D metaclass stuff vs C++23

[Andrei Alexandrescu]

On 5/13/19 11:25 AM, Alex wrote:
> On Monday, 13 May 2019 at 09:06:07 UTC, Russel Winder wrote:
>> Hi,
>>
>> In the "show what can be done, not what should be done" category, 
>> someone has done a C++ version of Hello World using the up-coming 
>> metaclass stuff:
>>
>> https://twitter.com/Cor3ntin/status/1127210941718962177
>>
>> I immediately (once I knew about it) replied with the Python version:
>>
>> https://twitter.com/russel_winder/status/1127859587393163265
>>
>>
>>    class Hello: pass
>>
>>    World = None
>>
>>    if __name__ == '__main__':
>>        print(" ".join([item for item in dir() if not 
>> item.startswith("_")]))
>>
>>
>> but clearly a better competitor would be a D version. Anyone done this 
>> sort of thing, i.e. compile time metaclass manipulation?
> 
> Something like this is possible (for a long time now, I think...),
> 
> ´´´
> import std.format;
> struct Hello;
> int World;
> 
> pragma(msg, format!"%s %s"(__traits(allMembers, mixin(__MODULE__))[$-2 
> .. $]));
> ´´´
> 
> but I'm sure, more cool versions are possible ;)

The idea is to just provide something simple that doesn't introduce a 
bunch of stuff. Here's what I wrote:

https://twitter.com/incomputable/status/1127891251666223105