The issue is here

http://jira.nhibernate.org/browse/NH-1393

Basically, the problem is we cannot do following

        [Test]
        public void CanAvgProjectionOnSqlFunction()
        {
            using (ISession s = OpenSession())
            {
                ISQLFunction arithmaticAddition = new VarArgsSQLFunction("(", "+", ")");
                ICriteria c = s.CreateCriteria(typeof(Person))
                    .SetProjection(
                    Projections.Avg(Projections.SqlFunction(arithmaticAddition,
                                                            NHibernateUtil.GuessType(typeof(double)),
                                                            Projections.Property("IQ"),
                                                            Projections.Property("ShoeSize"))));
                IList list = c.List();
                Assert.AreEqual( 334/5,list[0]);
            }
        }

What I want to ask is, should I go for SqlFunction projections, or any projection. Projection is not error-safe, I mean user may try to aggregate on Subquery projection, which is illegal in MSSQL(and probably the others.
So my question is, Should I go for Projection or restrict it to SqlFunctionProjection.