[SPARK-16836][SQL] Add support for CURRENT_DATE/CURRENT_TIMESTAMP literals

## What changes were proposed in this pull request?
In Spark 1.6 (with Hive support) we could use `CURRENT_DATE` and `CURRENT_TIMESTAMP` functions as literals (without adding braces), for example:
```SQL
select /* Spark 1.6: */ current_date, /* Spark 1.6  & Spark 2.0: */ current_date()
```
This was accidentally dropped in Spark 2.0. This PR reinstates this functionality.

## How was this patch tested?
Added a case to ExpressionParserSuite.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #14442 from hvanhovell/SPARK-16836.

1 files changed, 10 insertions, 1 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala
index 6e485a8f5b..8e7c8d7f07 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala
@@ -18,7 +18,7 @@
 package org.apache.spark.sql
 
 import java.math.MathContext
-import java.sql.Timestamp
+import java.sql.{Date, Timestamp}
 
 import org.apache.spark.{AccumulatorSuite, SparkException}
 import org.apache.spark.sql.catalyst.analysis.UnresolvedException
@@ -3017,4 +3017,13 @@ class SQLQuerySuite extends QueryTest with SharedSQLContext {
         data.selectExpr("`part.col1`", "`col.1`"))
     }
   }
+
+  test("current_date and current_timestamp literals") {
+    // NOTE that I am comparing the result of the literal with the result of the function call.
+    // This is done to prevent the test from failing because we are comparing a result to an out
+    // dated timestamp (quite likely) or date (very unlikely - but equally annoying).
+    checkAnswer(
+      sql("select current_date = current_date(), current_timestamp = current_timestamp()"),
+      Seq(Row(true, true)))
+  }
 }