Do not append from element when already in from

src/NHibernate.Test/Async/NHSpecificTest/GH3465/Fixture.cs

@@ -0,0 +1,31 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by AsyncGenerator.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+
+using System.Linq;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH3465
+{
+	using System.Threading.Tasks;
+	[TestFixture]
+	public class FixtureAsync : BugTestCase
+	{
+		[Test]
+		public void ThetaJoinSubQueryAsync()
+		{
+			using (var session = OpenSession())
+			using (session.BeginTransaction())
+			{
+				var query = session.CreateQuery("select e.Id from EntityA e where exists (from e.Children b, EntityC c)");
+				Assert.DoesNotThrowAsync(() => query.ListAsync());
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH3465/Entities.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+
+namespace NHibernate.Test.NHSpecificTest.GH3465
+{
+	class EntityA
+	{
+		public virtual Guid Id { get; set; }
+		public virtual ISet<EntityB> Children { get; set; }
+	}
+	class EntityB
+	{
+		public virtual Guid Id { get; set; }
+		public virtual EntityA Parent { get; set; }
+	}
+	class EntityC
+	{
+		public virtual Guid Id { get; set; }
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH3465/Fixture.cs

@@ -0,0 +1,20 @@
+using System.Linq;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH3465
+{
+	[TestFixture]
+	public class Fixture : BugTestCase
+	{
+		[Test]
+		public void ThetaJoinSubQuery()
+		{
+			using (var session = OpenSession())
+			using (session.BeginTransaction())
+			{
+				var query = session.CreateQuery("select e.Id from EntityA e where exists (from e.Children b, EntityC c)");
+				Assert.DoesNotThrow(() => query.List());
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH3465/Mappings.hbm.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernate.Test"
+                   namespace="NHibernate.Test.NHSpecificTest.GH3465">
+  <class name="EntityA">
+    <id name="Id" generator="guid.comb" />
+    <set name="Children" cascade="all-delete-orphan" inverse="true">
+      <key column="EntityAId" />
+      <one-to-many class="EntityB" />
+    </set>
+  </class>
+  <class name="EntityB">
+    <id name="Id" generator="guid.comb" />
+    <many-to-one name="Parent" column="EntityAId" class="EntityA" not-null="true" />
+  </class>
+  <class name="EntityC">
+    <id name="Id" generator="guid.comb" />
+  </class>
+</hibernate-mapping>

src/NHibernate/Hql/Ast/ANTLR/Tree/FromElement.cs

@@ -716,7 +716,7 @@ public void SetOrigin(FromElement origin, bool manyToMany)
 					JoinSequence.SetUseThetaStyle(true);
 				}
 			}
-			else
+			else if (Walker.CurrentClauseType != HqlSqlWalker.FROM)
 			{
 				FromClause.AppendFromElement(this);
 			}