diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
index 913d846a8c23..f987ed880176 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
@@ -2234,7 +2234,9 @@ object EliminateUnions extends Rule[LogicalPlan] {
 /**
  * Cleans up unnecessary Aliases inside the plan. Basically we only need Alias as a top level
  * expression in Project(project list) or Aggregate(aggregate expressions) or
- * Window(window expressions).
+ * Window(window expressions). Notice that if an expression has other expression parameters which
+ * are not in its `children`, e.g. `RuntimeReplaceable`, the transformation for Aliases in this
+ * rule can't work for those parameters.
  */
 object CleanupAliases extends Rule[LogicalPlan] {
   private def trimAliases(e: Expression): Expression = {
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
index b847ef7bfaa9..74c4cddf2b47 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
@@ -241,6 +241,10 @@ trait RuntimeReplaceable extends UnaryExpression with Unevaluable {
   override def nullable: Boolean = child.nullable
   override def foldable: Boolean = child.foldable
   override def dataType: DataType = child.dataType
+  // As this expression gets replaced at optimization with its `child" expression,
+  // two `RuntimeReplaceable` are considered to be semantically equal if their "child" expressions
+  // are semantically equal.
+  override lazy val canonicalized: Expression = child.canonicalized
 }
 
 
diff --git a/sql/core/src/test/resources/sql-tests/inputs/sql-compatibility-functions.sql b/sql/core/src/test/resources/sql-tests/inputs/sql-compatibility-functions.sql
index 2b5b692d29ef..f1461032065a 100644
--- a/sql/core/src/test/resources/sql-tests/inputs/sql-compatibility-functions.sql
+++ b/sql/core/src/test/resources/sql-tests/inputs/sql-compatibility-functions.sql
@@ -23,3 +23,7 @@ SELECT float(1), double(1), decimal(1);
 SELECT date("2014-04-04"), timestamp(date("2014-04-04"));
 -- error handling: only one argument
 SELECT string(1, 2);
+
+-- SPARK-21555: RuntimeReplaceable used in group by
+CREATE TEMPORARY VIEW tempView1 AS VALUES (1, NAMED_STRUCT('col1', 'gamma', 'col2', 'delta')) AS T(id, st);
+SELECT nvl(st.col1, "value"), count(*) FROM from tempView1 GROUP BY nvl(st.col1, "value");
diff --git a/sql/core/src/test/resources/sql-tests/results/sql-compatibility-functions.sql.out b/sql/core/src/test/resources/sql-tests/results/sql-compatibility-functions.sql.out
index 732b11050f46..e035505f15d2 100644
--- a/sql/core/src/test/resources/sql-tests/results/sql-compatibility-functions.sql.out
+++ b/sql/core/src/test/resources/sql-tests/results/sql-compatibility-functions.sql.out
@@ -1,5 +1,5 @@
 -- Automatically generated by SQLQueryTestSuite
--- Number of queries: 13
+-- Number of queries: 15
 
 
 -- !query 0
@@ -122,3 +122,19 @@ struct<>
 -- !query 12 output
 org.apache.spark.sql.AnalysisException
 Function string accepts only one argument; line 1 pos 7
+
+
+-- !query 13
+CREATE TEMPORARY VIEW tempView1 AS VALUES (1, NAMED_STRUCT('col1', 'gamma', 'col2', 'delta')) AS T(id, st)
+-- !query 13 schema
+struct<>
+-- !query 13 output
+
+
+
+-- !query 14
+SELECT nvl(st.col1, "value"), count(*) FROM from tempView1 GROUP BY nvl(st.col1, "value")
+-- !query 14 schema
+struct<nvl(tempview1.`st`.`col1` AS `col1`, 'value'):string,FROM:bigint>
+-- !query 14 output
+gamma	1