[Patch] Use variadic templates for tr1::is_function

[Paolo Carlini]

Doug Gregor wrote:

> That's not a valid use of result_of.
>
> result_of<F(A1, A2, ..., AN)>::type is the return type when calling a  
> function object of type F with arguments a1, a2, ..., aN of types A1,  
> A2, ..., AN, respectively. In your example use, F is "int", which is  
> not a valid function object.

Humm, ok, thanks, I'm still trying to digest the whole idea...

> Now, you could do something like:
>
>     result_of<int(double)(double)>::type
>
> That would be int.

But, unless I'm doing some stupid mistake, it doesn't compile...

Paolo.