[GitHub] [spark] cloud-fan 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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cloud-fan 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_r274380851
 
 

 ##########
 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
 ##########
 @@ -116,22 +116,23 @@ 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 cast the `from` type to `to` type without any truncating or
+   * precision lose, e.g. int -> long, date -> timestamp.
    */
-  def mayTruncate(from: DataType, to: DataType): Boolean = (from, to) match {
-    case (from: NumericType, to: DecimalType) if !to.isWiderThan(from) => true
-    case (from: DecimalType, to: NumericType) if !from.isTighterThan(to) => true
-    case (from, to) if illegalNumericPrecedence(from, to) => true
-    case (TimestampType, DateType) => true
-    case (StringType, to: NumericType) => true
+  def canSafeCast(from: DataType, to: DataType): Boolean = (from, to) match {
+    case _ if from == to => true
 
 Review comment:
   added support for complex types

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