address comments

Author: jiangxb1987

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

@@ -42,7 +42,7 @@ case class WindowSpecDefinition(
     orderSpec: Seq[SortOrder],
     frameSpecification: WindowFrame) extends Expression with WindowSpec with Unevaluable {
 
-  override def children: Seq[Expression] = partitionSpec ++ orderSpec ++ Seq(frameSpecification)
+  override def children: Seq[Expression] = partitionSpec ++ orderSpec :+ frameSpecification
 
   override lazy val resolved: Boolean =
     childrenResolved && checkInputDataTypes().isSuccess &&

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -688,8 +688,6 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
     case id: FunctionIdentifier => true
     case spec: BucketSpec => true
     case catalog: CatalogTable => true
-    case boundary: SpecialFrameBoundary => true
-    case frame: WindowFrame => true
     case partition: Partitioning => true
     case resource: FunctionResource => true
     case broadcast: BroadcastMode => true

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

@@ -1133,6 +1133,17 @@ class TypeCoercionSuite extends AnalysisTest {
         Seq(SortOrder(Literal.default(DateType), Ascending)),
         SpecifiedWindowFrame(RangeFrame, Literal(10.0), Literal(2147483648L)))
     )
+    // Should not cast SpecialFrameBoundary.
+    ruleTest(WindowFrameCoercion,
+      windowSpec(
+        Seq(UnresolvedAttribute("a")),
+        Seq(SortOrder(Literal(1L), Ascending)),
+        SpecifiedWindowFrame(RangeFrame, CurrentRow, Unbounded)),
+      windowSpec(
+        Seq(UnresolvedAttribute("a")),
+        Seq(SortOrder(Literal(1L), Ascending)),
+        SpecifiedWindowFrame(RangeFrame, CurrentRow, Unbounded))
+    )
   }
 }