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https://issues.apache.org/jira/browse/LUCENE-6228?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16255483#comment-16255483
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Robert Muir commented on LUCENE-6228:
-------------------------------------

I don't agree, sorry. We don't need to prevent anyone from calling 
getChildren() on a FakeScorer, it can just return empty which is the default 
implementation.

What we need to prevent is someone calling advance() based on the results of 
getChildren, that is key: and honestly if we can't prevent that with this 
issue, then the abstraction isn't right, its the whole point of doing this :)

Stuff like what getChildren() on a FakeScorer or BulkScorer does, that stuff is 
harmless, and those things are the separate issues that we shouldnt worry about.

> Do not expose full-fledged scorers in LeafCollector.setScorer
> -------------------------------------------------------------
>
>                 Key: LUCENE-6228
>                 URL: https://issues.apache.org/jira/browse/LUCENE-6228
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Adrien Grand
>            Assignee: Adrien Grand
>             Fix For: 5.2, 6.0
>
>         Attachments: LUCENE-6228.patch, LUCENE-6228.patch, LUCENE-6228.patch, 
> LUCENE-6228.patch
>
>
> Currently LeafCollector.setScorer takes a Scorer, which I don't like because 
> several methods should never be called in the context of a Collector (like 
> nextDoc or advance).
> I think it's even more trappy for methods that might seem to work in some 
> particular cases but will not work in the general case, like getChildren 
> which will not work if you have a specialized BulkScorer or iterating over 
> positions which will not work if you are in a MultiCollector and another leaf 
> collector consumes positions too.
> So I think we should restrict what can be seen from a collector to avoid such 
> traps.



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