Creating a template mixin for explicit casts.

[Sjoerd Nijboer via Digitalmars-d-learn]

Given the following code
`struct Foo(T)
        if(isNumeric!T)
{
        T t;
        .. other code
}
struct Bar(T)
        if(isNumeric!T)
{
        T t;
        .. other code
}

Foo!float foo_float;

Foo!double foo_double;

Bar!float bar_float;
`

I want to make a template mixin that is able to cast one of these 
generic structs to the other explicitly. I have a bunch of these 
structs and therefore I thought it would make sense to do it by 
template mixin. I just don't know what language or library 
features I need to use and how to apply them.

`
fooDouble = cast (Foo!double) foo_float;		// case [1] if 
possible, done by
                                                        // an implicit cast. 
Else by
                                                        //explicit.

fooFloat = cast (Foo!float) foo_double;                 // case [2]

barFloat = cast (Bar!float) foo_float;                  // case [3]
`

How would I do this in D?