What does this compile-time error mean?

[Don]

On Sunday, 24 February 2013 at 19:13:47 UTC, Jonathan M Davis 
wrote:
> On Sunday, February 24, 2013 14:05:26 Timon Gehr wrote:
>> On 02/24/2013 01:59 PM, Maxim Fomin wrote:
>> > On Sunday, 24 February 2013 at 09:00:17 UTC, Jonathan M 
>> > Davis wrote:
>> >> Because main_cont is module-level variable, it must be 
>> >> initialized with a
>> >> value at compile time. Classes can be used at compile time 
>> >> (at least
>> >> some of
>> >> the time), but they cannot stick around between compile 
>> >> time and runtime,
>> >> meaning that you could potentially use them in a CTFE 
>> >> function, but
>> >> you can't
>> >> initialize a module-level or static variable (or enum) with 
>> >> them, and
>> >> you're
>> >> attempting to initialize maint_cont with a RedBlackTree, 
>> >> which is a
>> >> class. It
>> >> won't work.
>> >> 
>> >> - Jonathan M Davis
>> > 
>> > I would say that it is arbitrary restriction
>> >
>> >...
>> 
>> Yes, IIRC Don once stated that it is a problem with DMD's 
>> architecture.
>
> Most restrictions in CTFE are a limitation of the current 
> implementation
> rather than being something that's literally impossible to do 
> (though some
> like the lack of source code aren't). In the case of a class, I 
> believe that
> it would have to basically serialize the class at compile time 
> and then
> deserialize it at runtime to do it. That's certainly possible 
> to do in theory,
> but it's well beyond what the current CTFE implementation can 
> do.
>
> - Jonathan M Davis

Yeah, this one is a limitation of the runtime, not of CTFE. CTFE 
has no problem with it.