Determine if a type if derived from a template parameter

[Alex]

e.g.,

class X(T,S)
{
    T x;
    S y;
}

Somehow determine if x's type is derived from the template 
parameter.

I doubt D has this capability but it would be nice for certain 
things. In my reflect library the types must be specified such as 
Reflect!(X!(int,float)) and ideally I would like to do 
Reflect!(X!(T,S)) to get generic reflection information. This 
helps reduce overhead as one could reflect once on the generic 
type, cache the results, then simply modify the results for 
specifics.