Templates and delegates trouble

[Lars Kyllingstad]

Hello,

I want to make a function (named 'call' in the example below) that calls 
another function (F). I want to allow F to be an ordinary function, a 
delegate, or a functor. I've been able to make it work with functions 
and functors, but not with delegates. Any tips would be greatly appreciated.

I have defined 'call' like this:

     T call(alias F, T=real)(T arg)
     {
         return F(arg);
     }

I can now do like this:

     call!(sqrt)(4.0);

or

     class Sqrt
     {
         real opCall(real x) { return sqrt(x); }
     }

     auto sqrt = new Sqrt;
     call!(sqrt)(4.0);

Both of these examples compile and the function returns 2.0 as expected. 
I can not, however, do this:

     class Sqrt
     {
         real eval(real x) { return sqrt(x); }
     }

     auto sqrt = new Sqrt;
     call!(sqrt.eval)(4.0);

To this, the compiler (GDC) says:

     Error: need 'this' to access member eval

So it would seem that even though eval is a method of the Sqrt class, F 
aliases it as a function pointer. Is there any way of making this work?

-Lars