stage (effectively making serialize() have side effects).