[SPARK-59643][SQL] Tighten CheckOverflow nullability to match MakeDecimal

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

@@ -117,7 +117,9 @@ case class CheckOverflow(
     dataType: DecimalType,
     nullOnOverflow: Boolean) extends UnaryExpression with SupportQueryContext {
 
-  override def nullable: Boolean = true
+  // When `nullOnOverflow` is false, an overflow throws instead of producing null, so the result is
+  // null only when the input is. When true, an overflow yields null regardless of the input.
+  override def nullable: Boolean = child.nullable || nullOnOverflow
 
   override def nullSafeEval(input: Any): Any =
     input.asInstanceOf[Decimal].toPrecision(
@@ -130,11 +132,13 @@ case class CheckOverflow(
   override protected def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val errorContextCode = getContextOrNullCode(ctx, !nullOnOverflow)
     nullSafeCodeGen(ctx, ev, eval => {
+      // Only assign isNull when nullable; nullSafeCodeGen makes it a literal otherwise.
+      val setIsNull = if (nullable) s"${ev.isNull} = ${ev.value} == null;" else ""
       // scalastyle:off line.size.limit
       s"""
          |${ev.value} = $eval.toPrecision(
          |  ${dataType.precision}, ${dataType.scale}, Decimal.ROUND_HALF_UP(), $nullOnOverflow, $errorContextCode);
-         |${ev.isNull} = ${ev.value} == null;
+         |$setIsNull
        """.stripMargin
       // scalastyle:on line.size.limit
     })

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/DecimalExpressionSuite.scala

@@ -78,6 +78,16 @@ class DecimalExpressionSuite extends SparkFunSuite with ExpressionEvalHelper {
       Literal.create(null, DecimalType(2, 1)), DecimalType(3, 2), false), null)
   }
 
+  test("SPARK-59643: CheckOverflow nullability") {
+    val d = Literal(Decimal("10.1"))                // non-nullable
+    val n = Literal.create(null, DecimalType(3, 1)) // nullable
+    assert(CheckOverflow(d, DecimalType(4, 1), nullOnOverflow = true).nullable)
+    assert(!CheckOverflow(d, DecimalType(4, 1), nullOnOverflow = false).nullable)
+    assert(CheckOverflow(n, DecimalType(4, 1), nullOnOverflow = true).nullable)
+    assert(CheckOverflow(n, DecimalType(4, 1), nullOnOverflow = false).nullable)
+    checkEvaluation(CheckOverflow(d, DecimalType(4, 1), nullOnOverflow = false), Decimal("10.1"))
+  }
+
   test("SPARK-39208: CheckOverflow & CheckOverflowInSum support query context in runtime errors") {
     val d = Decimal(101, 3, 1)
     val query = "select cast(d as decimal(4, 3)) from t"