[SPARK-24165][SQL][branch-2.3] Fixing the failing test and addressing review comments

Author: mn-mikke

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

@@ -679,8 +679,7 @@ trait ComplexTypeMergingExpression extends Expression {
    * A collection of data types used for resolution the output type of the expression. By default,
    * data types of all child expressions. The collection must not be empty.
    */
-  @transient
-  lazy val inputTypesForMerging: Seq[DataType] = children.map(_.dataType)
+  @transient lazy val inputTypesForMerging: Seq[DataType] = children.map(_.dataType)
 
   /**
    * A method determining whether the input types are equal ignoring nullable, containsNull and
@@ -707,12 +706,14 @@ trait ComplexTypeMergingExpression extends Expression {
       StructType(newFields)
   }
 
-  override def dataType: DataType = {
+  @transient override lazy val dataType: DataType = {
     require(
       inputTypesForMerging.nonEmpty,
       "The collection of input data types must not be empty.")
     require(
-      areInputTypesForMergingEqual,
+      inputTypesForMerging.sliding(2, 1).forall {
+        case Seq(dt1, dt2) => DataType.equalsIgnoreCaseAndNullability(dt1, dt2)
+      },
       "All input types must be the same except nullable, containsNull, valueContainsNull flags.")
     inputTypesForMerging.reduceLeft(mergeTwoDataTypes)
   }

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

@@ -34,8 +34,7 @@ import org.apache.spark.sql.types._
 case class If(predicate: Expression, trueValue: Expression, falseValue: Expression)
   extends ComplexTypeMergingExpression {
 
-  @transient
-  override lazy val inputTypesForMerging: Seq[DataType] = {
+  @transient override lazy val inputTypesForMerging: Seq[DataType] = {
     Seq(trueValue.dataType, falseValue.dataType)
   }
 
@@ -125,8 +124,7 @@ case class CaseWhen(
   override def children: Seq[Expression] = branches.flatMap(b => b._1 :: b._2 :: Nil) ++ elseValue
 
   // both then and else expressions should be considered.
-  @transient
-  override lazy val inputTypesForMerging: Seq[DataType] = {
+  @transient override lazy val inputTypesForMerging: Seq[DataType] = {
     branches.map(_._2.dataType) ++ elseValue.map(_.dataType)
   }