Details
Major Description
When you use TopDocs.merge today it always overrides the ScoreDoc#shardIndex value. The assumption that is made here is that all shard results are merges at once which is not necessarily the case. If for instance incremental merge phases are applied the shard index doesn't correspond to the index in the outer TopDocs array. To make this a backwards compatible but yet non-controversial change we could change the internals of TopDocs#merge to only assign this value unless it's not been assigned before to a non-default (-1) value to allow multiple or sparse top docs merging.