cloud-fan commented on a change in pull request #27991: [SPARK-31227][SQL] 
Non-nullable null type in complex types should not coerce to nullable type

 File path: 
 @@ -210,8 +206,15 @@ object Cast {
     case _ => false  // overflow
+  /**
+   * Returns `true` iff it should change the nullability of this type in map 
+   * array type, expressions such as cast, etc.
+   *
+   * Note that you should take the nullability context into account.
+   * For example, it should not force to nullable type when null type in array
+   * type is non-nullable which means an empty array of null type.
+   */
   def forceNullable(from: DataType, to: DataType): Boolean = (from, to) match {
-    case (NullType, _) => true
 Review comment:
   so adding a `case (NullType, _) => false` should be good to go?
   I feel it's weird if `concat(array(), array(1))` is allowed but 
`concat(array(), array(date'xxx'))` is not.

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