Github user sujithjay commented on a diff in the pull request:
https://github.com/apache/spark/pull/20002#discussion_r158586350
--- Diff: core/src/main/scala/org/apache/spark/Partitioner.scala ---
@@ -21,6 +21,8 @@ import java.io.{IOException, ObjectInputStream,
ObjectOutputStream}
import scala.collection.mutable
import scala.collection.mutable.ArrayBuffer
+import scala.language.existentials
--- End diff --
Without this import, there was a compiler warning:
```
Warning:(63, 29) inferred existential type
Option[org.apache.spark.rdd.RDD[_$2]]( forSome { type _$2 }), which cannot be
expressed by wildcards, should be enabled
by making the implicit value scala.language.existentials visible.
This can be achieved by adding the import clause 'import
scala.language.existentials'
or by setting the compiler option -language:existentials.
See the Scaladoc for value scala.language.existentials for a discussion
why the feature should be explicitly enabled.
```
Spark build failed because of this.
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