Simple model:

[Serializable]
[HierarchyRoot]
public class MyEntity : Entity
{
    [Field, Key]
    public int Id { get; private set; }

    [Field(Length = 100)]
    public string T1 { get; set; }

    [Field(Length = 100)]
    public string T2 { get; set; }
}

and simple query:

session.Query.All<MyEntity>()
    .GroupBy(a => new { a.T1, a.T2 }, (a, b) => new { a.T1, a.T2, Count = b.Count() })
    .ToArray();

works ok, but if I replace anonymous types with Named:

session.Query.All<MyEntity>()
    .GroupBy(a => new K { T1 = a.T1, T2 = a.T2 }, (a, b) => new S { T1 = a.T1, T2 = a.T2, Count = b.Count() })
    .ToArray();

...

public class K
{
    public string T1 { get; set; }
    public string T2 { get; set; }
}
public class S
{
    public string T1 { get; set; }
    public string T2 { get; set; }
    public int Count { get; set; }
}

I got exception:

Xtensive.Orm.QueryTranslationException was unhandled
  Message=Unable to translate 'Query.All().GroupBy(
  a => new Program.K() {
    T1 = a.T1,
    T2 = a.T2
  },
  (a, b) => new Program.S() {
    T1 = a.T1,
    T2 = a.T2,
    Count = b.Count()
  }
)' expression. See inner exception for details.
  Source=Xtensive.Orm
  StackTrace:
       at Xtensive.Orm.Linq.QueryProvider.Translate[TResult](Expression expression) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\QueryProvider.cs:line 85
       at Xtensive.Orm.Linq.QueryProvider.Execute[TResult](Expression expression) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\QueryProvider.cs:line 69
       at Xtensive.Orm.Linq.Queryable`1.GetEnumerator() in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Queryable.cs:line 65
       at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
       at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
       at Project28.Program.Main(String[] args) in c:\users\klekovkin\documents\visual studio 2010\Projects\Project28\Project28\Program.cs:line 35
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: System.InvalidOperationException
       Message=Internal error: Unable to remap KeyExpression.
       Source=Xtensive.Orm
       StackTrace:
            at Xtensive.Orm.Linq.Expressions.KeyExpression.Remap(Int32[] map, Dictionary`2 processedExpressions) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\KeyExpression.cs:line 69
            at Xtensive.Orm.Linq.Expressions.EntityExpression.Remap(Int32[] map, Dictionary`2 processedExpressions) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\EntityExpression.cs:line 73
            at Xtensive.Orm.Linq.Expressions.FieldExpression.Remap(Int32[] map, Dictionary`2 processedExpressions) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\FieldExpression.cs:line 77
            at Xtensive.Orm.Linq.ConstructorExpression.<>c__DisplayClass1a.<Remap>b__15(IMappedExpression mapped) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\ConstructorExpression.cs:line 75
            at Xtensive.Orm.Linq.Expressions.Visitors.GenericExpressionVisitor`1.VisitGenericExpression(T generic) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\Visitors\GenericExpressionVisitor.cs:line 52
            at Xtensive.Orm.Linq.Expressions.Visitors.GenericExpressionVisitor`1.VisitUnknown(Expression e) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\Visitors\GenericExpressionVisitor.cs:line 35
            at Xtensive.Linq.ExpressionVisitor`1.Visit(Expression e) in c:\Projects\dataobjects.net\Xtensive.Core\Xtensive.Core\Linq\ExpressionVisitor{TResult}.cs:line 128
            at Xtensive.Orm.Linq.Expressions.Visitors.GenericExpressionVisitor`1.Process(Expression target, Func`2 genericProcessor) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\Visitors\GenericExpressionVisitor.cs:line 25
            at Xtensive.Orm.Linq.ConstructorExpression.<>c__DisplayClass1a.<Remap>b__17(KeyValuePair`2 kvp) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\ConstructorExpression.cs:line 78
            at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
            at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector)
            at Xtensive.Orm.Linq.ConstructorExpression.Remap(Int32[] map, Dictionary`2 processedExpressions) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\ConstructorExpression.cs:line 78
            at Xtensive.Orm.Linq.Expressions.ItemProjectorExpression.<>c__DisplayClass4.<Remap>b__3(IMappedExpression mapped) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\ItemProjectorExpression.cs:line 58
            at Xtensive.Orm.Linq.Expressions.Visitors.GenericExpressionVisitor`1.VisitGenericExpression(T generic) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\Visitors\GenericExpressionVisitor.cs:line 52
            at Xtensive.Orm.Linq.Expressions.Visitors.GenericExpressionVisitor`1.VisitUnknown(Expression e) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\Visitors\GenericExpressionVisitor.cs:line 35
            at Xtensive.Linq.ExpressionVisitor`1.Visit(Expression e) in c:\Projects\dataobjects.net\Xtensive.Core\Xtensive.Core\Linq\ExpressionVisitor{TResult}.cs:line 128
            at Xtensive.Orm.Linq.Expressions.Visitors.GenericExpressionVisitor`1.Process(Expression target, Func`2 genericProcessor) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\Visitors\GenericExpressionVisitor.cs:line 28
            at Xtensive.Orm.Linq.Expressions.ItemProjectorExpression.Remap(RecordQuery dataSource, Int32[] columnMap) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Expressions\ItemProjectorExpression.cs:line 58
            at Xtensive.Orm.Linq.Translator.VisitGroupBy(Type returnType, Expression source, LambdaExpression keySelector, LambdaExpression elementSelector, LambdaExpression resultSelector) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Translator.Queryable.cs:line 751
            at Xtensive.Orm.Linq.Translator.VisitQueryableMethod(MethodCallExpression mc, QueryableMethodKind methodKind) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Translator.Queryable.cs:line 142
            at Xtensive.Linq.QueryableVisitor.VisitMethodCall(MethodCallExpression mc) in c:\Projects\dataobjects.net\Xtensive.Core\Xtensive.Core\Linq\QueryableVisitor.cs:line 34
            at Xtensive.Orm.Linq.Translator.VisitMethodCall(MethodCallExpression mc) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Translator.Expressions.cs:line 401
            at Xtensive.Linq.ExpressionVisitor`1.Visit(Expression e) in c:\Projects\dataobjects.net\Xtensive.Core\Xtensive.Core\Linq\ExpressionVisitor{TResult}.cs:line 106
            at Xtensive.Orm.Linq.Translator.Visit(Expression e) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Translator.Expressions.cs:line 84
            at Xtensive.Orm.Linq.Translator.Translate[TResult]() in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\Translator.Materialization.cs:line 33
            at Xtensive.Orm.Linq.QueryProvider.Translate[TResult](Expression expression) in c:\Projects\dataobjects.net\Xtensive.Orm\Xtensive.Orm\Orm\Linq\QueryProvider.cs:line 80
       InnerException: