add negative scale test in DecimalSuite

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

@@ -555,7 +555,9 @@ case class Round(child: Expression, scale: Expression)
   override def foldable: Boolean = child.foldable
 
   override lazy val dataType: DataType = child.dataType match {
-    case DecimalType.Fixed(p, s) => DecimalType(p, _scale)
+    // if the new scale is bigger which means we are scaling up,
+    // keep the original scale as `Decimal` does
+    case DecimalType.Fixed(p, s) => DecimalType(p, if (_scale > s) s else _scale)
     case t => t
   }
 

sql/catalyst/src/test/scala/org/apache/spark/sql/types/decimal/DecimalSuite.scala

@@ -24,14 +24,14 @@ import org.scalatest.PrivateMethodTester
 import scala.language.postfixOps
 
 class DecimalSuite extends SparkFunSuite with PrivateMethodTester {
-  test("creating decimals") {
-    /** Check that a Decimal has the given string representation, precision and scale */
-    def checkDecimal(d: Decimal, string: String, precision: Int, scale: Int): Unit = {
-      assert(d.toString === string)
-      assert(d.precision === precision)
-      assert(d.scale === scale)
-    }
+  /** Check that a Decimal has the given string representation, precision and scale */
+  private def checkDecimal(d: Decimal, string: String, precision: Int, scale: Int): Unit = {
+    assert(d.toString === string)
+    assert(d.precision === precision)
+    assert(d.scale === scale)
+  }
 
+  test("creating decimals") {
     checkDecimal(new Decimal(), "0", 1, 0)
     checkDecimal(Decimal(BigDecimal("10.030")), "10.030", 5, 3)
     checkDecimal(Decimal(BigDecimal("10.030"), 4, 1), "10.0", 4, 1)
@@ -53,6 +53,15 @@ class DecimalSuite extends SparkFunSuite with PrivateMethodTester {
     intercept[IllegalArgumentException](Decimal(1e17.toLong, 17, 0))
   }
 
+  test("creating decimals with negative scale") {
+    checkDecimal(Decimal(BigDecimal("98765"), 5, -3), "9.9E+4", 5, -3)
+    checkDecimal(Decimal(BigDecimal("314.159"), 6, -2), "3E+2", 6, -2)
+    checkDecimal(Decimal(BigDecimal(1.579e12), 4, -9), "1.579E+12", 4, -9)
+    checkDecimal(Decimal(BigDecimal(1.579e12), 4, -10), "1.58E+12", 4, -10)
+    checkDecimal(Decimal(103050709L, 9, -10), "1.03050709E+18", 9, -10)
+    checkDecimal(Decimal(1e8.toLong, 10, -10), "1.00000000E+18", 10, -10)
+  }
+
   test("double and long values") {
     /** Check that a Decimal converts to the given double and long values */
     def checkValues(d: Decimal, doubleValue: Double, longValue: Long): Unit = {