[GitHub] [spark] gatorsmile commented on a change in pull request #21586: [SPARK-24586][SQL] Upcast should not allow casting from string to other types

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gatorsmile commented on a change in pull request #21586: [SPARK-24586][SQL] Upcast should not allow casting from string to other types
URL: https://github.com/apache/spark/pull/21586#discussion_r284842999
 
 

 ##########
 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
 ##########
 @@ -120,35 +120,36 @@ object Cast {
   }
 
   /**
-   * Return true iff we may truncate during casting `from` type to `to` type. e.g. long -> int,
-   * timestamp -> date.
+   * Returns true iff we can safely up-cast the `from` type to `to` type without any truncating or
+   * precision lose or possible runtime failures. For example, long -> int, string -> int are not
+   * up-cast.
    */
-  def mayTruncate(from: DataType, to: DataType): Boolean = (from, to) match {
 
 Review comment:
   I know this will be ugly, but I still prefer to adding a conf for falling back to the original behavior. 

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