Complete the update to Cast for UDT.

Author: viirya

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala

@@ -19,6 +19,7 @@ package org.apache.spark.sql.catalyst.expressions
 
 import java.math.{BigDecimal => JavaBigDecimal}
 
+import org.apache.spark.SparkException
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
 import org.apache.spark.sql.catalyst.expressions.codegen._
@@ -434,6 +435,11 @@ case class Cast(child: Expression, dataType: DataType) extends UnaryExpression {
     case array: ArrayType => castArray(from.asInstanceOf[ArrayType].elementType, array.elementType)
     case map: MapType => castMap(from.asInstanceOf[MapType], map)
     case struct: StructType => castStruct(from.asInstanceOf[StructType], struct)
+    case udt: UserDefinedType[_]
+      if udt.userClass == from.asInstanceOf[UserDefinedType[_]].userClass =>
+      identity[Any]
+    case _: UserDefinedType[_] =>
+      throw new SparkException(s"Cannot cast $from to $to.")
   }
 
   private[this] lazy val cast: Any => Any = cast(child.dataType, dataType)
@@ -479,6 +485,8 @@ case class Cast(child: Expression, dataType: DataType) extends UnaryExpression {
     case udt: UserDefinedType[_]
       if udt.userClass == from.asInstanceOf[UserDefinedType[_]].userClass =>
       (c, evPrim, evNull) => s"$evPrim = $c;"
+    case _: UserDefinedType[_] =>
+      throw new SparkException(s"Cannot cast $from to $to.")
   }
 
   // Since we need to cast child expressions recursively inside ComplexTypes, such as Map's