add more test cases for negative scale and higher precision

spark/src/test/scala/org/apache/comet/CometCastSuite.scala

@@ -896,17 +896,38 @@ class CometCastSuite extends CometTestBase with AdaptiveSparkPlanHelper {
   }
 
   test("cast between decimals with different precision and scale") {
-    // cast between default Decimal(38, 18) to Decimal(9,1)
+    // cast between default Decimal(38, 18) to Decimal(7,2)
     val values = Seq(BigDecimal("12345.6789"), BigDecimal("9876.5432"), BigDecimal("123.4567"))
     val df = withNulls(values).toDF("a")
     castTest(df, DataTypes.createDecimalType(7, 2))
   }
 
-  test("cast two between decimals with different precision and scale") {
+  test("cast between decimals with lower precision and scale") {
     // cast between Decimal(10, 2) to Decimal(9,1)
     castTest(generateDecimalsPrecision10Scale2(), DataTypes.createDecimalType(9, 1))
   }
 
+  test("cast between decimals with higher precision than source") {
+    // cast between Decimal(10, 2) to Decimal(10,4)
+    withSQLConf("spark.comet.explainFallback.enabled" -> "true") {
+      castTest(generateDecimalsPrecision10Scale2(), DataTypes.createDecimalType(10, 4))
+    }
+  }
+
+  test("cast between decimals with negative precision") {
+    // cast to negative scale
+    checkSparkMaybeThrows(
+      spark.sql("select a, cast(a as DECIMAL(10,-4)) from t order by a")) match {
+      case (expected, actual) =>
+        assert(expected.contains("PARSE_SYNTAX_ERROR") === actual.contains("PARSE_SYNTAX_ERROR"))
+    }
+  }
+
+  test("cast between decimals with zero precision") {
+    // cast between Decimal(10, 2) to Decimal(10,4)
+    castTest(generateDecimalsPrecision10Scale2(), DataTypes.createDecimalType(10, 0))
+  }
+
   private def generateFloats(): DataFrame = {
     withNulls(gen.generateFloats(dataSize)).toDF("a")
   }

spark/src/test/scala/org/apache/spark/sql/CometTestBase.scala

@@ -231,11 +231,9 @@ abstract class CometTestBase
       df: => DataFrame): (Option[Throwable], Option[Throwable]) = {
     var expected: Option[Throwable] = None
     withSQLConf(CometConf.COMET_ENABLED.key -> "false") {
-      val dfSpark = Dataset.ofRows(spark, df.logicalPlan)
-      expected = Try(dfSpark.collect()).failed.toOption
+      expected = Try(Dataset.ofRows(spark, df.logicalPlan).collect()).failed.toOption
     }
-    val dfComet = Dataset.ofRows(spark, df.logicalPlan)
-    val actual = Try(dfComet.collect()).failed.toOption
+    val actual = Try(Dataset.ofRows(spark, df.logicalPlan).collect()).failed.toOption
     (expected, actual)
   }