Github user maropu commented on a diff in the pull request:

    https://github.com/apache/spark/pull/21704#discussion_r200216296
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala
 ---
    @@ -2007,7 +2007,14 @@ case class Concat(children: Seq[Expression]) extends 
Expression {
         }
       }
     
    -  override def dataType: DataType = 
children.map(_.dataType).headOption.getOrElse(StringType)
    +  override def dataType: DataType = {
    +    val dataTypes = children.map(_.dataType)
    +    dataTypes.headOption.map {
    +      case ArrayType(et, _) =>
    +        ArrayType(et, 
dataTypes.exists(_.asInstanceOf[ArrayType].containsNull))
    +      case dt => dt
    +    }.getOrElse(StringType)
    +  }
    --- End diff --
    
    +1 for the @mn-mikke idea


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