Address comments.

Author: viirya

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -728,8 +728,8 @@ object FoldablePropagation extends Rule[LogicalPlan] {
           stop = true
           j
 
-        // Operators that operate on objects should only have expressions from encoders, which
-        // should never have foldable expressions.
+        // These 3 operators take attributes as constructor parameters, and these attributes
+        // can't be replaced by alias.
         case m: MapGroups =>
           stop = true
           m

sql/core/src/test/scala/org/apache/spark/sql/DatasetSuite.scala

@@ -884,14 +884,12 @@ class DatasetSuite extends QueryTest with SharedSQLContext {
 
     import df.sparkSession.implicits._
 
-    assertResult(Seq()) {
+    checkDataset(
       df.withColumn("b", lit(0)).as[ClassData]
-        .groupByKey(_.a).flatMapGroups { case (x, iter) => List[Int]() }.collect().toSeq
-    }
-    assertResult(Seq()) {
+        .groupByKey(_.a).flatMapGroups { case (x, iter) => List[Int]() })
+    checkDataset(
       df.withColumn("b", expr("0")).as[ClassData]
-        .groupByKey(_.a).flatMapGroups { case (x, iter) => List[Int]() }.collect().toSeq
-    }
+        .groupByKey(_.a).flatMapGroups { case (x, iter) => List[Int]() })
   }
 }