Filter AA at compile time

[Andrey via Digitalmars-d-learn]

Hi,
I want to filter AA at compile time and do this:
void main()
{
    // datamap is some AA
    enum qaz = "qq";
    enum types = datamap.byKeyValue.filter!(pair => 
qaz.isGood(pair)).assocArray();

    types.writeln;
}

But compiler says:
Error: _aaRange cannot be interpreted at compile time, because 
it has no available source code

Why, where is a problem? And how to filter it correctly? Write my 
own foreach-loop?