Using delegates in callbacks for extern(C) functions

[simendsjo]

I have a C function taking a callback function as a parameter. My thought  
was to wrap this up using a template, but I cannot get it to work:

extern(C) alias void function() Callback;
template Wrap(alias dg)
{
    extern(C) void Wrap()
    {
        dg();
    }
}

void main()
{
    Callback cb = &Wrap!( () {} );
}

Error: cannot implicitly convert expression (&Wrap) of type void  
delegate() pure nothrow @safe to extern (C) void function()