Choosing arity of a template function

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On Friday, 26 February 2016 at 23:09:52 UTC, Andrei Alexandrescu 
wrote:
> So, what's an elegant solution to this? I looked up std.traits 
> but nothing seems to help. (Tried arity, no avail.)

There's this[1] PR which implements exactly what you want, but 
it's currently not passing the auto-tester.

1. https://github.com/D-Programming-Language/dmd/pull/5201