Constraining template with function signature

[Carl Vogel via Digitalmars-d-learn]

Hi,

What's the best way, when using a Callable as a template 
parameter, to constrain it by signature?

For example, if you have a function that sorts an array like so:

T[] sortArray(alias less, T)(T[] arr) { ... }

Then you know that you'd want `less` to have signature (T, T) -> 
bool.

Now, I can use something like isCallable std.traits to make sure 
the predicate is a Callable, and there are various function 
traits in the module that I could combine with `is` clauses to 
enforce the signature it seems, but that seems very clunky.  Is 
there a better way?

I note that I don't find many examples of this in phobos, so I'm 
wondering if there actually is a good solution...

Thanks,
-c.