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digitalmars.D.learn - Filter AA at compile time

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 => 
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?
Dec 26 2018