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https://issues.apache.org/jira/browse/SPARK-27681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16838799#comment-16838799
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Marcelo Vanzin commented on SPARK-27681:
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bq. The case to consider is, roughly, where Seq is an argument not a return 
type.

If this change is restricted to that it's probably fine. But it's still a 
tricky change. You'll probably have to explicitly use that class in the problem 
spots (i.e. no importing at the top level or it may change things you don't 
want to change). Then people will have to remember to do that (or reviewers 
remember to catch it during review). It just feels a little too brittle.

> Use scala.collection.Seq explicitly instead of scala.Seq alias
> --------------------------------------------------------------
>
>                 Key: SPARK-27681
>                 URL: https://issues.apache.org/jira/browse/SPARK-27681
>             Project: Spark
>          Issue Type: Sub-task
>          Components: ML, MLlib, Spark Core, SQL, Structured Streaming
>    Affects Versions: 3.0.0
>            Reporter: Sean Owen
>            Assignee: Sean Owen
>            Priority: Major
>
> {{scala.Seq}} is widely used in the code, and is an alias for 
> {{scala.collection.Seq}} in Scala 2.12. It will become an alias for 
> {{scala.collection.immutable.Seq}} in Scala 2.13. In many cases, this will be 
> fine, as Spark users using Scala 2.13 will also have this changed alias. In 
> some cases it may be undesirable, as it will cause some code to compile in 
> 2.12 but not in 2.13. In some cases, making the type {{scala.collection.Seq}} 
> explicit so that it doesn't vary can help avoid this, so that Spark apps 
> might cross-compile for 2.12 and 2.13 with the same source.



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