How can I test at compile time whether T is an instance of an interface ?

[data pulverizer]

On Wednesday, 23 September 2020 at 20:19:04 UTC, data pulverizer 
wrote:
> This has prompted me to write a data structure that I thought 
> would be impossible until now. [...SNIP...]

Here is the function with the correct template constraint:

```
auto makeChain(Args...)(Args args)
if(Args.length > 2 && is(Args[0] == Start) && is(Args[Args.length 
- 1] == End))
{
   args[0].next = args[1];
   static foreach(i; 1..(Args.length - 1))
   {
     args[i].prev = args[i - 1];
     args[i].next = args[i + 1];
   }
   args[$ - 1].prev = args[$ - 2];
   return args[0];
}
```