In SERVER-45447 we found that $bucketAuto does not override the optimize() method, which means expressions inside a $bucketAuto were not being constant-folded.  We've fixed this in 4.4.

This has caused us to lose some test coverage, because queries using $bucketAuto in 4.2 vs 4.4 can return different results.  A lot of the cases involve one query erroring and the other not, but some cases may involve two slightly different results.

Giving $bucketAuto an optimize() method on 4.2 would be a small code change, would make some queries faster, and would make our 4.4 tests greener.  But it could also cause subtle behavior changes.