[SPARK-58296][SQL] Fix to_time returning STRING type when the format is a foldable NULL

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

@@ -98,7 +98,7 @@ case class ToTime(str: Expression, format: Option[Expression])
     case Some(expr) if expr.foldable =>
       Option(expr.eval())
         .map(f => invokeParser(Some(f.toString), Seq(str)))
-        .getOrElse(Literal(null, expr.dataType))
+        .getOrElse(Literal(null, TimeType()))
     case _ => invokeParser()
   }
 

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

@@ -59,6 +59,20 @@ class TimeExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
       parameters = Map("input" -> "'100:50'", "format" -> "'mm:HH'"))
   }
 
+  test("SPARK-58296: to_time reports TimeType even when a foldable format is NULL") {
+    // A foldable format that evaluates to NULL must not change the output type: to_time
+    // still produces a TIME, matching the non-null and no-format branches. Before the fix
+    // the replacement fell back to the format argument's own type (STRING/NULL).
+    assert(new ToTime(Literal("00:00:00"), Literal.create(null, StringType)).dataType ===
+      TimeType())
+    assert(new ToTime(Literal("00:00:00"), Literal.create(null)).dataType === TimeType())
+    // The value is still NULL; assert type and value together for this exact case.
+    checkEvaluation(new ToTime(Literal("00:00:00"), Literal.create(null, StringType)), null)
+    // Sanity: the branches that were already correct.
+    assert(new ToTime(Literal("00:00:00")).dataType === TimeType())
+    assert(new ToTime(Literal("00:00:00"), Literal("HH:mm:ss")).dataType === TimeType())
+  }
+
   test("HourExpressionBuilder") {
     // Empty expressions list
     checkError(

sql/core/src/test/resources/sql-tests/analyzer-results/time.sql.out

@@ -48,6 +48,20 @@ Project [to_time(time_str#x, Some(fmt_str#x)) AS to_time(time_str, fmt_str)#x]
             +- OneRowRelation
 
 
+-- !query
+select typeof(to_time('00:12:00', null))
+-- !query analysis
+Project [typeof(to_time(00:12:00, Some(null))) AS typeof(to_time(00:12:00, NULL))#x]
++- OneRowRelation
+
+
+-- !query
+select typeof(to_time('00:12:00', cast(null as string)))
+-- !query analysis
+Project [typeof(to_time(00:12:00, Some(cast(null as string)))) AS typeof(to_time(00:12:00, CAST(NULL AS STRING)))#x]
++- OneRowRelation
+
+
 -- !query
 select to_time("11", "HH")
 -- !query analysis

sql/core/src/test/resources/sql-tests/inputs/time.sql

@@ -11,6 +11,10 @@ select time '16:39:45\t';
 select to_time(null), to_time('01:02:03'), to_time('23-59-59.999999', 'HH-mm-ss.SSSSSS');
 select to_time(time_str, fmt_str) from time_view;
 
+-- SPARK-58296: a NULL format must not change the result type of `to_time` (stays TIME).
+select typeof(to_time('00:12:00', null));
+select typeof(to_time('00:12:00', cast(null as string)));
+
 -- missing fields in `to_time`
 select to_time("11", "HH");
 -- invalid: there is no 13 hours

sql/core/src/test/resources/sql-tests/results/time.sql.out

@@ -47,6 +47,22 @@ struct<to_time(time_str, fmt_str):time(6)>
 11:53:26.038344
 
 
+-- !query
+select typeof(to_time('00:12:00', null))
+-- !query schema
+struct<typeof(to_time(00:12:00, NULL)):string>
+-- !query output
+time(6)
+
+
+-- !query
+select typeof(to_time('00:12:00', cast(null as string)))
+-- !query schema
+struct<typeof(to_time(00:12:00, CAST(NULL AS STRING))):string>
+-- !query output
+time(6)
+
+
 -- !query
 select to_time("11", "HH")
 -- !query schema