[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, 13 insertions, 0 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala
index f2cc8d3624..679adf2717 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala
@@ -1023,6 +1023,19 @@ class AstBuilder extends SqlBaseBaseVisitor[AnyRef] with Logging {
   }
 
   /**
+   * Create a current timestamp/date expression. These are different from regular function because
+   * they do not require the user to specify braces when calling them.
+   */
+  override def visitTimeFunctionCall(ctx: TimeFunctionCallContext): Expression = withOrigin(ctx) {
+    ctx.name.getType match {
+      case SqlBaseParser.CURRENT_DATE =>
+        CurrentDate()
+      case SqlBaseParser.CURRENT_TIMESTAMP =>
+        CurrentTimestamp()
+    }
+  }
+
+  /**
    * Create a function database (optional) and name pair.
    */
   protected def visitFunctionName(ctx: QualifiedNameContext): FunctionIdentifier = {