[SPARK-8831][SQL] Support AbstractDataType in TypeCollection.

Otherwise it is impossible to declare an expression supporting DecimalType.

Author: Reynold Xin <rxin@databricks.com>

Closes apache#7232 from rxin/typecollection-adt and squashes the following commits:

934d3d1 [Reynold Xin] [SPARK-8831][SQL] Support AbstractDataType in TypeCollection.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/HiveTypeCoercion.scala

@@ -708,8 +708,6 @@ object HiveTypeCoercion {
         case (NullType, target) => Cast(e, target.defaultConcreteType)
 
         // Implicit cast among numeric types
-        // If input is decimal, and we expect a decimal type, just use the input.
-        case (_: DecimalType, DecimalType) => e
         // If input is a numeric type but not decimal, and we expect a decimal type,
         // cast the input to unlimited precision decimal.
         case (_: NumericType, DecimalType) if !inType.isInstanceOf[DecimalType] =>

sql/catalyst/src/main/scala/org/apache/spark/sql/types/AbstractDataType.scala

@@ -53,10 +53,12 @@ private[sql] abstract class AbstractDataType {
  *
  * This means that we prefer StringType over BinaryType if it is possible to cast to StringType.
  */
-private[sql] class TypeCollection(private val types: Seq[DataType]) extends AbstractDataType {
+private[sql] class TypeCollection(private val types: Seq[AbstractDataType])
+  extends AbstractDataType {
+
   require(types.nonEmpty, s"TypeCollection ($types) cannot be empty")
 
-  private[sql] override def defaultConcreteType: DataType = types.head
+  private[sql] override def defaultConcreteType: DataType = types.head.defaultConcreteType
 
   private[sql] override def isParentOf(childCandidate: DataType): Boolean = false
 
@@ -68,9 +70,9 @@ private[sql] class TypeCollection(private val types: Seq[DataType]) extends Abst
 
 private[sql] object TypeCollection {
 
-  def apply(types: DataType*): TypeCollection = new TypeCollection(types)
+  def apply(types: AbstractDataType*): TypeCollection = new TypeCollection(types)
 
-  def unapply(typ: AbstractDataType): Option[Seq[DataType]] = typ match {
+  def unapply(typ: AbstractDataType): Option[Seq[AbstractDataType]] = typ match {
     case typ: TypeCollection => Some(typ.types)
     case _ => None
   }

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/HiveTypeCoercionSuite.scala

@@ -71,6 +71,12 @@ class HiveTypeCoercionSuite extends PlanTest {
 
     shouldCast(IntegerType, TypeCollection(StringType, BinaryType), StringType)
     shouldCast(IntegerType, TypeCollection(BinaryType, StringType), StringType)
+
+    shouldCast(
+      DecimalType.Unlimited, TypeCollection(IntegerType, DecimalType), DecimalType.Unlimited)
+    shouldCast(DecimalType(10, 2), TypeCollection(IntegerType, DecimalType), DecimalType(10, 2))
+    shouldCast(DecimalType(10, 2), TypeCollection(DecimalType, IntegerType), DecimalType(10, 2))
+    shouldCast(IntegerType, TypeCollection(DecimalType(10, 2), StringType), DecimalType(10, 2))
   }
 
   test("ineligible implicit type cast") {