Choosing arity of a template function

[Andrei Alexandrescu via Digitalmars-d]

A generic function receives an argument called "partition" by alias. 
That may work in one of the following ways:

partition(range);

partition!less(range);

partition!less(range, n); // n is a number

I tried this:

static if (is(partition == function) || is(partition == delegate))
   partition(r);
else if (__traits(compiles, partition!less(r, n)))
   partition!less(r, n);
else
   partition!less(r);

The first test works very nice. The second does not; the compiler 
attempts to instantiate the template wrongly and spits a bunch of errors 
before giving up, in spite of the whole "let me know silently whether 
this compiles" thing.

So, what's an elegant solution to this? I looked up std.traits but 
nothing seems to help. (Tried arity, no avail.)


Thanks,

Andrei