Support count distinct on function in hql (#2502)

Fixes #1285

Author: bahusoid

src/NHibernate.Test/Async/Hql/Ast/HqlFixture.cs

@@ -306,5 +306,14 @@ public async Task UnaryMinusBeforeParenthesesHandledCorrectlyAsync()
 				Assert.That(actualWorkaround, Is.EqualTo(-2));
 			}
 		}
+
+		//NH-3249 (GH-1285)
+		[Test]
+		public async Task CountDistinctOnFunctionAsync()
+		{
+			var hql = @"SELECT COUNT(DISTINCT DATE(m.birthdate)) FROM Mammal m";
+			using(var s = OpenSession())
+				await (s.CreateQuery(hql).ListAsync());
+		}
 	}
 }

src/NHibernate.Test/Async/Linq/ByMethod/CountTests.cs

@@ -40,6 +40,21 @@ public async Task CountDistinctProperty_ReturnsNumberOfDistinctEntriesForThatPro
 			Assert.That(result, Is.EqualTo(387));
 		}
 
+		//NH-3249 (GH-1285)
+		[Test]
+		public async Task CountDistinctFunc_ReturnsNumberOfDistinctEntriesForThatFuncAsync()
+		{
+			if (!TestDialect.SupportsCountDistinct)
+				Assert.Ignore("Dialect does not support count distinct");
+
+			var result = await (db.Orders
+				.Select(x => x.ShippingDate.Value.Date)
+				.Distinct()
+				.CountAsync());
+
+			Assert.That(result, Is.EqualTo(387));
+		}
+
 		[Test]
 		public async Task CountProperty_ReturnsNumberOfNonNullEntriesForThatPropertyAsync()
 		{

src/NHibernate.Test/Hql/Ast/HqlFixture.cs

@@ -337,5 +337,14 @@ public void UnaryMinusBeforeParenthesesHandledCorrectly()
 				Assert.That(actualWorkaround, Is.EqualTo(-2));
 			}
 		}
+
+		//NH-3249 (GH-1285)
+		[Test]
+		public void CountDistinctOnFunction()
+		{
+			var hql = @"SELECT COUNT(DISTINCT DATE(m.birthdate)) FROM Mammal m";
+			using(var s = OpenSession())
+				s.CreateQuery(hql).List();
+		}
 	}
 }

src/NHibernate.Test/Linq/ByMethod/CountTests.cs

@@ -28,6 +28,21 @@ public void CountDistinctProperty_ReturnsNumberOfDistinctEntriesForThatProperty(
 			Assert.That(result, Is.EqualTo(387));
 		}
 
+		//NH-3249 (GH-1285)
+		[Test]
+		public void CountDistinctFunc_ReturnsNumberOfDistinctEntriesForThatFunc()
+		{
+			if (!TestDialect.SupportsCountDistinct)
+				Assert.Ignore("Dialect does not support count distinct");
+
+			var result = db.Orders
+				.Select(x => x.ShippingDate.Value.Date)
+				.Distinct()
+				.Count();
+
+			Assert.That(result, Is.EqualTo(387));
+		}
+
 		[Test]
 		public void CountProperty_ReturnsNumberOfNonNullEntriesForThatProperty()
 		{

src/NHibernate/Hql/Ast/ANTLR/Hql.g

@@ -627,9 +627,14 @@ aggregateArgument
 	;
 
 aggregateDistinctAll
-	: ( ( DISTINCT | ALL )? ( path | collectionExpr ) )
+	: ( distinctAll aggregateArgument ) => (distinctAll aggregateArgument)
+	| aggregateArgument
 	;
-	
+
+distinctAll
+	: ( DISTINCT | ALL ) 
+	;
+
 //## collection: ( OPEN query CLOSE ) | ( 'elements'|'indices' OPEN path CLOSE );
 
 collectionExpr